Glutamine and leucine nitrogen kinetics and their relation to urea nitrogen in newborn infants

Prabhu S. Parimi, Srisatish Devapatla, Lourdes Gruca, Alicia M. O'Brien, Richard W. Hanson, and Satish C. Kalhan

Departments of Pediatrics and Biochemistry, Robert Schwartz M.D. Center for Metabolism and Nutrition, MetroHealth Medical Center, Case Western Reserve University School of Medicine, Cleveland, Ohio 44109-1998


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Glutamine kinetics and its relation to transamination of leucine and urea synthesis were quantified in 16 appropriate-for-gestational-age infants, four small-for-gestational-age infants, and seven infants of diabetic mothers. Kinetics were measured between 4 and 5 h after the last feed (fasting) and in response to formula feeding using [5-15N]glutamine, [1-13C,15N]leucine, [2H5]phenylalanine, and [15N2]urea tracers. Leucine nitrogen and glutamine kinetics during fasting were significantly higher than those reported in adults. De novo synthesis accounted for ~85% of glutamine turnover. In response to formula feeding, a significant increase (P = 0.04) in leucine nitrogen turnover was observed, whereas a significant decrease (P = 0.002) in glutamine and urea rate of appearance was seen. The rate of appearance of leucine nitrogen was positively correlated (r2 = 0.59, P = 0.001) with glutamine turnover. Glutamine flux was negatively correlated (r2 = 0.39, P = 0.02) with the rate of urea synthesis. These data suggest that, in the human newborn, glutamine turnover is related to a high anaplerotic flux into the tricarboxylic acid cycle as a consequence of a high rate of protein turnover. The negative relationship between glutamine turnover and the irreversible oxidation of protein (urea synthesis) suggests an important role of glutamine as a nitrogen source for other synthetic processes and accretion of body proteins.

small for gestational age; appropriate for gestational age; phenylalanine; stable isotopes; infant of diabetic mother


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

GLUTAMINE ALONG WITH ALANINE is a major carrier of nitrogen from the periphery to the liver. In addition, it has been shown to participate in several key metabolic processes, such as ammonia production by the kidney, precursor for purine and pyrimidine biosynthesis, and as a major respiratory fuel for enterocytes and lymphocytes. Studies of chronically catheterized sheep fetus in utero have shown that glutamine constitutes the largest component of amino nitrogen transferred from the mother to the fetus (25). Furthermore, the rate of plasma glutamine turnover in the sheep fetus, measured by the isotopic tracer dilution method, has been estimated to be high (~1,200 µmol · kg-1 · h-1; see Refs. 2 and 26). A unique placental and fetal hepatic metabolism of glutamine and glutamate has been described wherein a large fraction (~45%) of net glutamine taken up by the fetal liver is converted to glutamate to be returned to the placenta and oxidized to CO2 (2, 37, 26). The physiological significance of the high rate of glutamine flux and its role in fetal metabolism and growth remains to be elucidated. Similar data are difficult to obtain in the human fetus because of ethical considerations. The concentration of plasma glutamine in the umbilical venous blood of the human neonate obtained at elective cesarean section delivery has been observed to be high when compared with umbilical arterial blood concentrations (3, 15). However, the umbilical venous-arterial gradient was not found to be significantly different from zero (3, 15), suggesting that glutamine metabolism in the human fetus at term gestation may not be comparable to that in the sheep. Furthermore, little information regarding glutamine kinetics and metabolism has been reported in human newborn infants. During adaptation to the extrauterine environment after birth, major changes in fuel-energy metabolism occur (18). These adaptive responses, triggered by surges in catecholamine, glucagon, thyroid-stimulating hormone, and the like could have a significant effect on glutamine metabolism (8, 13, 32). The impact of these metabolic adaptations on glutamine metabolism in the human newborn has not been examined. Such data are important, since several investigators have suggested glutamine to be a "conditionally essential" amino acid, especially for prematurely born newborn infants and particularly when they are acutely sick (24, 30). In the present study, by using stable isotopic tracer methods, we have quantified the kinetics of glutamine in healthy full-term infants. Because branched-chain amino acids are the major source of amino nitrogen for glutamine synthesis (7, 12), and because glutamine nitrogen is an important nitrogen source for hepatic urea synthesis, we have quantified the relationship between the rates of transamination of leucine, urea synthesis, and glutamine turnover. The contribution of glutamine release from proteolysis to whole body glutamine kinetics was quantified simultaneously by measuring the whole body rate of protein turnover. Glutamine and nitrogen kinetics were also quantified in infants of diabetic mothers and those born small for gestational age to examine the impact of sufficient and insufficient nutrients, respectively, during intrauterine life.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The study group consisted of sixteen appropriate-for-gestational-age (AGA) infants, four small-for-gestational-age (SGA) infants, and seven infants of diabetic mothers (IDM). Six infants in the IDM group were born to gestationally diabetic mothers. All infants, except two in the AGA group and two in the IDM group, were studied within the first 48 h after birth. Study characteristics are shown in Table 1. All infants were healthy and were cared for in the normal newborn nursery; they had no risk factors for neonatal sepsis. The Apgar scores of the infants were within the normal range. They were not receiving any medications, including antibiotics, nor any parenteral fluids. All infants were on a three-hourly feeding regimen, as per our clinical practice. The AGA and IDM infants were being fed a 20 kcal/30 ml infant formula (Similac; Ross Laboratories, Columbus, OH), whereas SGA infants were receiving a 22 kcal/30 ml formula (NeoSure; Ross Laboratories). Written informed consent was obtained from the mother, and the father when available, after full explanation of the risks and benefits of the procedure. The study protocol was approved by the Institutional Review Board of MetroHealth Medical Center (Cleveland, OH). Verbal consent from the primary physician of the individual infant was also obtained before the study. The investigators were not responsible for the clinical care of the infants.

                              
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Table 1.   Clinical characteristics of study subjects

L-[1-13C,15N]leucine (99% 13C,15N), [2H5]phenylalanine (98% 2H), and [15N2]urea (99% 15N) were purchased from Merck (Dorvall, Canada), and the L-[5-15N]glutamine (99% 15N) was from Isotec (Miamisburg, OH).

Three hours after the last meal, the babies were transferred to the study nursery in the General Clinical Research Center. Two intravenous cannulas were placed in the infants, one in the dorsum of the hand to infuse the isotopic tracers and the other in the saphenous vein to draw blood samples. The sampling site was kept patent by continuous infusion of 0.9% NaCl at 2-3 ml/h. Weighed amounts of isotopic tracers were mixed in 0.45% NaCl, sterilized by Millipore filtration, and tested for sterility and pyrogenicity. The tracer solution was infused at 3 ml/h. The actual rate of infusion was determined gravimetrically upon completion of the study, using the same infusion tubing, cannula, and infusion pump. The isotopic tracers were administered as prime constant-rate infusions as follows: [1-13C,15N]leucine, prime 7.5 µmol/kg and constant infusion 7.5 µmol · kg-1 · h-1; [5-15N]glutamine, prime 30 µmol/kg and constant infusion 30 µmol · kg-1 · h-1; [15N2]urea, prime 33 µmol/kg and constant rate infusion 3.3 µmol · kg-1 · h-1; [2H5]phenylalanine, prime 6 µmol/kg and constant rate infusion 4 µmol · kg-1 · h-1. Blood samples (1-1.5 ml, depending on the weight of the infant) were obtained in heparinized syringes before the start of tracer infusion and at 150, 165, and 180 min. At 180 min, the infants were fed a commercial infant formula (20 kcal/30 ml) at a rate of 10 ml · kg-1 · h-1 for the next 2 h. The infants were offered formula every 30 min, and the ingested volume was recorded. Additional blood samples were obtained at 270, 285, and 300 min; the blood was mixed with cold trichloroacetic acid (10%) and centrifuged, and the separated plasma was stored at -70° for later analysis. Because of difficulty in continued blood sampling, the feeding portion of the study was not completed in some infants. These are identified in Tables 3 and 4. In addition, three AGA and two IDM infants did not receive tracer urea. Blood glucose concentrations were monitored in all infants at the bedside throughout the study and remained in the normal range.

Because studies by others (35) have shown that the isotopic steady state for glutamine may not be achieved in a short time because of intracellular compartmentation, studies were conducted in three infants for 300 min, during fasting, without formula feeding.

Analytical Procedures

Plasma glucose and urea nitrogen concentrations were measured by the glucose oxidase and urease methods, respectively, using commercial analyzers (Beckman Instruments, Fullerton, CA). Plasma amino acids were measured using an HPLC equipped with a fluorescent detector using the OPDA derivative and precolumn derivatization (34).

Mass spectrometric analysis. The mass spectrometric methods for the measurement of isotopic enrichment of leucine, alpha -ketoisocaproic acid (KIC), and urea have been described previously in publications from this laboratory (9, 11, 33). Amino acids and urea were separated from the plasma by preparatory ion exchange chromatography using a mini column. An N-acetyl,N-propyl ester derivative of leucine and phenylalanine was prepared according to the method of Adams (1) with certain modifications (9). A Hewlett-Packard model 5970 or model 5870 gas chromatography-mass spectrometry (GC-MS) system was used. Methane chemical ionization was used, and mass-to-charge ratios (m/z) 216 and 218, representing the unlabeled and di-labeled leucine, respectively, were monitored using the selected ion chromatography software. For phenylalanine, m/z 250 and 255 were monitored, representing unlabeled phenylalanine and [2H5]phenylalanine, respectively. The 13C enrichment of plasma KIC was measured using the quinoxalone derivative (11). Standard solutions of known isotopic enrichment were run along with unknowns to correct for analytical and instrumental variations.

Glutamine and glutamate in the amino acid eluate were derivatized according to the method of Haisch and colleagues (14). A tri-tert-butyldimethylsilyl derivative was prepared by adding 50 µl N-methyl-N-(tert-butyldimethylsilyl) trifluroacetamide plus 1% tert-butyldimethylchlorosilane and 50 µl acetonitrile to the dry eluate. GC-MS analysis was performed in electron-impact ionization mode using a 0.25-mm-ID, 30-m HP-1 column (Agilent, Palo Alto, CA) with a film thickness of 0.25 µm. Helium was used as the carrier gas. Glutamine eluted at ~8 min. Ion clusters m/z 431 (M-57) and m/z 432, representing the unlabeled glutamine and [15N]glutamine, respectively, were monitored for quantification of the 15N enrichment of glutamine. The 15N enrichment of glutamate was quantified similarly by minor modification in the gas chromatograph settings and monitoring m/z 432 and 433.

15N enrichment of urea was quantified using the trifluoracetohydroxypyrimidine derivative in the electron-impact mode as described (33).

Calculation

The rates of appearance (Ra) of leucine, phenylalanine, glutamine, and urea were calculated by tracer dilution with steady-state kinetics. Ra = I × [(Ei/Ep- 1], where I is the rate of infusion of the tracer (µmol · kg-1 · h-1) and Ei and Ep represent the enrichment of infusate and of plasma amino acid at steady state, respectively. The coefficient of variation for enrichment data for various tracers in individual subjects was between 3 and 5%; the slope was not different from zero.

The kinetic data between 150 and 180 min were designated as "fasting," and those between 270 and 300 min were designated the "fed state". In newborn infants fed every 3 h, the fasting period as designated here, i.e., 5.5-6 h after the last feed, may not be comparable to the postabsorptive period in adults because of the large variability in gastric emptying and gut motility. However, for clinical and ethical considerations, the babies cannot be starved for more prolonged periods.

Leucine carbon flux (QC) was calculated using 13C enrichment of plasma KIC, whereas leucine nitrogen flux (QN) was calculated using m2 enrichment (13C,15N) of plasma leucine during isotopic steady state (27). During fasting, QN is the sum of leucine released from protein breakdown and that formed from the reamination of KIC. QC, in contrast, is predominantly the result of protein breakdown, since leucine carboxyl carbon (1-13C) is not lost during transamination of leucine to and from KIC. The difference between QN and QC provides an estimate of the rate of reamination of leucine KIC (21, 27). As discussed previously (21), inasmuch as [1-13C,15N]leucine enrichment is measured in the plasma, the calculated QN is an underestimation, since the intracellular enrichment would be less than that in the plasma.

The contribution of glutamine nitrogen to urea nitrogen was calculated from the m1 enrichment of urea during isotopic steady state using the precursor-product relationship. This estimate also will include a small, if any, amount of 15N reincorporated in urea after the hydrolysis of infused [15N2]urea in the gut. However, theoretical estimates suggest that such reincorporation of 15N will be negligible (17, 20). Because the [15N2]urea was infused at a rate corresponding to 1.5-2% of the endogenous rate of urea synthesis, the maximal contribution of the recycled nitrogen would be only 0.04 µmol · kg-1 · h-1, or 0.02% of urea synthesized (assuming the rate of urea synthesis to be ~200 µmol · kg-1 · h-1 and a maximal 20% rate of hydrolysis in the gut). Nevertheless, the hydrolysis of urea in the gut and its reincorporation in urea in the newborn infant have not been confirmed (16, 17, 20). The m1 enrichment of urea could also be the result of 15N incorporation from alanine and aspartate as a result of transamination from labeled leucine. This is also anticipated to be small because of the tremendous dilution of 15N from leucine in the intermediate pools. For these reasons, the m1 enrichment of urea mostly represents the incorporation of amide 15N of glutamine into urea.

The Ra of glutamine in plasma was measured by [5-15N]glutamine tracer dilution. The m1 enrichment of glutamine measured by the tri-tert-butyldimethylsilyl derivative includes both amide and amino nitrogen. Since the contribution of 15N from the infused [1-13C,15N]leucine to m1 (amino) glutamine is anticipated to be small because of transamination of leucine with multiple amino acids, the measured m1 enrichment of glutamine is mostly due to the infused (amide) glutamine tracer. The measured m1 enrichment of the glutamate pool in four babies, because of transfer of 15N from leucine to glutamate, was <0.5%. This is in contrast to the m1 enrichment of glutamine, which was 5.1 ± 0.7% in AGA infants during fasting. The Ra of glutamine in plasma as measured here is the sum of the de novo synthesis of glutamine and the glutamine released from protein breakdown. Assuming that glutamine represents a fixed proportion of whole body protein, the rate of release of glutamine from protein breakdown (BGln) was calculated as follows: BGln = Ra phenylalanine × 1.07. The fraction 1.07 represents the relationship between glutamine and phenylalanine in mixed-muscle protein (23). The rate of de novo synthesis of glutamine is the difference between the Ra of glutamine and BGln.

Statistics

All data are reported as means ± SD. Statistical analyses were performed using commercial software (Statistix 7.0; Analytical Software, Tallahassee, FL). The data were initially analyzed for skew and kurtosis using descriptive statistics. The difference among the groups was analyzed by Kruskall-Wallis one-way nonparametric ANOVA or by Wilcoxon's signed-rank test. The response to feeding was evaluated using a two-tailed paired t-test. Pearson correlations were done for linear regression analysis.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

All infants were healthy, had no risk factors for sepsis, and were not receiving any medications, including antibiotics (Table 1). Their Apgar scores at birth were within the normal range (>8 at 1 and 5 min), and the infants were cared for in the newborn nursery. As anticipated, the birth weight of SGA infants was significantly lower than AGA infants.

Plasma amino acid concentrations during fasting are displayed in Table 2. There was no significant difference in the concentration of any amino acids among the three groups.

                              
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Table 2.   Plasma amino acid concentrations in newborn infants

Leucine and Urea Kinetics

The Ra of QN during fasting in AGA infants was 337 µmol · kg-1 · h-1 (Table 3). There was no difference in QN between normal and IDM infants. However, when expressed per kilogram body weight, QN was significantly higher in SGA infants during fasting. Formula feeding resulted in a significant increase in QN (average increase 45 µmol · kg-1 · h-1, P = 0.04) in the entire study group.

                              
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Table 3.   Leucine and urea kinetics in newborn infants

QC, estimated from the dilution of tracer in the plasma KIC pool, was ~237 µmol · kg-1 · h-1 in AGA infants; it was similar in IDM infants. SGA infants showed a higher QC compared with AGA infants (P = 0.001) and remained higher after formula feeding (P = 0.015). Formula feeding had no impact on the Ra of leucine carbon.

The rate of reamination of leucine calculated from the difference between QN and QC was similar in the three groups. In response to formula feeding, there was a significant (P = 0.01) increase in the rate of reamination of leucine.

Ra in the AGA infants was similar to that reported by us previously (17, 19). Urea synthesis decreased significantly in all babies after the administration of formula. There was no significant difference in the rate of urea synthesis among AGA, SGA, and IDM infants, either during fasting or during the fed state.

Glutamine and Phenylalanine Kinetics

Ra of glutamine measured by [5-15N]glutamine tracer dilution was similar in AGA and IDM infants (Table 4). Ra glutamine was significantly higher in SGA infants (P = 0.007) when compared with AGA infants. Formula feeding resulted in a significant decrease (P = 0.002) in the Ra of glutamine in all infants. Glutamine Ra remained significantly higher (P = 0.03) in SGA infants during the fed state. Other investigators have suggested that prolonged tracer infusion may be necessary to achieve an isotopic steady state in the glutamine pool (35). To be certain of the observed decrease in glutamine Ra in response to feeding, we studied three infants (two AGA and one SGA) during the fasting state. Tracer infusion for 300 min showed that an isotopic steady state was achieved by 180 min, with no further increase in enrichment in plasma glutamine. Therefore, the observed decrease in glutamine Ra during feeding appeared to be a true response to nutrient administration.

                              
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Table 4.   Glutamine and phenylalanine kinetics

Ra of phenylalanine in the plasma was 79.7 ± 18.0 µmol · kg-1 · h-1 in healthy term infants. There was no significant difference in phenylalanine Ra between normal and IDM infants. Feeding the formula had no impact on phenylalanine turnover in these infants. In contrast to AGA infants, phenylalanine Ra, and therefore the whole body rate of protein breakdown, was significantly higher (P = 0.01) in SGA infants when compared with AGA infants. There was no effect of formula feeding on the turnover of phenylalanine in SGA infants. A statistically significant positive correlation was observed between the Ra of phenylalanine and QC (Fig. 1).


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Fig. 1.   Correlation between phenylalanine flux and leucine carbon flux during fasting in newborn infants. Broken line, 95% confidence interval (y = -31.0 + 3.245x; r2 = 0.66; P = 0.001); open circles, small-for-gestational-age (SGA) babies; filled circles, appropriate-for-gestational-age (AGA) babies; gray circles, infants of diabetic mothers (IDM).

The rate of glutamine release from proteolysis was calculated from phenylalanine kinetics and reflects those measurements. The rate of de novo synthesis of glutamine was calculated by subtracting the glutamine released from protein breakdown from glutamine Ra. As shown (Table 4), de novo synthesis was a major component of the measured glutamine turnover and contributed ~85% to glutamine Ra. Formula feeding resulted in a significant decrease in de novo synthesis of glutamine.

Measurements of the 15N appearance (m1) in plasma urea were used to calculate the contribution of glutamine nitrogen to urea. The fraction of urea nitrogen derived from [5-15N]glutamine ranged between 1 and 9% (average 5.5%).

15N enrichment of plasma glutamate was measured in four AGA and one IDM infant. These data show that 25-30% of glutamate nitrogen was derived from leucine nitrogen during fasting. The fraction of glutamate nitrogen derived from leucine increased to 27-37% during feeding.

Correlation

A positive correlation was observed between QN and glutamine flux during fasting (r2 = 0.59, P = 0.001; Fig. 2). During feeding, when there was an increase in QN and a decrease in glutamine Ra, the correlation between QN and glutamine Ra was not as significant (r2 = 0.47, P = 0.04). A negative correlation was also observed between glutamine flux and the rate of urea synthesis (r2 = 0.39, P = 0.02; Fig. 3). Because SGA infants have higher rates of protein turnover and higher rates of energy consumption than AGA infants, the relation between birth weight and glutamine flux normalized for phenylalanine flux was also examined (Fig. 4). As shown, there was no correlation between birth weight and the ratio of glutamine and phenylalanine turnover rates in SGA infants.


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Fig. 2.   Correlation between rate of appearance of leucine nitrogen and glutamine. Broken line, 95% confidence interval (y = 215.7 + 1.07 x; r2 = 0.59; P = 0.001); open circles, SGA babies; filled circles, AGA babies; gray circles, IDM babies.



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Fig. 3.   Correlation between the rates of appearance of glutamine and urea during fasting. Broken line, 95% confidence interval (y = 316.7 - 0.21x; r2 = 0.39; P = 0.03); open circles, SGA babies; filled circles, AGA babies; gray circles, IDM babies.



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Fig. 4.   Relationship between birth weight and ratio of the rates of turnover of glutamine and phenylalanine. Open circles, SGA babies; filled circles, AGA babies; gray circles, IDM babies.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The present data show that, in healthy full-term infants in the immediate neonatal period, 1) the rates of leucine kinetics and transamination are high compared with healthy adults (28); 2) there is a high rate of glutamine turnover, the majority (~85%) of which is contributed by de novo synthesis; 3) systemic glutamine nitrogen contributes only ~5% to urea nitrogen; 4) there is a positive correlation between leucine nitrogen turnover and glutamine turnover; 5) there is a negative correlation between the Ra of glutamine and urea synthesis; and 6) enteral feeding results in an increased rate of branched-chain amino acid transamination but a decreased Ra of glutamine while having no effect on the Ra of leucine C or phenylalanine. These data are the first to systematically examine the relationship between branched-chain amino acid transamination, glutamine turnover, and urea synthesis in healthy newborn infants and provide the normative data against which studies in premature infants or the impact of nutrient and other interventions can be examined. We selected the study population carefully so that only healthy infants who experienced no adverse perinatal events and had no neonatal problems were studied. However, because these babies had not started to gain weight and possibly did not accrete nitrogen, these data may not be a true representation of growth and may have been influenced by the hormonal and other changes associated with transition to extrauterine life. The data on SGA infants show that, on a weight-specific basis, most of the measured parameters of protein and nitrogen metabolism were higher in these infants when compared with AGA infants.

The data of phenylalanine and leucine C kinetics in the present study are of similar magnitude, as reported previously in full-term infants, and are significantly higher than those reported in normal healthy adults after an overnight fast (5, 9, 10, 28). A positive correlation between leucine QC and phenylalanine flux shows that either tracer could be used to quantify whole body rates of protein turnover. The relative ratio of leucine and phenylalanine turnover (slope 3.25) is higher than that observed in healthy adults and similar to that reported previously in low-birth-weight infants (36). The higher leucine-to-phenylalanine ratio suggests a high turnover rate of proteins enriched with leucine relative to phenylalanine in the newborn infant.

No measurable change in leucine C or phenylalanine kinetics was observed in response to formula feeding. The unchanged Ra of these amino acids in the presence of enteral intake would suggest a decrease in the whole body rate of proteolysis, a response similar to that seen in adults (29). The lack of any effect of feeding on leucine C kinetics could also be the consequence of the first-pass extraction of enterally administered amino acids in the splanchnic compartment and an inhibitory effect of nutrient administration on whole body proteolysis. However, these effects cannot be separated without the simultaneous administration of labeled tracers via the orogastric route. Additionally, the short duration of the study and the relatively large variance in these measurements may not have permitted the examination of the effects, if any, of the orally administered nutrients on the kinetics of leucine C and phenylalanine.

The present data are the first to quantify Ra of leucine nitrogen and the rate of transamination of leucine in newborn infants. Both of these rates were higher than those reported in the healthy adult population (21, 27), reflecting a higher rate of overall whole body nitrogen kinetics. In response to feeding, as anticipated, there was a significant increase in QN and transamination (21), indicating a higher rate of transfer of nitrogen between various nonessential amino acids.

The measurements of the Ra of glutamine by tracer dilution methods in the plasma have been discussed in relation to cellular compartmentation of glutamine and the possible lack of an isotopic steady state (22, 35). To ensure that the observed response to feeding was not a consequence of lack of isotopic steady state, tracer studies were performed during a prolonged fast in three babies. These data confirmed that isotopic steady state was achieved during the study period. The high rate of glutamine turnover observed during fasting was, for the most part (~85%), the consequence of de novo synthesis. The major source of glutamine carbon for de novo synthesis is likely to be the anaplerotic flux into the tricarboxylic acid cycle as a result of the high rate of endogenous protein breakdown (turnover) observed in these infants (Table 4). Because branched-chain amino acids are the major source of glutamine amino nitrogen (7, 4), it was of note that a linear correlation between leucine nitrogen turnover and glutamine turnover was observed during fasting (Fig. 2). A negative relationship between glutamine turnover and urea synthesis (Fig. 3) suggests that glutamine's role involves transfer of nitrogen, not only for urea synthesis but also for other synthetic processes such as hepatic protein synthesis.

Enteral nutrient administration was associated with an increase in branched-chain amino acid transamination and a decrease in the Ra of glutamine. The increase in leucine transamination and decrease in glutamine turnover during feeding resulted in a less significant correlation between QN and glutamine Ra. These data suggest that glutamate, the immediate precursor of glutamine formed by transamination of branched-chain amino acids, was being transaminated with several other amino acids, resulting in a redistribution of nitrogen and ultimately a decrease in glutamine turnover. In addition, the decrease in Ra glutamine in response to feeding could be the result of the decrease in proteolysis. The primary site of these processes cannot be determined. However, at the hepatic level, it could result in a decrease in N-acetylglutamine, which should cause a decrease in urea synthesis during feeding, as was observed in the present study.

IDM infants did not show any significant differences from AGA infants in any of the kinetic parameters examined. This was not surprising, because the IDM babies were also appropriate for gestational age, i.e., they were not macrosomic, the consequence of a contemporary practice of rigorous regulation of maternal metabolism during pregnancy.

The data on SGA infants are of interest since all of the measured parameters (leucine nitrogen, phenylalanine turnover, glutamine turnover, and rate of transamination of leucine) were increased on a weight-specific basis, whereas the rate of urea synthesis was lower when compared with AGA infants. Because the rate of energy consumption as expressed per kilogram body weight is increased in these babies and because protein turnover has been shown to be significantly related to the rate of energy consumption (38), we normalized the glutamine turnover data for the rate of phenylalanine (protein) turnover. As shown in Fig. 4, glutamine turnover expressed in relation to phenylalanine turnover had no relationship with birth weight, suggesting that energy consumption is an important determinant of a number of metabolic fluxes. Of significance, in the presence of a higher rate of glutamine flux, the rate of urea synthesis was lower in the smaller infants, suggesting an important regulatory mechanism for conservation (and accretion) of nitrogen (Fig. 3).

In summary, in the present study, we have examined the relation between protein turnover, glutamine turnover, and urea synthesis in healthy normal and SGA infants. The data show that, in the presence of high anaplerotic flux into the tricarboxylic acid cycle, as a result of a high rate of protein turnover and energy consumption or as a result of nutrient administration, there is an increased rate of cataplerosis (glutamine turnover). These mechanisms may be important for protein accretion and for the conservation of body nitrogen, as evidenced by the lower rate of oxidation of protein (urea synthesis) in the smaller infants.


    ACKNOWLEDGEMENTS

We thank the nursing staff of the General Clinical Research Center for help with these studies and Dr. Saeid Amini for statistical analysis. The secretarial help of Joyce Nolan is gratefully appreciated.


    FOOTNOTES

The work was supported by National Institutes of Health Grants HD-11089, RR-00080, and in part by a metabolism training grant (DK-07319) to P. S. Parimi.

Address for reprint requests and other correspondence: S. C. Kalhan, Schwartz Center for Metabolism & Nutrition, MetroHealth Medical Center, 2500 MetroHealth Dr., Cleveland, OH 44109-1998 (E-mail: sck{at}po.cwru.edu).

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.

10.1152/ajpendo.00403.2001

Received 10 September 2001; accepted in final form 1 November 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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5.   Clarke, JTR, and Bier DM. The conversion of phenylalanine to tyrosine in man. Direct measurement by continuous intravenous tracer infusions of L-[ring-2H5]phenylalanine and L-[1-13C]tyrosine in the postabsorptive state. Metabolism 31: 999-1005, 1982[ISI][Medline].

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