(Received for publication, October 11, 1996, and in revised form, February 10, 1997)
From the Department of Pathology,
§ Department of Medicine, and ¶ Department of Cell
Biology, Baylor College of Medicine, Houston, Texas 77030
We have investigated the metabolism of
leukotriene C4 (LTC4) in -glutamyl
transpeptidase (GGT)-deficient mice (Lieberman, M. W., Wiseman, A. L.,
Shi, Z-Z., Carter, B. Z., Barrios, R., Ou, C-N., Chevez-Barrios, P.,
Wang, Y., Habib, G. M., Goodman, J. C., Huang, S. L., Lebovitz, R. M.,
and Matzuk, M. M. (1996) Proc. Natl. Acad. Sci. U. S. A.
93, 7923-7926) and have found substantial conversion of
LTC4 to leukotriene D4 by high performance liquid chromatography and continuous flow fast atom bombardment-tandem mass spectrometric analyses. LTC4-converting activity has a
tissue distribution different from GGT with highest activity in spleen followed by small intestine, kidney, and pancreas and lower activity in
liver and lung. The activity is membrane-bound and is inhibited by
acivicin, a known inhibitor of GGT. The enzyme was partially purified
from the small intestine of GGT-deficient mice by papain treatment and
gel filtration chromatography. The partially purified fragment released
by papain has an apparent molecular mass of 65-70 kDa and the same
substrate specificity as the tissue homogenate. In addition to
LTC4, S-decyl-GSH is also cleaved. GSH itself, oxidized GSH, and the synthetic substrates used to analyze GGT activity
(
-glutamyl-p-nitroanilide and
-glutamyl-4-methoxy-2-naphthylamide) are not substrates for this
newly discovered enzyme. These data demonstrate that in addition to GGT
at least one other enzyme cleaves LTC4 in mice. To reflect
this enzyme's preferred substrate, we suggest that it be named
-glutamyl leukotrienase.
Peptidyl leukotrienes, cysteine-containing derivatives of
arachidonic acid, are potent inducers of airway constriction,
vasoconstriction, smooth muscle contraction, edema, and inflammation
(1-4). LTC41 is formed by
conjugation of leukotriene A4 with GSH (5) and is known to
be cleaved by GGT, which removes the glutamyl moiety to form
LTD4 (6). LTC4 conversion to LTD4
has long been thought to be mediated solely by GGT (7, 8). Recently,
however, the existence of an activity termed GGT-rel has been
identified in humans (9). GGT-rel shares an overall 40% amino acid
sequence identity with human GGT and is capable of cleaving the
-glutamyl linkage of LTC4, but it is unable to hydrolyze
synthetic substrates that are commonly used for assaying GGT. This
activity has been reported to be absent in mice (9).
The role of GGT in leukotriene metabolism is of interest because of the great potency of these compounds in responses to injury. However, the distribution of GGT activity and the sites of peptidyl leukotriene actions are not concordant. In the mouse, GGT tends to be expressed at very high levels in epithelia concerned with catabolism of GSH and reabsorption of its constituent amino acids (kidney and small intestine) and other ductular and secretory epithelia (pancreas and seminal vesicles) (7, 10-12). It is characteristically low in organs in which leukotrienes may play a role in responses to injury (e.g. lung, heart, and lymphoid tissue (spleen)). In addition, the types of responses mediated by the peptidyl leukotrienes (vasoconstriction, bronchoconstriction, increases in vascular permeability, and mucus formation) do not occur in close proximity to sites where GGT is most abundant (13). Although there are reports of GGT activity in endothelium (14-16), the discordance between peptidyl leukotriene targets and GGT activity raises the possibility that there are other LTC4-cleaving enzymes in the mouse.
We have recently used homologous recombination to inactivate GGT in the mouse (12). Mice homozygous for this mutation are completely GGT-deficient and have the expected phenotype (glutathionemia and massive glutathionuria). Thus, they represent a useful model to evaluate the conversion of LTC4 to LTD4 in the absence of GGT. In this paper we report the results of our initial experiments in which we demonstrate the presence of an additional LTC4/LTD4-converting enzyme in the mouse.
GGT-deficient mice were developed in our
laboratory (12). All mice used in this study were 6-7 weeks old.
[glycine-2-3H]GSH was purchased from DuPont
NEN. Bathophenanthroline disulfonic acid was obtained from GFS
Chemicals (Columbus, OH), and HPLC grade methanol from Baxter
Healthcare Corp. (McGaw Park, IL). LTC4, LTD4,
LTE4, GSH, S-decyl-GSH,
-glutamyl-p-nitroanilide, papain, and other chemicals
were purchased from Sigma unless otherwise indicated.
Total homogenates from different tissues of both wild type and GGT-deficient female mice and the partially purified enzyme from the small intestine of GGT-deficient mice (see below) were incubated with 5 µM LTC4 in a total of 0.2 ml of 0.1 M Tris-HCl buffer, pH 8.0, at 37 °C for a preset time. The reaction was terminated by adding 0.8 ml of methanol and then centrifuged. The supernatant was adjusted to 13% methanol with water, and the solution was loaded onto a 3-ml octadecyl disposable extraction column (J. T. Baker Inc., Philipburg, NJ) conditioned as recommended by the manufacturer. The column was washed with water followed by 10% methanol to remove proteins and other impurities. LTC4 and its conversion products were eluted with methanol, dried by Speedvac, and separated by HPLC on a C18 reversed-phase column (Customsil ODS, 4.6 × 150 mm, 3-µm particles, Custom LC, Houston, TX) as described (17). The mobile phase consisted of methanol:water:acetic acid (65:35:0.1), pH 5.6, and the column effluent was monitored at 280 nm. We expressed the specific activity of LTC4-converting activity as nmol of LTC4 converted/h/mg of protein by measuring the formation of products (LTD4 and sometimes LTE4; LTE4 is formed by the action of dipeptidase on LTD4 in some preparations (see also "Results")). The protein concentration was determined using the Pierce BCA protein assay kit (Pierce). For LTC4 to LTD4 conversion, the Km of the newly discovered enzyme from GGT-deficient mouse spleen and GGT from wild type mouse kidney was determined by measuring LTC4-converting activity with increasing LTC4 concentration from a double-reciprocal plot (1/v and 1/[S]) (18).
Assay for GGT ActivityVarious tissues from both wild type
and GGT-deficient female mice were homogenized with a Polytron
Tissuemizer in 0.1 M Tris-HCl buffer, pH 8.0. GGT
activity was assayed by incubating the homogenates from various tissues
with -glutamyl-p-nitroanilide and Gly-Gly for a preset
time at 37 °C as described (12). The specific activity is expressed
as µmol of p-nitroanilide released/min/mg of protein.
Tissue homogenates from either wild type
mouse kidney or GGT-deficient mouse spleen were incubated with 0.2 mM GSH, 0.2 mM Gly-Gly and 0.2 mM
dithiothreitol in a total of 0.5 ml of 0.1 M Tris-HCl
buffer, pH 8.0, at 37 °C for various lengths of time. In some
experiments, 1 µl of [glycine-2-3H]GSH (43.8 Ci/mmol, 1 µCi/µl) was added to the reaction. The reaction was
stopped by adding 100 µl of 70% perchloric acid, 50 µl of 15 mM bathophenanthroline disulfonic acid, and 50 µl of 2.5 mM -Glu-Glu as an internal standard. Following
derivatization with 2,4-dinitrofluorobenzene, the reaction was analyzed
by a reversed-phase ion exchange HPLC using a 3-aminopropyl column (4.6 × 200 mm, 5-µm particles; CEL Associates, Houston, TX) as described (19). The column effluent was monitored at 365 nm. The
fractions were collected and counted using a Beckman LS scintillation counter.
LTC4, LTD4, and
LTE4 standards and preparations from incubations of
LTC4 and homogenates from both wild type and GGT-deficient mice were analyzed by reversed-phase HPLC as described above. The
fractions corresponding to retention times of LTC4,
LTD4, and LTE4 were collected, dried, and
subjected to FAB-MS/MS analysis. Tandem mass spectrometric analyses
were performed using a VG ZAB-SEQ (VG Analytical, Manchester, UK)
hybrid mass spectrometer of BEqQ configuration (B, magnetic sector; E,
electric sector; q, radio frequency-only quadrupole; Q, quadrupole mass
filter), equipped with a VG 11-250J data system. The methods employed
were similar to those previously described (20, 21). Briefly,
ionization was accomplished by continuous flow FAB using xenon atoms (8 keV) for the primary atom beam. Delivery of solvent (water:acetonitrile (50:50) containing 1% saturated aqueous oxalic acid and 1%
2,2-dithiodiethanol) was via a fused silica capillary (50 µm inner
diameter). Flow rate was maintained at 4.5 µl/min using a Waters
prototype syringe pump controlled by the modified output of a Waters
model 680 gradient controller. Samples (10 µl) were injected using a
Rheodyne model 7125 injector. The accelerating voltage was 8 kV, and
the ion source temperature was 70 °C. Narrow range parent ion
scanning analyses were obtained by operating the instrument under
hardware control. For LTC4, the instrument was set to
detect parents of ions with a mass-to-charge
(m/z) ratio of 308; similarly, the instrument was
set to detect parents of m/z 319 for
LTD4 and LTE4.
Homogenates from either wild type mouse kidney or GGT-deficient mouse spleen were incubated with 0.2 mM S-decyl-GSH and 0.2 mM Gly-Gly in a final volume of 0.2 ml of 0.1 M Tris-HCl buffer, pH 8.0, at 37 °C for a preset time. The reactions were stopped by adding 100 µl of 70% perchloric acid, 50 µl of 50 mM bathophenanthroline disulfonic acid, derivatized as described (19), and analyzed by the same reversed-phase HPLC column under the same conditions as for leukotriene analysis and monitored at 365 nm. To determine the Km of S-decyl-GSH cleavage by the newly discovered enzyme, the formation of S-decyl-Cys-Gly was measured in GGT-deficient mouse spleen homogenates, and Km was calculated from a double-reciprocal plot (1/v and 1/[S]) (18).
Partial Purification of the LTC4-converting Activity from the Small Intestine of GGT-deficient MiceBased on preliminary experiments that showed the activity to be membrane-bound, we adapted the method of Meister with minor modification (22). Briefly, small intestine from GGT-deficient mice was homogenized with 2 mM Tris-HCl, pH 7.4, containing 50 mM D-mannitol; 1 M CaCl2 solution was then added to the homogenate to a final concentration of 10 mM. The homogenate was placed on ice for 10 min and centrifuged at 43,000 × g for 20 min. The pellet obtained was suspended in the above buffer using an all glass Dounce homogenizer. This step was repeated once, and the pellet was resuspended in 10 mM sodium phosphate, pH 7.4, containing 0.15 M NaCl to obtain the membrane fraction. The membrane fraction was treated with papain (17 units/mg of protein) at a final concentration of 1 mg of papain/15 mg of total membrane protein at 25 °C for 2 h and centrifuged at 43,000 × g for 30 min. Ammonia sulfate was added to the supernatant to achieve 90% of saturation. The solution was set at 4 °C for 3 h, and the precipitate was collected by centrifugation and dissolved in 50 mM Tris-HCl, pH 8.0. The solution was subjected to gel filtration chromatography on a Sephadex G-150 column equilibrated and developed with 50 mM Tris-HCl, pH 8.0. Fractions containing the LTC4-converting activity were pooled and concentrated by a Centricon concentrator with a 30-kDa molecular mass cut-off. This procedure resulted in about 10-fold purification of the enzyme.
-Glutamyl-p-nitroanilide is the standard
substrate used to assay GGT activity in mammalian tissues (22). As
shown in Table I, GGT-deficient mice fail to cleave this
substrate, while in wild type mice GGT levels are high in kidney,
pancreas, and small intestine and low in spleen, lung, and liver (see
also Ref. 12). These data indicate not only that GGT is absent from
these mutant mice but also that no other enzyme in mice cleaves this
substrate. Similarly,
-glutamyl-4-methoxy-2-naphthylamide, the usual
histochemical substrate for GGT (23), was inactive as a histochemical
substrate in GGT-deficient mice (data not shown), and thus, like
-glutamyl-p-nitroanilide, it is not cleaved by other
enzymes in mice.
|
Since our
previous studies did not detect any GGT activity in the GGT-deficient
mice (12), we hypothesized that tissues from these mice would be unable
to cleave LTC4. Our initial experiments with
LTC4 were performed on wild type kidney as a positive
control. When we examined homogenates of wild type kidney for the
ability to metabolize LTC4, we found, as expected, that
LTC4 was readily cleaved to LTD4: homogenates
containing 50 µg of protein cleaved 39% of LTC4 (0.39 nmol) in 1 min with the appearance of a peak corresponding to
LTD4 (Fig. 1A). Under similar
conditions we were unable to detect degradation of LTC4 in
homogenates of GGT-deficient kidney (not shown); however, longer
incubation times (over 1 h) revealed conversion of
LTC4 to LTD4 (Fig. 1B). This result
indicates that LTC4-converting activity other than GGT
exists in mice.
Next we surveyed a variety of tissues from GGT-deficient mice to
determine the tissue distribution of this newly detected activity. We
started by assaying mouse tissues that normally express little or no
GGT activity such as spleen, adult liver, and lung for LTC4
metabolism (Fig. 2). We found that incubation of
LTC4 with homogenates from these tissues resulted in a
decrease in LTC4 (peaks 1) and the appearance of
products having the same retention times as LTD4
(peaks 2) and LTE4 (peaks 3) (Fig. 2, left). These data indicate that LTC4 is
metabolized by GGT-deficient mouse spleen, liver, and lung homogenates.
Under the same conditions, homogenates from wild type mouse spleen,
liver, and lung also cleave LTC4 (Fig. 2,
right). Further, the cleavage of LTC4 by homogenates from GGT-deficient mice occurs at rates comparable with
those seen in preparations from wild type mice. In many tissues, we
observed two new peaks following incubation, one corresponding to the
retention time for LTD4 and the other to that for
LTE4. The latter product is the result of the conversion of
LTD4 to LTE4 by dipeptidase activity present in
some tissues (2, 24).
We evaluated homogenates of pancreas and small intestine for the
ability to cleave LTC4 and found results similar to those from kidney: homogenates from GGT-deficient mice cleaved
LTC4 at much reduced rates compared with pancreas and small
intestine homogenates from wild type mice (Fig. 3).
Results from these and similar experiments were used to calculate the
specific activity of LTC4 conversion for various tissues of
GGT-deficient and wild type mice (Table I). In GGT-deficient mice, the
highest level of this new activity is found in spleen. Kidney, small
intestine, and pancreas have activity levels that are about 30-40%
that of spleen, while liver and lung have somewhat lower levels. In
contrast, in wild type mice the highest level is found in kidney,
followed by pancreas and then small intestine. This order of activity
is similar to that of GGT when assayed by
-glutamyl-p-nitroanilide (Table I; see also Ref. 12). It is of interest, however, that in tissues with low GGT activity (liver,
lung, and spleen), 35-95% of LTC4 cleavage is accounted for by this newly detected enzyme (Table I). Thus, in spleen and liver
almost all of the metabolism of LTC4 appears to result from
this enzyme activity rather than from GGT.
HPLC peaks corresponding to the retention times of LTC4, LTD4, and LTE4 from reactions run with homogenates from both wild type and GGT-deficient mice were analyzed by a FAB-MS/MS (20, 21). Our studies revealed that the presumed LTC4, LTD4, and LTE4 peaks produced the same [M + H]+ ions and fragment ions as standard LTC4, LTD4, and LTE4 (results not shown). These data confirm that the reactions we have monitored by HPLC represent conversion of LTC4 to LTD4 and LTE4.
Evaluation of GSH and GSSG Cleavage by LTC4-converting ActivityGGT is believed to be the major enzyme responsible for the cleavage of GSH. We were curious to know if the newly identified LTC4-converting activity in GGT-deficient mice could also metabolize GSH. This question was examined by two approaches. The first involved monitoring the disappearance of GSH enzymatically from homogenates of spleens of GGT-deficient mice (25). We were unable to demonstrate any loss of GSH using this method (data not shown).
The second method involved HPLC and was adapted from Heisterkamp
et al. (9). These investigators identified an enzyme from human material termed GGT-rel (GGT-related) that cleaved
LTC4 and suggested that it might cleave the -glutamyl
moiety from GSH. We modified the assay conditions of Heisterkamp
et al. and used nonradioactive GSH and
[glycine-2-3H]GSH in the incubation mixtures
to improve assay sensitivity. We also added 0.2 mM
dithiothreitol in the reaction to prevent the conversion of GSH to
GSSG. We monitored the appearance of products (Cys-Gly, Cys, and Gly)
and the disappearance of GSH spectrophotometrically or by liquid
scintillation counting of HPLC effluent. No metabolism of GSH was
observed with either method in the absence of homogenate (Fig.
4, A1 and B1), while kidney homogenates from wild type mice readily cleaved GSH (positive control;
Fig. 4, A2 and B2). Fig. 4, A3 and
B3, illustrates profiles of homogenates of spleen from
GGT-deficient mice in which no loss of GSH or appearance of Cys-Gly,
Cys, or Gly was detected spectrophotometrically or by liquid
scintillation counting even after a 3-h incubation.
To confirm our results, we also used a partially purified enzyme preparation (see "Experimental Procedures") to assay for the hydrolysis of GSH. We standardized conditions so that a kidney homogenate from wild type (GGT-positive) mouse and our partially purified preparation catalyzed the same amount of LTC4 to LTD4 conversion and used these to assay for GSH cleavage. We could not detect any GSH cleavage with the partially purified enzyme from the small intestine of GGT-deficient mice, while under the same conditions, the wild type kidney homogenate cleaved 13.5% (6.75 nmol) of GSH. Thus, the newly identified LTC4-converting enzyme does not metabolize GSH or does so at a negligible rate.
Cleavage of GSSG was also evaluated. It is likely that the dipeptidase
(membrane-bound dipeptidase, renal dipeptidase) that cleaves Cys-Gly
generated by the action of GGT actually uses Cys-bis-Gly rather than
Cys-Gly as a substrate (26). Thus, we modified both the enzymatic
approach and the nonradioactive HPLC approach described above to
determine if our LTC4-cleaving activity could metabolize GSSG. In GGT-deficient mouse spleen homogenates, we could not detect
cleavage of GSSG; however, when we used homogenates of wild type kidney
as a positive control, GSSG cleavage was detected (data not shown).
Thus, unlike GGT, the activity we have detected in GGT-deficient mice
cleaves LTC4 and does not cleave GSH or GSSG. To reflect
the substrate specificity of this enzyme, we suggest that the newly
identified enzyme be designated -glutamyl leukotrienase (GGL).
GGL is a membrane-bound enzyme.
After centrifugation of spleen homogenates from GGT-deficient mice at
45,000 × g for 30 min, we recovered all of the GGL
activity in the pellet. Treatment of similar homogenates with papain
prior to centrifugation (22) released more than 90% of the GGL
activity into the supernatant. The papain-cleaved enzyme has an
apparent molecular mass of ~65 kDa by gel filtration chromatography
on Sephadex G-150 and 65-70 kDa by sedimentation velocity
centrifugation using linear 5-20% sucrose gradient (27). Incubation
at 60 °C for 5 min completely inactivated cleavage of
LTC4, but at 37 °C the enzyme was active for as long as
5 h. Acivicin, a commonly used inhibitor of GGT (28, 29), also
inhibited GGL (data not shown). We determined the Km
of LTC4 to LTD4 conversion by GGL to be
2.2 × 106 M. In contrast, the
Km for this conversion by GGT was found to be an
order of magnitude higher (2.0 × 10
5
M). Inclusion of Gly-Gly, a
-glutamyl acceptor, in the
incubation mixture did not significantly enhance the conversion of
LTC4 to LTD4 (data not shown). GSH does not
appear to be a competitive inhibitor of GGL, since even an 80-fold
molar excess of GSH in the reaction mixture does not alter the rate of
cleavage of LTC4 by GGL (data not shown).
Studies from Goffinet and Nguyen (30) demonstrated that mouse brain
LTC4 binding sites are also S-alkyl-GSH binding
sites. S-Decyl-GSH has the strongest binding among all of
the S-alkyl-GSH analyzed and had binding activity similar to
LTC4. We hypothesized that S-decyl-GSH might be
a substrate for GGL and examined the cleavage of S-decyl-GSH
by GGL. Our results indicated that, like GGT, GGL indeed hydrolyzes
S-decyl-GSH (results not shown). We compared the
Km of GGL using S-decyl-GSH as a
substrate with the value we obtained when LTC4 was used as
a substrate. We found that the Km for
S-decyl-GSH is about 3.0 × 105
M, more than an order of magnitude higher than the value of
2.2 × 10
6 M we found for
LTC4. The fact that GGL cleaves LTC4 and
S-decyl-GSH but not GSH, GSSG,
-glutamyl-p-nitroanilide, and
-glutamyl-4-methoxy-2-naphthylamide suggests that GGL prefers
hydrophobic GSH conjugates.
Using transgenic mice that lack GGT, we have detected an enzyme
that cleaves the -glutamyl group from LTC4 and generates LTD4. The activity is highest in spleen followed by small
intestine and kidney. Relatively high levels are also found in liver
and lung, organs in which GGT activity is less than 1% of that seen in
kidney (Table I). GGL cleaves LTC4, but GSH and GSSG, which are the major substrates for GGT in organs such as kidney, do not
appear to be cleaved by GGL. Support for this conclusion also comes
from our work with GGT-deficient mice: urine levels of GSH in
GGT-deficient mice are ~15 mM compared with urine levels
of ~6 µM in wild type mice (12), suggesting that GGL
metabolizes little, if any, of the GSH filtered or secreted by the
kidney. The fact that the Km of GGL (2.2 × 10
6 M) is an order of magnitude lower than
that of GGT (2.0 × 10
5 M) indicates
that GGL has higher affinity for LTC4 than GGT. Thus,
LTC4 is likely to be a physiologically significant
substrate for GGL. Several possibilities may explain why the level of
GGL activity is low under our assay conditions. First, the expression of GGL may be confined to certain cell types within organs, and analysis of GGL activity from homogenates of whole organs may underestimate GGL activity. Second, the GGL activity may be
constitutively low, but inducible under appropriate conditions
including responses to physiological stimuli or injury. Third, we
cannot exclude the possibility that compounds other than
LTC4 may be better substrates for GGL.
In addition to its cleavage of LTC4, GGL is capable of
cleaving S-decyl-GSH. Although we have not tested S-GSH
conjugates exhaustively, relatively long chain S-GSH conjugates of
aliphatic (hydrophobic) moieties (LTC4 and
S-decyl-GSH) seem to be preferred substrates for GGL. GGL
will not cleave GSH, GSSG, -glutamyl-nitroanilide, or
-glutamyl-naphthylamide; all of them are good substrates for GGT.
Thus, based on the substrates examined to date, GGL appears to have a
more restricted substrate specificity than GGT and cleaves only some of
the
-glutamyl compounds that GGT metabolizes. At present, we have
not found a compound that is a specific substrate for GGL.
In humans another enzyme, termed GGT-rel, that cleaves LTC4 has been identified (9). It appears to be a different enzyme from GGL, since GGT-rel is not found in the mouse, and it may cleave GSH as well (9). However, at present neither GGT-rel nor GGL has been extensively characterized, and it is not inconceivable that they may represent the human and mouse counterparts of the same enzyme. Because we have not yet extensively purified GGL, it is conceivable that GGL represents more than one enzyme. However, we view this possibility as unlikely.
In addition to a partial overlap in substrate specificity, GGL and GGT share some properties. Both are membrane-bound and are inhibited by acivicin. The papain-released fragment of GGL from mouse small intestine has an apparent molecular mass of 65 kDa compared to 68 kDa for GGT from rat kidney (22). This observation must be viewed with some caution, since GGT at least is known to exist in many glycosylated forms. However, all of these observations suggest that like human GGT and GGT-rel, mouse GGT and GGL may be related. Nevertheless, GGL and GGT are clearly separate enzymes, as evidenced by the fact that we have characterized GGL in tissues of GGT-deficient mice. In addition, GGL and GGT have different substrate preferences and different tissue distributions.
The tissue distribution of the two enzymes raises several interesting questions about their functions. It is unclear, for example, why GGL is expressed in kidney, pancreas, and small intestine, where GGT is expressed at very high levels. One possibility is that GGL has other as yet unidentified functions; for example, it may cleave substrates that are not metabolized by GGT. Another possibility is that within these organs GGL and GGT have different cellular distributions. It is known, for example, that GGT is confined to the proximal tubular cells of the kidney and the villous epithelium of the small intestine (31, 32) and may function at these sites to initiate the degradation of circulating or biliary LTC4 to LTD4. However, much of the physiologic action of LTC4/LTD4 occurs at sites where responses to injury occur, such as the smooth muscle of vessels and bronchi/bronchioles, and in inflammatory exudates. GGL function may be related to these responses and occur at the sites of these responses. Thus, it is possible that in the kidney and the small intestine, for example, the metabolism of LTC4 may be divided into two "compartments," one concerned with physiological function in these organs and the other with the clearance of peptidyl leukotrienes from the body. GGL activity accounts for 35-95% of the LTC4-metabolizing activity in spleen, lung, and liver. It may be that the substrate specificity of GGL compared with GGT is an advantage in these organs. Because GSH/GSSG is not cleaved by GGL, the function of this enzyme may be important in environments in which the extracellular reducing capability of GSH is necessary but generation of LTD4 is also required. Such conditions are met in the lung, in which GSH plays an important role in protection from oxidative injury and the generation of LTD4 may be essential as part of the response to injury in this organ. All of the above considerations suggest that this newly identified enzyme may play a significant role in the metabolism of leukotrienes.
We thank Drs. Mark L. Entman, Hiram F. Gilbert, Richard N. Sifers, and Russell M. Lebovitz for helpful discussion, Yan Liu for helping with some experiments, and Cheryl L. Rognerud for technical assistance with HPLC.