(Received for publication, November 5, 1996, and in revised form, December 30, 1996)
From the Merck Frosst Centre for Therapeutic Research, Pointe
Claire-Dorval, Quebec, Canada H9R 4P8 and the Department
of Pharmacology and Therapeutics, McGill University, Montreal, Quebec,
Canada H3G 1Y6
Microsomal glutathione S-transferase-II (GST-II) has recently been discovered and characterized as a member of the 5-lipoxygenase-activating protein (FLAP)/5(S)-hydroxy-6(R)-S-glutathionyl-7,9-trans-11,14-cis-eicosatetraenoic acid (LTC4) synthase gene family, which also includes microsomal glutathione S-transferase-I (GST-I) as a distant member of this gene family. This new enzyme is unique as it is the only member of this family capable of efficiently conjugating reduced glutathione to both 5,6-oxido-7,9,11,14-eicosatetraenoic acid (LTA4) and 1-chloro-2,4-dinitrobenzene. Although microsomal GST-II has been demonstrated to display both general glutathione S-transferase (GST) and specific LTC4 synthase activities, its biological function remains unknown. In this study, we investigated the physiological location of microsomal GST-II as well as the relative importance of this enzyme versus LTC4 synthase for the production of LTC4 in various human tissues and cells that have been previously demonstrated to possess LTC4 synthase activity. As determined by Western blot, microsomal GST-II was predominantly expressed in human liver microsomes, human endothelial cell membranes, and sparsely detected in human lung membranes. In contrast, LTC4 synthase was prevalent in human lung membranes, human platelet homogenates, and human kidney tissue. Concomitant to the formation of LTC4, microsomal GST-II also produces a new metabolite of LTA4, a postulated LTC4 isomer. This isomer was used to distinguish between microsomal GST-II and LTC4 synthase activities involved in the biosynthesis of LTC4. Based on the relative production of LTC4 to the LTC4 isomer, microsomal GST-II was demonstrated to be the principal enzyme responsible for LTC4 production in human liver microsomes and human endothelial cells and played a minor role in the formation of LTC4 in human lung membranes. In comparison, LTC4 synthase was the main enzyme capable of catalyzing the conjugation of reduced glutathione to LTA4 in human lung membranes and human platelet homogenates. Therefore, microsomal GST-II appears to be an integral component in the detoxification of biological systems due to its marked presence in human liver, in accordance with its known GST activity. Microsomal GST-II, however, may also be pivotal for cysteinyl leukotriene formation in endothelial cells, and this could change our current understanding of the regulation of leukotriene biosynthesis in inflammatory disorders such as asthma.
Microsomal glutathione S-transferase (GST)-II1 is the newest member of the FLAP/LTC4 synthase gene family discovered to date. This novel enzyme displays 33% identity to FLAP, 44% identity to LTC4 synthase, and limited sequence identity to microsomal GST-I at the amino acid level (1). These four proteins may also have similar structural configurations based on analogous hydrophobicity plots. Microsomal GST-II is a 16.6-kDa protein with a calculated pI of 10.4, possesses both LTC4 synthase as well as conventional GST activities as shown through its ability to conjugate reduced glutathione (GSH) with both LTA4 and 1-chloro-2,4-dinitrobenzene, and was consequently characterized as a member of the membrane bound GSTs (1). This new enzyme, accordingly, may have important roles in both leukotriene biosynthesis and in cellular detoxification by GST activity.
GSTs are a family of enzymes that catalyze the conjugation of reduced
GSH to a variety of electrophilic substrates. The GSTs belong to a gene
superfamily in which four different gene families encode the cytosolic
GSTs (, µ,
, and
) and two encode the microsomal forms of
the enzyme (1, 2). Biologically, these enzymes are responsible for
detoxification of xenobiotics by catalyzing GSH conjugation to
generated metabolites and for protection from endogenous
hydroperoxides produced during oxidative stress via their
GSH-dependent peroxidase activity (3).
LTC4 synthase is a unique membrane bound enzyme that has been distinguished from all previously known cytosolic and microsomal GSTs (4, 5) through its narrow substrate specificity for LTA4. LTC4 synthase and FLAP are important components of the leukotriene biosynthetic pathway. FLAP is required for cellular LTA4 formation, possibly through the presentation of arachidonate to 5-lipoxygenase (6-9). LTC4 synthase is the first committed enzyme for the conversion of LTA4 to the cysteinyl leukotrienes, LTC4, LTD4, and LTE4, which have significant roles in immediate hypersensitivity reactions (see reviews in Refs. 10-15).
Prior to the molecular characterization of LTC4 synthase and microsomal GST-II, a number of studies have described LTC4 synthase activity in various cell types and tissues (15-24). The present study addresses the relative importance of these two enzymes in the synthesis of LTC4 in various human tissues.
Human venous endothelial frozen cell pellet was purchased from Cell Systems (Kirkland, WA). Human liver and lung tissues were obtained from the International Institute for the Advancement of Medicine (IIAM) (Exton, PA). Protein medleys were from Clontech (Palo Alto, CA). Bovine serum albumin, phenylmethylsulfonyl fluoride (PMSF), Hepes, and dithiothreitol were purchased from Sigma. Aprotinin, leupeptin, and pepstatin A were from Boehringer Mannheim GmbH, (Mannheim, Germany). Tris base was also from Boehringer Mannheim Corporation (Indianapolis, IN). Taurocholic acid, sodium salt was obtained from Calbiochem Corporation (La Jolla, CA). LTA4-methyl ester, 2-(2-(1-(4-chlorobenzyl)-4-methyl-6-((5-phenylpyridin2-yl)-methoxy)-4,5-dihydro-1H-thiopyrano(2,3,4-c, d)indol-2-yl)ethoxy)butanoic acid (L-699, 333), and 3-(1-(p-chlorophenyl)-5-isopropyl-3-tert-butylthio-1H-indol-2-yl)-2,2-dimethylpropanoic acid (MK-886) were synthesized by the Department of Medicinal Chemistry at the Merck Frosst Centre for Therapeutic Research. Novapak C18 HPLC columns were from Waters Chromatography (Milford, MA). HPLC solvents were from BDH (Toronto, Ontario). The Department of Biochemistry and Molecular Biology at the Merck Frosst Center for Therapeutic Research provided the specific polyclonal antibodies to FLAP, LTC4 synthase, and microsomal GST-II (25, 26). The peptide antibody for microsomal GST-II was raised to microsomal GST-II amino acid sequence 42-57 (note: Asp57 was replaced with His). Microsomal GST-I polyclonal antibody was a kind gift from Dr. John Hayes, and purified microsomal GST-I was a kind gift from Dr. Ralf Morgenstern. Partially purified (137-fold) LTC4 synthase from THP-1 cell extracts, HiLoad Q anion exchange fraction, was prepared as described previously (27). Renaissance Western blot enhanced chemiluminescence reagent was purchased from DuPont NEN Research Products. Horseradish-peroxidase-linked donkey anti-rabbit antibody was obtained from Amersham Life Sciences (Buckinghamshire, United Kingdom). Blotting grade blocker non-fat dry milk and Tween-20 were from Bio-Rad Laboratories. Sodium dodecyl sulfate and phosphate-buffered saline without calcium and magnesium (PBS) were from Life Technologies, Inc. All other reagents were of analytical grade and were purchased from Sigma.
Isolation of Human Peripheral Blood Platelets and Preparation of HomogenatesHuman peripheral blood was collected from healthy
volunteers who had not taken any medication for the previous 7 days.
One-tenth the volume of 3.8% (w/v) trisodium citrate, in 0.9% (w/v)
saline, was immediately added to the collected blood as anticoagulant, and the mixture was centrifuged at 200 × g for 10 min
at 22 °C. The platelet-rich plasma was transferred to a tube
containing 50% (v/v) Tris-buffered saline (TBS) containing 6.2 mM EDTA and 30% (v/v) of 3.8% (w/v) trisodium citrate.
After gentle mixing by inversion, the diluted platelet-rich plasma was
centrifuged at 650 × g for 10 min at 22 °C. The
pellet was washed twice with TBS containing 6.2 mM EDTA,
and cells were counted using a Coulter counter. The final concentration
was adjusted to 1.0 × 108 platelets/ml in cold lysis
buffer containing 10 mM Hepes/KOH (pH 7.4), 2 mM EDTA, 2% (w/v) taurocholate, with freshly added dithiothreitol (5 mM), PMSF (1 mM, from a fresh
200 mM stock in EtOH), leupeptin (20 µg/ml), pepstatin A
(10 µg/ml, from a fresh 1 mg/ml stock in EtOH), and aprotinin (10 µg/ml). Lysates were shaken for 20 min at 4 °C, sonicated for
5 s at 0 °C, frozen in liquid nitrogen, and stored at
80 °C. Bradford (28) protein assay (Bio-Rad) was performed on the
homogenate.
A frozen human venous endothelial cell pellet
containing 1 × 107 cells was thawed on ice and
resuspended in PBS containing 2 mM EDTA and 2 mM PMSF. The cells were sonicated three times for 15 s
each on ice and subjected to differential centrifugation at 1,000 × g for 15 min and 100,000 × g for 1 h, both at 4 °C. The pellets were resuspended in 50 µl of PBS
containing 2 mM EDTA, frozen in liquid nitrogen, and stored
at 80 °C. Subcellular fractions of frozen human liver and lung
tissues were prepared according to standard procedures (see Refs. 29
and 30, respectively).
Western blot analyses were performed similarly to those described previously (26). Briefly, SDS-containing sample buffer (31) was added to all samples. The samples were subsequently heated for 5 min at 95 °C, electrophoresed through SDS-polyacrylamide gels (Novex), and electroblotted onto nitrocellulose. Ponceau S staining was used to visualize the efficiency of transfer. Membranes were then soaked for 1 h at 25 °C in Tris-buffered saline containing 0.1% (v/v) Tween 20 (0.1% T-TBS) (20 mM Tris/HCl (pH 7.5), 0.5 M NaCl) containing 5% (w/v) Bio-Rad blotting grade blocker non-fat dry milk. Blots were washed twice for 5 min each with 0.1% T-TBS and subsequently treated for 1 h at 25 °C with the indicated specific primary polyclonal antibody (dilution 1:500) in 0.05% T-TBS containing 5% dry milk. After washing the blots 3 times for 5 min each with 0.1% T-TBS, the membranes were incubated for 1 h at 25 °C with a horseradish-peroxidase-linked donkey anti-rabbit antibody (dilution 1:3,000) in 0.05% T-TBS containing 1% dry milk. The blots were washed 3 times for 5 min each with 0.3% T-TBS and then 3 times for 5 min each with 0.1% T-TBS and subsequently developed using enhanced chemiluminescence (Renaissance Western blot chemiluminescence reagent, DuPont NEN) according to the manufacturer instructions.
Measurement of LTC4 Synthase and Microsomal GST-II Enzymatic Activities by Reverse-phase HPLCLTC4 synthase and microsomal GST-II activities were assayed as described previously (1) by measuring the amount of LTC4 and an isomer of LTC4 produced in incubations at 25 °C from various samples in 0.1 M potassium phosphate buffer (pH 7.4) containing reduced glutathione (5 mM) and 60 µM LTA4 (free acid) stabilized by the presence of 0.05% (w/v) bovine serum albumin in a final volume of 100 µl. After 15 min, the reaction was terminated by the addition of an equivalent volume of acetonitrile/methanol/acetic acid at 50:50:1, and the precipitated proteins were removed by centrifugation at 16,000 × g for 15 min at 4 °C. The amount of LTC4 and an isomer of LTC4 synthesized were resolved by isocratic reverse-phase HPLC on a Waters Associates Novapak C18 column (3.0 × 150 mm, 4 µm particle size) with the mobile phase (acetonitrile/methanol/water/acetic acid at 28:14:54:1 (pH 5.6)) at a flow rate of 1.2 ml/min. Quantification of the amount of products formed was based on the measurement of the peak absorbance at 280 nm from known amounts of injected LTC4. The LTC4 and the LTC4 isomer peaks were identified by comparison to retention time of synthetic LTC4 and on-line analysis of the UV absorbance spectra of the eluted compounds using a Waters 991 diode-array spectrophotometer.
Preparation of Microsomal GST-II, LTC4 Synthase, and FLAP Membranes from Baculovirus-infected Sf9 CellsMicrosomal
GST-II, LTC4 synthase, and FLAP proteins were obtained from
baculovirus-infected Sf9 cell membranes as described previously (1, 8).
Briefly, Sf9 cells (Invitrogen) were infected with recombinant or
wild-type virus and cultured for 72 h at 28 °C in Grace's
insect media (Life Technologies, Inc.) supplemented with 10% fetal
bovine serum (v/v), gentamycin (50 µg/ml), and fungizone (2.5 µg/ml). The cells were then harvested, washed, resuspended in PBS,
and sonicated three times for 10 s each on ice. The sonicates were
subsequently subjected to differential centrifugation at 500 × g for 10 min and 100,000 × g for 1 h, both at 4 °C. The pellets were resuspended in PBS and stored at 80 °C. Coomassie protein (Pierce) assay was performed according to
the manufacturer instructions.
Polyclonal antibodies were used as tools to
determine the relative contribution of LTC4 synthase
versus microsomal GST-II for the production of
LTC4 in biological systems. To determine the presence of
either of these proteins, the specificity of these antibodies with
respect to one another and to the other members of this family, FLAP
and microsomal GST-I, were tested. The antibodies used to detect
microsomal GST-II, LTC4 synthase, and FLAP had been raised
against peptides in the region displaying the highest identity to one
another, the FERV region (Fig. 1). The microsomal GST-I
polyclonal antibody was raised against the full-length protein, which
also contains a similar region of homology to the other peptides. As
the different peptides contain homologous regions, the four antibodies
were tested for possible cross-reactivity by Western blot analyses.
Fig. 2 demonstrates that microsomal GST-I, microsomal
GST-II, and FLAP antibodies were specific. However, the
LTC4 synthase antibody displayed some cross-reactivity by detecting microsomal GST-II and FLAP proteins. These results indicate that we have specific polyclonal antibodies for the detection of
microsomal GST-I, microsomal GST-II, and FLAP and a polyclonal antibody
that cannot distinguish between LTC4 synthase and
microsomal GST-II.
Expression of Microsomal GST-I, Microsomal GST-II, LTC4 Synthase, and FLAP in Various Human Tissues
After establishing
the specificity of the above polyclonal antibodies, they were
subsequently used to search for their respective protein targets in
various human tissues (Fig. 3). As expected, microsomal
GST-I was markedly detected in liver tissue, as well as in adrenal
gland, kidney, and to a lesser extent mammary gland tissues. Microsomal
GST-II was highly expressed in liver (Fig. 3) and in lower amounts in
adrenal gland, kidney, pancreas, and thymus tissues (after prolonged
exposure, data not shown). The LTC4 synthase antibody
detected ~17-kDa proteins intensely in kidney, adrenal gland, liver,
and lymph and weakly in skeletal and stomach samples, whereas, the FLAP
antibody displayed strong ~18-kDa bands in lymph and thymus tissues
and weak bands in adrenal gland, liver, and skeletal tissues. These
results have identified many important human tissues in which these
proteins may be found.
Expression of LTC4 Producing Enzymes in Human Liver, Lung, Endothelial Cells, and Platelets
Fig. 3 demonstrates that
microsomal GST-II was primarily present in human liver tissue. To
confirm and extend this finding, the subcellular localization of
microsomal GST-II was investigated in human liver tissue by probing
liver samples prepared by differential centrifugation with the
microsomal GST-II and LTC4 synthase polyclonal antibodies
(Fig. 4). Both microsomal GST-II and LTC4
synthase antibodies displayed an ~17-kDa band in the liver
homogenate, the 10,000 × g supernatant, and the
100,000 × g pellet fractions. The intensity of
detection increased with increasing fractionation of the membrane
samples, and no bands were observed in the liver 100,000 × g supernatant fraction. These results indicate that microsomal GST-II is present in human liver as a membrane bound protein.
Also, the presence of microsomal GST-II versus LTC4 synthase was investigated in human lung, platelets, and endothelial cells. The LTC4 synthase antibody recognized an ~17-kDa band in the lung 10,000 × g pellet and a more intense band in the 100,000 × g pellet, as well as an ~17-kDa band in platelet homogenates. The LTC4 synthase antibody also displayed a band in endothelial cell membranes after a prolonged exposure time (20 min, data not shown). In contrast, microsomal GST-II was only slightly detected in lung membranes, not observed in platelet homogenates, and detected in endothelial cell membranes. These results indicate that LTC4 synthase is the predominant enzyme present in human lung tissue and human platelets, whereas, microsomal GST-II appears to be the principal enzyme found in human endothelial cell membranes.
LTC4 Synthase and Microsomal GST-II Biosynthetic Activities in Human Tissues Can Be Distinguished by Product ProfilesMicrosomal GST-II catalyzes the formation of
LTC4 and a possible LTC4 isomer from
LTA4 and glutathione. LTC4 and the putative LTC4 isomer have retention times of 9.0 and 7.6 min,
respectively, and distinct maximum UV absorbencies of 281 and 283 nm,
respectively (1) (Fig. 5). LTC4 synthase,
however, mainly catalyzes the synthesis of LTC4. Therefore,
the ratio of the production of LTC4 to the postulated
LTC4 isomer was used to distinguish between the presence of
microsomal GST-II activity and LTC4 synthase activity (Table I). To confirm, as well as discern, the presence
of either microsomal GST-II or LTC4 synthase as
demonstrated by Western blot (Fig. 4), activity assays were performed.
Lung 100,000 × g pellet, platelet homogenates, and
LTC4 synthase derived from both THP-1 cell extracts and Sf9
cell membranes all displayed an LTC4:LTC4
isomer ratio of >50. In contrast, liver 100,000 × g
pellet, endothelial 100,000 × g pellet, and microsomal
GST-II from Sf9 cell membranes all demonstrated ratios of <5. These
ratios indicate that LTC4 synthase activity is mainly in
human lung and platelets, whereas, microsomal GST-II activity resides
predominantly in human liver and endothelial cells.
|
Due to the similarities in structure between microsomal GST-II, LTC4 synthase, and FLAP and their involvement in leukotriene biosynthesis, activity assays were also performed on microsomal GST-II in the presence of known inhibitors of leukotriene biosynthesis. MK-886 is a potent inhibitor of FLAP (nanomolar range), while both L-699 333 and MK-886 are capable of inhibiting LTC4 synthase activity in the micromolar range. L-699 333 and MK-886, were both found to inhibit LTC4 formation by microsomal GST-II expressed in Sf9 cell membranes with IC50 values in the 10 µM range (data not shown). The inhibition of microsomal GST-II activity by these leukotriene biosynthesis inhibitors suggests a similar active site for these three proteins.
Recently, a novel membrane bound protein, microsomal GST-II, was discovered that displayed homology at the amino acid level to FLAP, LTC4 synthase, and to a lesser extent microsomal GST-I (1). These proteins all display similar hydrophobicity patterns and consequently may have similar structures. Due to the above similarities, these four proteins appear to be members of a gene family that encodes membrane bound proteins important for either leukotriene production or cellular detoxification by GSH conjugation. Microsomal GST-II is a unique member of this family due to its ability to efficiently conjugate reduced GSH to both LTA4 and to 1-chloro-2,4-dinitrobenzene (1). Hence, microsomal GST-II may be more important than LTC4 synthase as a catalyst for the formation of LTC4 in certain tissues. To try to understand the biological function (importance) of this unique enzyme, its location in human cells and tissues compared with other family members was determined by Western blot analyses. In addition, a new method was developed for distinguishing between previously determined LTC4 synthase activity and what may actually be microsomal GST-II activity.
Microsomal GST-I, microsomal GST-II, and FLAP polyclonal antibodies were determined to be specific for the recognition of their respective proteins even though the peptides used to raise these antibodies contained regions of homology. Only the LTC4 synthase antibody displayed cross-reactivity by recognizing FLAP, microsomal GST-II, and LTC4 synthase. This nonspecific association of the LTC4 synthase antibody to FLAP may be explained by a combination of high antibody concentration (1:500 dilution) and high expression of FLAP from baculovirus-infected Sf9 cells resulting in its high abundance on the blot. The recognition of FLAP by the LTC4 synthase antibody could be discerned from LTC4 synthase and microsomal GST-II due to the higher molecular weight of FLAP and was therefore not a problem in determining the presence of FLAP in various tissues. This antibody, however, could not discriminate between LTC4 synthase and microsomal GST-II, thus, the presence of LTC4 synthase was inferred based on the absence of specific microsomal GST-II recognition by the microsomal GST-II polyclonal antibody in identical tissues.
Interestingly, microsomal GST-II expressed from Sf9 cells was detected as a doublet, which may indicate the phosphorylation of this enzyme or a downstream degradation product. Phosphorylation of microsomal GST-II could be a possible regulatory mechanism for this enzyme as it has also been postulated for LTC4 synthase (27). LTC4 synthase has two potential protein kinase C phosphorylation sites which are not, however, present in microsomal GST-II. Microsomal GST-II from Sf9 cells was also inhibited by leukotriene biosynthesis inhibitors with a similar IC50 value as that for LTC4 synthase (32). As these inhibitors have been shown to inhibit FLAP and LTC4 synthase, these proteins may have a similar active site. FLAP amino acids in the region Ser41 to Val61 have been shown to be critical for binding of leukotriene biosynthesis inhibitors (33). This region is the most homologous region between the members of this family including microsomal GST-I and is, therefore, postulated to be the lipid binding site of these proteins.
As expected, human microsomal GST-I is mainly expressed in human liver tissue in accordance with previous findings of location and its function as a phase II detoxifying enzyme (3). The significant detection of microsomal GST-I in the adrenal gland and kidney substantiates previous findings of microsomal GST activity, protein expression, or mRNA expression and probably serves to protect these extrahepatic tissues from endogenous or exogenous toxicants (34, 35). Microsomal GST-II is also primarily expressed in human liver membranes and to a lesser extent in endothelial cells and minimally in lung membranes, displaying a very narrow tissue distribution. The marked detection of microsomal GST-II in the liver along with its glutathione S-transferase activity suggests that it may have a similar biological function to microsomal GST-I. Microsomal GST-II also appears to be post-transcriptionally regulated since the protein was not significantly detected in many of the tissues that exhibit mRNA expression (1). However, this lack of protein detection may also be a matter of sensitivity in the Western blot analyses. FLAP and LTC4 synthase were widely distributed with expression in overlapping tissues as might be anticipated for two proteins involved in the biosynthesis of cysteinyl leukotrienes. However, there were also tissues (kidney, stomach, and thymus) that only expressed either FLAP or LTC4 synthase. Consequently, these tissues demonstrate either the requirement of transcellular metabolism for leukotriene biosynthesis or in the case of those tissues that only express FLAP, may solely produce LTB4. These observations may also indicate the recognition of other putative members of this gene family by the cross-reactive LTC4 synthase antibody. Contamination of tissue preparations by infiltrating peripheral blood cells could also explain any discrepancies in tissue distributions. FLAP was distinctly found in human lymph and thymus tissues, confirming previous findings of FLAP in B- and T-lymphocytes (36), whereas, LTC4 synthase is significantly detected in human kidney, lung, platelets, and to a lesser extent in skeletal tissue, identical to earlier observations of mouse LTC4 synthase mRNA expression (37) and human LTC4 synthase protein detection (38). The LTC4 synthase antibody appears to detect a 17-kDa band in the liver with an equivalent intensity to that detected by the microsomal GST-II antibody. These band intensities are most likely saturated and have reached a plateau due to overexposure of the blots. Therefore, the band intensities displayed in the liver by different antibodies cannot be quantitatively compared.
Microsomal GST-II possesses both generalized GST and LTC4 synthase activities thereby contributing to the formation of LTC4 in biological systems. Microsomal GST-II concomitantly produces LTC4 and a new LTA4 metabolite, a possible isomer of LTC4, that can be separated from LTC4 by reverse-phase HPLC. LTC4 synthase stereoselectively produces LTC4, thus, the production ratio of LTC4 to the LTC4 isomer clearly differentiates microsomal GST-II activity versus LTC4 synthase activity. Identification of microsomal GST-II activity corresponds directly to the detection of microsomal GST-II in Western blot analyses. There is significant microsomal GST-II activity in liver 100,000 × g membrane samples. Microsomal GST-II is accordingly the predominant membrane bound enzyme responsible for LTC4 formation in human liver and may subsequently be a fundamental enzyme responsible for detoxification of lipid epoxides. Based on the above data, however, the possibility that LTC4 synthase still exists in the liver in an inactive form cannot be excluded.
LTC4 was selectively synthesized by lung membranes with a minor production of the LTC4 isomer, demonstrating LTC4 synthase as the favored enzyme for LTC4 production in human lung tissue with a minimal contribution from microsomal GST-II. In the lung, microsomal GST-II may play a minor part in LTC4 production, however, it may have another function such as protection from oxidative stress and inhaled xenobiotics. The limited formation of LTC4 from liver cytosol most likely represents the ability of cytosolic GSTs (mostly µ family GSTs) to conjugate reduced GSH to LTA4 (39).
Platelet homogenates produced significant amounts of LTC4 in the absence of notable quantities of the LTC4 isomer, which together with the Western blot data indicate that the LTC4 synthase enzyme predominates for LTC4 production in platelets. In contrast, microsomal GST-II was detected in endothelial cell membranes by Western blot and activity assays and may be the enzyme responsible for converting exogenous LTA4 to LTC4 during transcellular metabolism in these cells. The presence of microsomal GST-II as the main enzyme for LTC4 production in endothelial cells could explain the findings of Habib and Maclouf (40). In these studies, endothelial cells and platelets were both demonstrated to produce LTC4 from exogenously added LTA4, however, platelets were more efficient at this conversion. Endothelial cells displayed a higher apparent Km for LTA4 in comparison to platelets. Similarly, microsomal GST-II has a higher Km for LTA4 than LTC4 synthase and is less stereoselective for the production of LTC4 (1). Analogous to our results, the HPLC chromatogram demonstrating LTC4 production by endothelial cells (40) also contains an extra peak where the LTC4 isomer would be predicted to appear. Such a peak is absent or less pronounced in the chromatogram from platelets. These observations may support the present findings, that LTC4 production is catalyzed by microsomal GST-II in endothelial cells and by LTC4 synthase in platelets.
Overall, microsomal GST-II is an integral membrane bound protein in human liver tissue that is potentially involved in detoxification of biological systems. Importantly, we have developed a method to distinguish microsomal GST-II activity from LTC4 synthase activity. This has led to the discovery that microsomal GST-II is predominantly responsible for LTC4 production in liver membranes and may be a key enzyme in the transcellular metabolism of LTA4 into cysteinyl leukotrienes in certain cells that lack all of the enzymes required for de novo leukotriene biosynthesis.
The authors thank Dr. Don Nicholson for help throughout these studies. We also wish to thank Nathalie Chauret and Dr. Deborah Nicoll-Griffith for preparing the human liver subcellular fractions, Dita Rasper for preparing the human lung membranes, and Joseph Mancini for helpful discussions and for supplying the membranes from Sf9 cells expressing FLAP.