Taurine kinetics assessed using [1,2-13C2]taurine in healthy adult humans

Benjamin Rakotoambinina,1,3 Lisa Marks,1 Abdul Monem Badran,1 Frank Igliki,1 François Thuillier,1 Pascal Crenn,1 Bernard Messing,1 and Dominique Darmaun2

1Institut National de la Santé et de la Recherche Médicale U290, Hôpital Lariboisière-Saint-Lazare, 75475 Paris; 2Institut National de la Santé et de la Recherche Médicale U539, Centre de Recherche en Nutrition Humaine, 44093 Nantes, France; and 3Department of Physiology, University of Antananarivo School of Medicine, Antananarivo, Madagascar

Submitted 18 July 2003 ; accepted in final form 27 February 2004


    ABSTRACT
 TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
To assess the dynamics of taurine metabolism in vivo, two sets of studies were carried out in healthy volunteers. First, pilot studies were carried in a single human subject to determine the time course of plasma and whole blood isotope enrichment over the course of an 8-h, unprimed continuous infusion of [1,2-13C2]taurine. Second, five healthy adult males received two tracer infusions on separate days and in randomized order: 1) a 6-h continuous infusion of [1,2-13C2]taurine (3.1 ± 0.2 µmol·kg–1·h–1) and 2) a bolus injection of [13C2]taurine (3.0 ± 0.1 µmol/kg). Isotope enrichments in plasma and whole blood taurine were determined by gas chromatography-mass spectrometry. The pilot experiments allowed us to establish that steady-state isotope enrichment was reached in plasma and whole blood by the 5th h of tracer infusion. The plateau enrichment reached in whole blood was lower than that obtained in plasma taurine (P < 0.02). In the second set of studies, the appearance rate (Ra) of plasma taurine, determined from continuous infusion studies was 31.8 ± 3.1 µmol·kg–1·h–1. After a bolus injection of tracer, the enrichment decay over the subsequent 2 h was best fitted by a two-exponential curve. Taurine Ra was {approx}85% higher when determined using the bolus injection technique compared with continuous infusion of tracer. We conclude that 1) taurine Ra into plasma is very low in healthy postabsorptive humans, and, due to taurine compartmentation between the extra- and intracellular milieus, may represent only interorgan taurine transfer and merely a small fraction of whole body taurine turnover; and 2) the bolus injection technique may overestimate taurine appearance into plasma. Further studies are warranted to determine whether alterations in bile taurine dynamics affect taurine Ra.

constant infusion; bolus injection technique


TAURINE (2-ETHANEAMINOSULFONIC ACID) IS, after glutamine, the second most abundant free amino acid in mammalian cells (2) and is involved in various physiological functions, including osmoregulation, defense against oxidative stress, detoxification of xenobiotics, membrane stabilization, retinal function, and bile conjugation (22). The two sources of body taurine are dietary intake and de novo taurine synthesis from methionine and cysteine, its sulfur amino acid precursors (22). Due to the limited taurine synthetic capacity of the newborn, taurine is considered a conditionally essential amino acid in infants (28). Taurine depletion has been described in adult patient populations as well (13, 50), as a result of either 1) increased utilization (11), 2) decreased intake (38), 3) decreased rate of de novo synthesis (45), or 4) various combinations of the three.

Most of the knowledge of the dynamics of taurine metabolism, however, is derived from the measurement of taurine concentration, which provides little information as to the mechanisms leading to taurine depletion and repletion (11, 35, 45, 48, 50).

In theory, the use of tracer dilution techniques should provide further insight into these mechanisms and help in designing strategies for taurine supplementation as well. Although tracer methodology has been applied extensively to the exploration of various amino acids, including methionine and cysteine (37), very few data have appeared with regard to in vivo taurine kinetics. The first estimates of taurine kinetics were reported from compartmental analysis using either [35S]taurine bolus injection in humans (44) or bolus injection of [35S]taurine and [13C]taurine in rhesus monkeys (33). In recent years, we developped a gas chromatography-mass spectrometry (GC-MS) assay to determine both the concentration and the stable isotope enrichment of taurine in blood and obtained preliminary estimates of taurine kinetics in human subjects by use of [1,2-13C2]taurine tracer (31). The aims of the present study were therefore 1) to expand on these preliminary data and assess the turnover rate of circulating taurine and the size of taurine tracer-miscible pools in healthy adults in the fasting state and 2) to determine whether the administration of labeled taurine as an intravenous bolus injection would yield kinetic parameters comparable to those obtained with constant infusion.


    METHODS
 TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Subjects and protocol design. Six healthy, 36 ± 8-yr-old men (mean ± SD; range 29–46 yr) with normal body weight (72.5 ± 3.6 kg) and body mass index (22.8 ± 1.4 kg/m2) were recruited (Table 1). They had taken neither vitamin (especially B6) supplements nor any medication for ≥3 mo before the initiation of the study (44). Over the week before study, they consumed a regular Western diet that was not supplemented with taurine (2,219 ± 291 kcal/day, 1.2 ± 0.15 g·kg–1·day–1 protein; 116 ± 16 mg taurine/day). Each subject gave informed, written consent before the study, according to protocols approved by the Lariboisière-Saint-Lazare Hospital ethics committee. All tests were performed in the morning while subjects were in the postabsorptive state as outpatients at the Institut National de la Santé et de la Recherche Médicale U290 Laboratory of Lariboisière-Saint-Lazare Hospital.


View this table:
[in this window]
[in a new window]
 
Table 1. Characteristics of study subjects

 
A first set of preliminary experiments (protocol A) was carried out in a single subject to determine whether an isotopic steady state could be achieved in blood taurine enrichment by using a constant intravenous infusion at rates of either 2 or 3 µmol·kg–1·h–1 with or without a priming dose (31).

A priming dose equivalent to 1 h of tracer infusion rate was chosen for several reasons: 1) assuming that taurine would be distributed throughout body water, taurine volume of distribution would be equivalent to total body water (0.6 liter/kg), and assuming the average taurine concentration in body tissues to be close to that measured in plasma ({approx}50 µmol/l), we estimated body taurine pool at 0.6 x 50 = 30 µmol/kg, so that a 3 µmol/kg labeled taurine prime would enable us to instantaneously achieve a taurine enrichment {approx}10%; and 2) in a first set of preliminary studies using a bolus injection, we had obtained an estimate of {approx}2.7 mmol (37 µmol/kg) for the "rapid" taurine pool, and because we aimed at a plateau enrichment ~8% during the primed, continuous infusion, we therefore elected to use 3 µmol/kg as a priming dose.

The priming dose was injected over 1–2 min and immediately followed by a continuous infusion using a syringe pump (PerfusorV, B Braun, Melsungen, Germany) at a rate of 6 ml/h. Blood samples were drawn at 0, 10, 20, and 30 min after the bolus and every 30 min thereafter until 480 min. Expired air was collected at hourly intervals throughout the study for 13CO2 determination.

In the second set of studies (protocol B), five subjects were studied on two separate occasions while receiving [1,2-13C2]taurine 1) as a primed continuous infusion on one day and 2) as a single intravenous bolus injection on the other day. The two studies were ≥1 wk apart and in random order.

For the primed, continuous infusion study, [13C2]taurine was administered at a rate of 3 µmol·kg–1·h–1 via a peripheral venous line, and a catheter placed in a dorsal vein of the contralateral hand was used for blood sampling. During the blood sampling period, the hand was kept in a radiant warmer at 60°C to obtain arterialized venous blood at 0, 240, 270, 300, 330, and 360 min (5).

On the day of the bolus injection study, baseline blood samples were obtained, immediately followed by the intravenous injection of 3 µmol/kg [13C2]taurine over 1 min. Immediately thereafter, the intravenous line was abundantly flushed with >25 ml of saline, and blood was sampled from the same venous line at 2, 5, 8, 10, 15, 30, 45, 60, and 120 min.

Materials. [13C2]taurine (99% atom 13C) was purchased from MassTrace (Woburn, MA). Tracer solutions were prepared using aseptic technique by dissolving accurately weighed amounts of labeled taurine in known volumes of sterile, 0.9% saline, passed through a 0.22-µm Millipore filter, and stored at 4°C until the infusion. All infusates were verified to be pyrogen free by the Pharmacie Centrale des Hôpitaux (Paris, France) using the limulus lysate method.

Breath CO2 analysis. To determine the 13C-to-12C ratio in expired CO2, aliquots of expired air were collected in 10-ml Vacutainer tubes at –20, –1, and 0 min (baseline) and again at 30-min intervals throughout the study and were analyzed by gas chromatography-isotope ratio mass spectrometry (Europa VG).

Blood taurine analysis. For the determination of plasma taurine enrichment, blood samples were immediately separated by centrifugation at room temperature at a minimum rotary speed of 800 g for 10 min. The residual plasma was centrifuged a second time at 300 g for 10 min to remove platelets. Platelet-free plasma samples were then stored at –80°C until the day of analysis. The method for preparing the N-pentafluorobenzoyl-di-n-butylamine derivative of taurine has been previously described in detail (31). All solvents and reagents used in the analysis of plasma taurine were, at minimum, of HPLC grade purity.

Both taurine concentration and isotopic enrichment were determined by electron impact (70 eV) GC-MS using a Hewlett-Packard 5973 (Palo Alto, CA) instrument. Aliquots were injected in triplicate into a GC (model 6890, Hewlett-Packard, Fullerton, CA) equipped with an OV1 fused silica capillary column (30 m x 0.32 mm ID, 0.25 µm film thickness). Ions at mass-to-charge ratios (m/z) = 302 and 304, representing natural and [13C2]taurine, respectively, were selectively monitored. A standard curve obtained by diluting natural taurine with graded amounts of [13C2]taurine was run before each sample series. When the observed 304-to-302 ion current ratios were plotted as a function of expected [13C2]taurine-to-natural taurine mole ratios, linear regression coefficients of >0.99 were consistently observed along with slopes of 0.82–0.92. Taurine concentration was assessed in the full-scan mode by monitoring masses ranging from m/z = 100–350 using methyltaurine as internal standard.

Body compostion analysis. Body composition was measured using a two-frequency (5 kHz, 1 MHz) bioelectrical impendance analyzer (IMP B01, Cachan, France) to estimate fat-free mass, i.e., lean body mass.

Calculations. The intracellular taurine concentration in "total blood cells" ([Tau]tbc), defined as the fraction of blood volume not accounted for by plasma, was estimated as

(1)
where Hct is hematocrit (expressed as a fraction of unity), and [Tau]wb and [Tau]p are the measured concentrations of taurine in whole blood and plasma, respectively. The isotope enrichment in intracellular taurine in total blood cells (Etbc) was then estimated as (6, 7)

(2)
For the continuous tracer intravenous infusion, the appearance rate (Ra, µmol·kg–1·h–1) of taurine into plasma in the postabsorptive state was calculated using the steady-state equation

(3)
where i is the tracer infusion rate (µmol·kg–1·h–1), and Ei and Epss are taurine enrichments in infusate and plasma at steady state, respectively. A steady state was defined as an insignificant change (i.e., slope not significantly different from zero), with time in both plasma isotopic enrichment and substrate level, along with a coefficient of variation (CV = 100 x SD/mean) of <10% for both parameters.

For unprimed infusion, the rise of tracer enrichment [Ep, mole% excess (MPE)] to plateau was fitted to an exponential curve using a nonlinear regression

(4)
where Epss is plasma taurine enrichment at plateau, and k is the rate constant (h–1). The size of the tracer-miscible taurine pool (pool) was calculated as pool = Ra/k (7).

For the bolus tracer intravenous injection study, the area under the curve (AUC, µmol·l–1·h) sustended by plasma [13C2]taurine concentration was calculated by manual trapezoidal integration or Tai's formula (46). Taurine metabolic clearance rate (MCR, l·h–1·kg–1) was calculated as MCR = d/AUC, where d is the amount of labeled taurine injected (µmol/kg), and taurine Ra was calculated as

(5)

Taurine pool determination was performed using a two-compartment model according to Sturman (43).

Statistics. Data are presented as means ± SD. Data comparisons were performed using SSPSS software (SPSS, Chicago, IL). Statistical significance was set at P < 0.05.


    RESULTS
 TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Protocol A. In this pilot study, the same individual underwent infusions of [13C2]taurine on three separate occasions, either as an unprimed infusion or as a primed infusion, at 2 and 3 µmol·kg–1·h–1 (Table 2). As shown in Fig. 1, the unprimed 2 µmol·kg–1·h–1 infusion rate resulted in a slow rise in plasma enrichment, which reached an apparent plateau between 240 and 480 min of tracer infusion. The rise of plasma enrichment during the unprimed infusion was better fitted to a curve with a double exponential than it was when a single exponential curve was used, as the r2 were 0.97 and 0.93 with the double and single exponential fits, respectively. The miscible taurine pool was determined to be 38 µmol/kg.


View this table:
[in this window]
[in a new window]
 
Table 2. Taurine kinetics during primed and unprimed continuous infusions of [1,2-13C2]taurine in a single subject

 


View larger version (19K):
[in this window]
[in a new window]
 
Fig. 1. Time course of isotope enrichment (mole% excess) in plasma ({circ}), whole blood ({blacktriangleup}), and total blood cell ({bullet}) taurine in a single subject receiving either unprimed (A) or primed (B) continuous infusion of [1,2-13C2]taurine. The dotted lines were drawn from visual inspection.

 
The use of a priming dose equivalent to a 1-h infusion rate did not help to achieve steady state any sooner. This result suggests that the dose used resulted in "underpriming." This is supported by the data shown in Fig. 1; as a matter of fact, the "peak" enrichment obtained after the prime appears to be significantly below the plateau enrichment subsequently achieved after 240 min of infusion. Although a larger priming dose might have solved the problem, the use of an excessive dose as a prime ("overpriming") would have been a worse choice yet: due to the relatively slow turnover rate of taurine, plasma taurine enrichment might have declined continuously without achieving a plateau over the course of the experiment. This would have obviously resulted in erroneous Ra values, as discussed by other authors in the case of urea (32).

At all time points, whole blood taurine concentrations were higher than plasma taurine (P < 0.01), due to considerably higher intracellular taurine levels, as estimated using hematocrit (Eq. 2). Regardless of the infusion rate and prime, isotopic enrichment remained considerably lower in whole blood than in plasma taurine (P < 0.02; Table 2). As shown in Fig. 1, a steady state was nevertheless observed in whole blood enrichment over the last hour of isotope infusion. The estimated total blood cell enrichment/plasma taurine enrichment ratio was 0.15–0.29 (95% confidence limit) between 300 and 480 min.

Regardless of the infusion protocol, no detectable 13C enrichment could be found in CO2 in expired air collected throughout the study period.

Protocol B: primed continuous infusion. In view of results from the pilot studies, five additional subjects received a 6-h primed continuous infusion of [13C2]taurine infusion at a rate of 3.1 ± 0.2 µmol·kg–1·h–1. Near-steady state was observed between 240 and 360 min, as attested by 1) coefficients of variation ranging between 6.2 and 2.1% (Table 3) in plasma enrichment and 2) slopes of regression curves not different from zero when enrichment was plotted as a function of sampling time. Yet the use of the 240- to 360-min sampling period resulted in a slightly but significantly (P = 0.03) lower calculated Ra than the use of the 300- to 360-min period (Table 3). Although the difference in flux values for the two tested periods was small (only 2 µmol·kg–1·h–1) and, most likely, of little biological significance, the steady state achieved during the 6th h of tracer infusion (300–360 min) was of better quality, since in every single subject the CV in plateau enrichment was lower than that during the corresponding 5th h: on average, the CV was 2.1% during the 6th h, vs. 6.2% during the 5th h of tracer infusion (Table 3).


View this table:
[in this window]
[in a new window]
 
Table 3. Estimates of taurine fluxes (Ra) calculated from different periods (240–300 and 300–360 min) during a 360-min primed continuous infusion (3 µmol·kg–1·h–1) of [13C2]taurine in 6 healthy adults

 
Taurine Ra, as calculated from the measurement obtained between 300 and 360 min was 31.8 ± 3.1 µmol·kg–1·h–1 when expressed per kilogram of body weight and 38.0 ± 1.8 µmol·kg–1·h–1 when expressed per kilogram of fat-free mass.

Bolus injection study. The time course of 13C2 enrichment in plasma taurine after a single intravenous bolus of 3.02 ± 0.07 µmol/kg [13C2]taurine is depicted in Fig. 2 for four of the same five subjects. Because the very early sampling points were missed for subject 5, the Ra calculation could not be performed for that subject. There was no significant rise in the total (unlabeled + labeled) concentration of plasma taurine following the pulse dose, and for each subject, taurine concentration remained relatively stable between 2 and 120 min.



View larger version (12K):
[in this window]
[in a new window]
 
Fig. 2. Isotopic enrichment decay curve after an intravenous [13C2]taurine bolus injection. The bold line represents the fitted curve for composite data points from 4 subjects [using a dual exponential equation: F(t) = a x ebt + c x e–dt], where t is time, and a, b, c, and d are fitted parameters obtained from curve filling.

 
The time-dependent decline of plasma [13C2]taurine enrichment was well fitted to a two-component exponential decay curve (Fig. 2), as the r2 for the four individual curves were 0.988, 0.995, 0.996, and 0.997 (mean {approx}0.994). The taurine production rates calculated from this set of data are shown in Table 4. Two taurine pool sizes were identified, the first one ranging from 2.7 to 8 mmol and the second from 51 to 88 mmol.


View this table:
[in this window]
[in a new window]
 
Table 4. Plasma [13C2]taurine kinetics in 4 fasted healthy adults

 
The noncompartmental analysis using the AUC describing the time course of the tracer-to-tracee curve between 2 and 120 min yields a mean value for taurine Ra of 58.0 ± 17.8 µmol·kg–1·h–1. The Ra determined by bolus injection was, on average, equal to 1.85 times the value obtained when continuous infusion was used (Table 4), and the difference was significant (P = 0.03) with a two-tailed paired t-test and close to statistical significance (P = 0.06) with a nonparametric Wilcoxon paired test.


    DISCUSSION
 TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
The present study demonstrates that the Ra of taurine into plasma can be assessed using a 6-h continuous intravenous infusion of [1,2-13C2]taurine in humans and ranks among the lowest turnover rates of all amino acids measured to date. The lack of equilibration of labeled taurine between plasma and circulating cells, along with the slow Ra observed, further implies that taurine metabolism is highly compartmentalized and suggests that taurine Ra mostly reflects the interorgan exchange of taurine. Finally, the data suggest that the use of a single bolus injection of labeled taurine overestimates taurine Ra.

Little is known about taurine kinetics in humans. Using [1,2-13C2]taurine, Vinton et al. (50) reported a taurine Ra {approx} 5.6 µmol·kg–1·h–1 in five healthy volunteers, one-sixth of the taurine flux measured in the current study. Differences in study design could have contributed to the discrepancy. Whereas Vinton et al. studied fed subjects, we kept our subjects in the postabsorptive state to avoid any potential effect of exogenous taurine on endogenous taurine production. In addition, we infused tracer for 6 h, compared with 21 days in the studies of Vinton et al., so tracer recycling cannot be excluded in the latter. Putative tracer recycling would result in higher enrichments in plasma taurine and, consequently, lower estimates of taurine Ra. Whereas protein breakdown cannot release label since taurine is not incorporated into protein (26), taurine conjugates to bile acids in liver and, once excreted into bile, is reabsorbed from intestinal lumen (21). Enterohepatic taurine cycling is therefore a potential source of tracer recycling and must increase with the two- to sixfold rise in bile flux associated with the fed state (1). We failed to detect any evidence of tracer recirculation, as judged by the lack of a secondary rise in enrichment during the last of 2 h of infusion; we therefore assume that recycling was minimal in our fasted subjects. We cannot, however, rule out the possibility that isotope enrichment would have continued to increase slowly, had we extended infusion time.

Moreover, achievement of plateau in plasma does not predict whether steady state had been reached inside cells where the bulk of taurine is located (2). We observed a much lower labeled taurine enrichment in total blood cells compared with plasma. Although "total blood cell" obviously is a loose term for a heterogeneous compartment that lumps taurine-rich (e.g., neutrophils and platelets) and taurine-poor cells such as erythrocytes (25, 26, 40, 51), lack of tracer equilibration between blood cells and plasma nevertheless suggests compartmentation between extra- and intracellular taurine.

Compared with the fluxes observed in the present report (32 µmol·kg–1·h–1), arteriovenous (a-v) gradients assessed across various vascular beds (9, 10, 16, 36) yield much lower estimates of taurine interorgan exchange: muscle releases 2.2 µmol·kg–1·h–1 taurine (36), and splanchnic bed (9) and brain (10) take up 1.9 and 1.3 µmol·kg–1·h–1 taurine, respectively, assuming blood flows of 1,200 ml/min in liver (9) and 2.2 and 60 ml·min–1·100 g–1 in muscle (36) and brain (39), respectively. Yet both approaches have intrinsic limitations. The a-v concentration differences are often of the same magnitude as measurement error, and determination of blood flow across organ beds is technically difficult. Moreover, a-v gradient methods measure net organ balance, so that simultaneous release and uptake in the same tissue can be missed. On the other hand, isotope dilution relies on enrichments measured in circulating taurine, although taurine may be produced and utilized inside cells without exchanging with plasma. The Ra obtained may thus reflect interorgan taurine exchange rather than whole body taurine production rate.

In the current study, estimates of taurine Ra derived from constant tracer infusion were consistently lower than those provided by bolus injection (Table 4), using the AUC describing the time course of [13C2]taurine level between 2 and 120 min. These cut-off times were chosen because of the difficulty in detecting isotope enrichments below 0.1 MPE by GC-MS and may have led to an underestimation of the AUC, resulting in higher estimates of taurine Ra. In addition, as the same intravenous line was used for bolus injection and for sampling, this technique could lead to "contamination" of initial blood samples by excess amounts of tracers. This source of error was quantitated in a recent study, albeit for a different substrate. When Boles Ponto et al. [Ponto et al. (35a)] infused 2-deoxy-2-[18F]fluoroglucose (18F-DFG) through a forearm vein and sampled blood from both the same line and a contralateral forearm vein, they found the 18F-DFG level to be, on average, 2% higher when sampling on the infused line, i.e., when using a 10-ml saline flush and discarding the initial 1-ml blood sampled. We believe the level of "contamination" was still less in the present studies, because 1) samples were obtained after a larger, 25-ml saline flush, and we discarded the initial 5 ml of blood; 2) we (data not shown) and others (14) have routinely used the same technique for intravenous glucose tolerance tests, a technique in which large doses of glucose are injected over 1 min and the same intravenous line is used for measuring blood glucose and insulin levels at 3 and 5 min after injection, and in the latter studies flushing of the intravenous line was sufficient to avoid contamination from the concentrated dextrose solution; and 3) if contamination had nevertheless occurred, it would have resulted in artificially high [13C2]taurine enrichment in the initial plasma samples obtained, which, in turn, would have increased the AUC describing the course of [13C2]taurine over time and, consequently, resulted in artificially decreased taurine Ra. This was obviously not the case, since the bolus injection technique yielded higher flux values than did the continuous infusion technique. Taurine Ra values obtained from [13C2]taurine continuous infusion are nevertheless close to the {approx}20–25 µmol·kg–1·h–1 obtained by monitoring the decay of taurine specific activity for 48 h after a bolus of [35S]taurine in humans (44) or monkeys (33). Taken together, these studies suggest that, whereas bolus injection may be appropriate for radioactive taurine, a continuous-infusion approach is preferable when stable isotopes are used.

Regardless of the methodological uncertainties, taurine Ra is lower than most amino acid Ra values assessed to date (3). The appearance of [18O]taurine after inhalation of 18O2 established the ability of humans to synthesize taurine de novo (24). Conversion of cysteine to taurine is the sole source of endogenous taurine and is, however, limited, due to 1) the low activity of cysteine sulfinate decarboxylase, the rate-limiting step in taurine synthesis (8, 26, 27); 2) the low Ra of cysteine ({approx}45 µmol·kg–1·h–1); and 3) the fact that two other major pathways, cysteine incorporation into body protein (22) and glutathione (12), compete for cysteine utilization (37, 42).

Regarding taurine disposal (Rd), irreversible taurine losses of 0.1 and 5.4–28.5 µmol·kg–1·day–1 have been reported in feces (47) and urine (38), respectively. The quantitative contribution of hepatic taurine conjugation to taurine utilization can be extrapolated from published data (15, 19, 41). Glycine is another amino acid moiety used for bile acid conjugation (18). Because 1) overall glycine Ra approximates 150 µmol·kg–1·h–1 (7) and 2) 2.3% of glycine flux is turning over for bile acid, then {approx}3.4 µmol·kg–1·h–1 glycine are used for bile conjugation in an adult human (17–19). Assuming a ratio of glycine to taurine of {approx}3.5 in bile acids (17, 19, 21), bile acid conjugation may therefore only account for {approx}1 µmol·kg–1·h–1, or <2% the taurine Rd measured in the current study. The fate of the bulk of taurine therefore remains unresolved.

One might argue that taurine is a "metabolic dead end," implying that, once it enters cells, it remains there, and the large intracellular taurine pool may be a vast "taurine sink." Taurine residence time in muscle was estimated at 427 h in healthy men, nine times that of glutamine (29). The low fractional turnover of taurine might account for the slow isotopic equilibration, as it took 4 h of a constant infusion of [13C2]taurine to reach isotopic equilibrium. A delay in tracer equilibration across cell membranes invariably occurs when a substrate has a large intracellular pool, such as was observed with [15N]glutamine in the skeletal muscle glutamine pool (49).

The two-component taurine pools obtained from the current data agree with previous reports (44, 50), and the small component pool size found in four subjects explored by bolus injection (2.7–8.0 mmol; Table 5) is in the same range as the tracer-miscible pool determined using continuous infusion (38 µmol/kg body wt, i.e., {approx}2.6 mmol). Assuming extracellular fluid to be 0.2 l/kg with a taurine content of 50 µmol/l, we can estimate extracellular taurine pool at {approx}10 µmol/kg. The extracellular taurine pool thus accounts for only 26% of tracer-miscible pool (10/38), and the rest of the taurine tracer-miscible pool must be accounted for by a fraction of intracellular taurine that exchanges with plasma. These pool sizes are far from whole body taurine content, since muscle alone contains >287 mmol (2), and large amounts of taurine are found in retina, interstitial spaces, or bound with intracellular peptides (22, 30). It is therefore safe to conclude that both taurine Ra and tracer-miscible pools represent a tiny fraction of whole body taurine.


View this table:
[in this window]
[in a new window]
 
Table 5. Estimated taurine pool sizes (mmol) derived from in vivo isotope dilution studies

 
Finally, no detectable 13CO2 appeared in breath over the course of an 8-h infusion of [13C2]taurine. In theory, a fraction of taurine released into bile and, subsequently, duodenum could be deconjugated from bile acids and oxidized by bacterial enzymes in the large intestine. The resulting CO2 could diffuse across the colonic wall and appear in breath, which could be an index of exposure of bile acids to bacterial deconjugating enzymes (21, 23). Although [35S]taurine is oxidized to inorganic [35S]sulfate by colonic flora (23), no exhaled 13CO2 was detected when [13C]taurine was used. This implies that either 1) there was no bacterial overgrowth in the healthy subjects enrolled in the current study or 2) the dose or duration of [13C2]taurine infusion was insufficient for bacterially produced 13CO2 to appear in breath since <1% of labeled taurine administered could be converted into [13C]cholyl taurine (19).

Taken in aggregate, data from the current studies using [13C]taurine, and from earlier studies, shed some light into the peculiar nature of taurine metabolism in humans. Like glutamine, taurine is highly concentrated in intracellular space (2, 7) and compartmentalized between extra- and intracellular milieus. Yet, contrary to glutamine, taurine has a slow turnover and exchanges very little with plasma (7, 22, 26, 44). In addition, whereas the active interorgan exchange of glutamine is known to serve as a shuttle of carbon and nitrogen between skeletal muscle and splanchnic bed (34), the physiological function of interorgan taurine exchange, if any, remains to be found. As a matter of fact, although taurine has major physiological functions in brain, retina, liver, and other tissues, the taurine needed for these functions may arise from de novo synthesis in situ rather from taurine taken up from systemic circulation. The precise significance of interorgan taurine exchange, measured using isotope dilution methods, therefore remains to be determined.


    GRANTS
 TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
This study was supported, in part, by grants from the Institut de Recherche sur les Maladies de l'Appareil Digestif, and from Astra Zeneca, Rueil Malmaison, France.


    ACKNOWLEDGMENTS
 
We acknowledge the help of M. C. Morin, our dietician at the parenteral and enteral nutrition unit of Lariboisière Hospital, for invaluable help.


    FOOTNOTES
 

Address for reprint requests and other correspondence: B. Messing, Hôpital Lariboisière-Saint-Lazare 75475 Paris cedex 10, France (E-mail: bernard.messing{at}lrb.ap-hop-paris.fr).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


    REFERENCES
 TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Angelin B, Björkhem I, Einarsson K, and Ewerth S. Hepatic uptake of bile acids in man. Fasting and postprandial concencration of individual bile acids in portal venous and systemic blood serum. J Clin Invest 70: 724–731, 1982.[ISI][Medline]
  2. Bergström J, Fürst P, Noree LO, and Vinnars E. Intracellular free amino acid concentration in human muscle tissue. J Appl Physiol 36: 693–697, 1974.[Free Full Text]
  3. Bier DM. Intrinsically difficult problems: the kinetics of body proteins and amino acids in man. Diabetes Metab Rev 5: 111–132, 1989.[ISI][Medline]
  4. Copeland KC, Kenney F, and Nair SK. Heated dorsal hand vein sampling for metabolic studies: a reappraisal. Am J Physiol Endocrinol Metab 263: E1010–E1014, 1992.[Abstract/Free Full Text]
  5. Darmaun D, Froguel P, Rongier M, and Robert JJ. Amino acid exchange between plasma and erythrocytes in vivo in humans. J Appl Physiol 67: 2383–2388, 1989.[Abstract/Free Full Text]
  6. Darmaun D, Matthews DE, and Bier DM. Glutamine and glutamate kinetics in humans. Am J Physiol Endocrinol Metab 251: E117–E126, 1986.[Abstract/Free Full Text]
  7. De la Rosa J and Stipanuk MH. Evidence for a rate-limiting role of cysteinesulfinate decarboxylase activity in taurine biosynthesis in vivo. Comp Biochem Physiol B 81: 565–571, 1985.[CrossRef][ISI][Medline]
  8. Felig P and Wahren J. Amino acid metabolism in exercising man. J Clin Invest 50: 2703–2714, 1971.[ISI][Medline]
  9. Felig P, Wahren J, and Ahlborg G. Uptake of individual amino acids by human brain. Proc Soc Exp Biol Med 142: 230–231, 1973.
  10. Franconi F, Bennardini F, Mattana A, Miceli M, Ciuti M, Mian M, Gironi A, Anichini R, and Seghieri G. Plasma and platelet taurine are reduced in subjects with insulin-dependent diabetes mellitus: effects of taurine supplementation. Am J Clin Nutr 61: 1115–1119, 1995.[Abstract]
  11. Fukagawa NK, Ajami AM, and Young VR. Plasma methionine and cysteine kinetics in response to an intravenous glutathione infusion in adult humans. Am J Physiol Endocrinol Metab 270: E209–E214, 1996.[Abstract/Free Full Text]
  12. Geggel HS, Ament ME, Heckenlively JR, Martin DA, and Kopple JD. Nutritional requirement for taurine in patients receiving long-term parenteral nutrition. N Engl J Med 312: 142–146, 1985.[Abstract]
  13. Grulet H, Durlach V, Hecart AC, Gross A, and Leutenegger M. Study of the rate of early glucose disappearance following insulin injection: insulin sensitivity index. Diabetes Res Clin Pract 20: 201–207, 1993.[ISI][Medline]
  14. Hardison WG. Relation of hepatic taurine pool size to bile acid conjugation in man and animals. Prog Clin Biol Res 125: 407–417, 1983.[Medline]
  15. Hashiguchi Y, Fukushima R, Saito H, Naka S, Inaba T, Lin MT, and Muto T. Interleukin-1 and tumor necrosis factor alter plasma concentration and interorgan fluxes of taurine in dogs. Shock 7: 147–153, 1997.[ISI][Medline]
  16. Hepner GW, Hofmann AF, and Thomas PJ. Metabolism of steroid and amino acid moieties of conjugated bile acids in man. II. Glycine-conjugated dihydroxy bile acids. J Clin Invest 51: 1898–1905, 1972.[ISI][Medline]
  17. Hepner GW, Hofmann AF, and Thomas PJ. Metabolism of steroid and amino acid moieties of conjugated bile acids in man. I. Cholyglycine. J Clin Invest 51: 1889–1897, 1972.[ISI][Medline]
  18. Hepner GW, Sturman JA, Hofmann AF, and Thomas PJ. Metabolism of steroid and amino acid moieties of conjugated bile acids in man. III. Cholyltaurine (taurocholic acid). J Clin Invest 52: 433–440, 1973.[ISI][Medline]
  19. Hickman MA, Rogers QR, and Morris JG. Effect of processing on fate of dietary [14C] taurine in cats. J Nutr 120: 995–1000, 1990.[ISI][Medline]
  20. Hofmann AF. The enterohepatic circulation of bile acids in man. Adv Intern Med 21: 501–534, 1976.[Medline]
  21. Huxtable RJ. Physiological actions of taurine. Physiol Rev 72: 101–163, 1992.[Free Full Text]
  22. Ikeda K, Yamada H, and Tanaka S. Bacterial degradation of taurine. J Biochem (Tokyo) 54: 312–316, 1963.[ISI][Medline]
  23. Irving CS, Marks L, Klein PD, Foster N, Gadde P, Chase TN, and Samuel D. New evidence for taurine biosynthesis in man obtained from 18O2 inhalation studies. Life Sci 38: 491–495, 1985.[CrossRef][ISI]
  24. Jacobson E, Kurzawaski G, and Tustanowski S. Synthesis and uptake of taurine by isolated human granulocytes. Folia Histochem Cytobiol 24: 179–185, 1986.[ISI][Medline]
  25. Jacobson JG and Smith LH. Biochemistry and physiology of taurine and taurine derivatives. Physiol Rev 48: 424–511, 1968.[Free Full Text]
  26. Jerkins AA, Jones DD, and Kohlhepp EA. Cysteine sulfinic acid decarboxylase mRNA abundance decreases in rats fed a high-protein diet. J Nutr 128: 1890–1895, 1998.[Abstract/Free Full Text]
  27. Lourenco R and Camilo ME. Taurine: a conditionally essential amino acid in humans? An overview in health and disease. Nutr Hosp 17: 262–270, 2002.[Medline]
  28. Lundholm K, Bennegard K, Zachrisson H, Lundgren F, Eden E, and Möller-Loswick AC Transport kinetics of amino acids across the resting human leg. J Clin Invest 80: 763–771, 1987.[ISI][Medline]
  29. Maggs DG, Jacob R, Rife F, Lange R, Leone P, During M, Tamborlane WV, and Sherwin R. Interstitial fluid concentrations of glycerol, glucose and amino acids in human quadriceps muscle and adipose tissue. Evidence for significant lipolysis in skeletal muscle. J Clin Invest 96: 370–377, 1995.[ISI][Medline]
  30. Marks L, Iglicki F, Rakotoambinina B, Thuillier F, and Messing B. A gas chromatographic/electron impact mass spectrometric method for the isolation and derivatization of plasma taurine for use in stable isotope tracer kinetic studies. J Mass Spectrom 30: 1687–1693, 1995.[ISI]
  31. Matthews DE and Downey RS. Measurement of urea kinetics in humans: a validation of stable isotope tracer methiods. Am J Physiol Endocrinol Metab 246: E519–E527, 1984.[Abstract/Free Full Text]
  32. Matsubara Y, Lin YY, Sturman JA, Gaull GE, Marks LM, and Irving CS. Stable isotope study of plasma taurine kinetics in rhesus monkey. Life Sci 36: 1933–1940, 1985.[CrossRef][ISI][Medline]
  33. Nurjhan N, Bucci A, Perriello G, Stumvoll M, Dailey G, Bier DM, Toft I, Jenssen TG, and Gerich JE. Glutamine: a major gluconeogenic precursor and vehicle for interorgan carbon transport in man. J Clin Invest 95: 272–277, 1995.[ISI][Medline]
  34. Paauw JD and Davis AT. Taurine supplementation at three different dosages and its effect on trauma patients. Am J Clin Nutr 60: 203–206, 1994.[Abstract]
  35. Ponto LL, Graham MM, Richmond JC, Ward CA, Clermont DA, Schmitt BA, Clark JA, Conklin A, Weldon L, Watkins GL, Madsen MT, and Hichwa RD. Contamination levels in blood samples drawn from the injection intravenous line. Mol Imaging Biol 4: 410–414, 2003.
  36. Pozefsky T, Felig P, Jobin J, Soeldner SJ, and Cahill GE. Amino acid balance across tissues of the forearm in the postabsorptive man. Effects of insulin at two dose levels. J Clin Invest 48: 2273–2282, 1969.[ISI][Medline]
  37. Raguso CA, Regan M, and Young VR. Cysteine kinetics and oxidation at different intakes of methionine and cystine in young adults. Am J Clin Nutr 71: 491–499, 2000.[Abstract/Free Full Text]
  38. Rana SK and Sanders TA. Taurine concentrations in the diet, plasma, urine and breast milk of vegans compared with omnivores. Br J Nutr 56: 17–27, 1986.[ISI][Medline]
  39. Rootwelt KJ, Dybevold S, Nyberg-Hansen R, and Russell D. Measurement of cerebral blood flow with 133Xe inhalation and dynamic single photon emission computer tomography: normal values. Scand J Clin Lab Invest 46: 97–105, 1986.[Medline]
  40. Soupart P. Free amino acids of blood and urine in the human. In: Amino Acid Pools, edited by Holden JT. New York: Elsevier, 1962, p. 220–262.
  41. Stahl E and Arnesjö B. Taurocholate metabolism in man. Scand J Gastroenterol 7: 559–566, 1972.[ISI][Medline]
  42. Stipanuk MH, Coloso RM, Garcia RA, and Banks MF. Cysteine concentration regulates cysteine metabolism to glutathione, sulfate and taurine in rat hepatocytes. J Nutr 122: 420–427, 1992.[ISI][Medline]
  43. Sturman JA. Taurine pool sizes in the rat: effects of vitamin B-6 deficiency and high taurine diet. J Nutr 103: 1566–1580, 1973.[ISI][Medline]
  44. Sturman JA, Hepner G, Hofmann AF, and Thomas PJ. Metabolism of [35S] taurine in man. J Nutr 105: 1206–1214, 1975.[ISI][Medline]
  45. Suliman ME, Anderstam B, and Bergstrom J. Evidence of taurine depletion and accumulation of cysteinesulfinic acid in chronic dialysis patients. Kidney Int 50: 1713–1717, 1996.[ISI][Medline]
  46. Tai MM. A mathematical model for the determination of total area under glucose tolerance and other metabolic curves. Diabetes Care 17: 152–154, 1994.[Abstract]
  47. Thompson GN. Excessive fecal taurine loss predisposes to taurine deficiency in cystic fibrosis. J Pediatr Gastroenterol Nutr 7: 214–219, 1988.[ISI][Medline]
  48. Trautwein EA and Hayes KC. Plasma and whole blood taurine concentrations respond differently to taurine supplementation (humans) and depletion (cats). Z Ernährungswiss 34: 137–142, 1995.[ISI][Medline]
  49. Van Acker B, Hulsewé KW, Wagenmakers AJ, Deutz NEP, Van Kreel B, Halliday D, Matthews DE, Soeters PB, and Von Meyenfeldt M. Absence of glutamine isotopic steady state: implications for the assessment of whole-body glutamine production rate. Clin Sci (Colch) 95: 339–346, 1998.[CrossRef][ISI][Medline]
  50. Vinton NE, Kopple J, Bier DM, Matthews DE, Laidlaw SA, and Ament ME. Kinetics of taurine metabolism in healthy adults and long term total parenteral nutrition patients (Abstract). Am J Clin Nutr 35: 864, 1987.
  51. Vinton NE, Laidlaw SA, Ament ME, and Kopple JD. Taurine concentrations in plasma, blood cells, and urine of children undergoing long-term total parenteral nutrition. Pediatr Res 21: 399–403, 1987.[Abstract]
  52. Wolfe RR. Radioactive and Stable Isotope Tracers in Biomedicine. New York: Wiley-Liss, 1992.




This Article
Abstract
Full Text (PDF)
All Versions of this Article:
287/2/E255    most recent
00333.2003v1
Alert me when this article is cited
Alert me if a correction is posted
Citation Map
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Download to citation manager
Search for citing articles in:
ISI Web of Science (1)
Google Scholar
Articles by Rakotoambinina, B.
Articles by Darmaun, D.
Articles citing this Article
PubMed
PubMed Citation
Articles by Rakotoambinina, B.
Articles by Darmaun, D.


HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2004 by the American Physiological Society.