Investigation of intra-domain and inter-domain interactions of glutathione transferase P1-1 by limited chymotryptic cleavage

Limited proteolysis of glutathione transferase P1-1 (GSTP1-1) by chymotrypsin performed at 20 degrees and 30 degrees C mainly generates two complementary peptides of 17 kDa and 6 kDa molecular mass with concomitant loss of catalytic capacity. Sequence analysis of these peptides showed that the peptide bond between Tyr47 and Gly48 was cleaved. The analysis of the recently resolved three-dimensional structure of GSTP1-1 [Reinemer, P., Dirr, H. W., Ladenstein, R., Huber, R., Lo Bello, M., Federici, G. & Parker, M. W. (1992) J. Mol. Biol. 227, 214-226] suggests that the proteolytically cleaved bond results located in a portion of the polypeptide chain lining the G-site which has been demonstrated to be part of an exposed and flexible region of the N-terminal domain (structural elements alpha B1 and alpha B2) [Aceto, A., Caccuri, A. M., Sacchetta, P., Bucciarelli, T., Dragani, B., Rosato, N., Federici, G. & Di Ilio, C. (1992) Biochem. J. 285, 241-245]. The fragments which are generated by proteolysis at 20 degrees C, remain linked by noncovalent interaction in a complex (nicked GSTP1-1) which is dissociated by incubation at higher temperatures. As shown by circular dichroic analysis, although inactive, nicked GSTP1-1 retains an overall secondary structure closely resembling that of the parent enzyme. However, the fluorescence data of the nicked GSTP1-1 indicate that the Trp38, which is near the chymotrypsin-cleavable bond, becomes exposed in a more polar environment. This indicates that, in the nicked enzyme, the polypeptide portion containing the structural elements alpha B1 and alpha B2 has more freedom of fluctuation. The fact that this polypeptide chain portion contains two essential amino acid residues of the G-site (Trp38 and Lys42) might account for the loss of ability to bind glutathione by the nicked enzyme which is consequently catalytically inactive. Proteolysis performed at 30 degrees C generated a homodimeric 17-kDa fragment. The structural analysis of this fragment suggests that the GSTP1-1 alpha C helix, which is located in the domain I and is thought to be involved in the inter-domain interaction, could exert a critical role in maintaining the native folding of domain II.

Affinity labeling of glutathione S-transferase, isozyme 4-4, by 4-(fluorosulfonyl)benzoic acid reveals Tyr115 to be an important determinant of xenobiotic substrate specificity

Incubation of 4-(fluorosulfonyl)benzoic acid (4-FSB), a xenobiotic substrate analogue, with the 4-4 isozyme of rat liver glutathione S-transferase at pH 7.5 and 25 degrees C results in a time-dependent inactivation of the enzyme. The rate of inactivation exhibits a nonlinear dependence on 4-FSB concentration from 0.50 to 7.85 mM, with kmax = 0.082 min-1 and a KI of 1.95 mM. Nearly 1 mol of reagent/mol of enzyme subunit is incorporated when the enzyme is maximally inactivated. Protection against incorporation and inactivation is provided by bromosulfophthalein, a competitive inhibitor with respect to the hydrophobic substrate, 1-chloro-2,4-dinitrobenzene (CDNB), suggesting that the reaction occurs in the binding site of the xenobiotic substrate. Fractionation by high-performance liquid chromatography of a tryptic digest of inactivated enzyme yields a single, modified, 14-residue peptide containing Tyr115 as the altered amino acid. Modified and control enzymes have comparable affinities for glutathione, as indicated by fluorescence titration. In contrast, as distinguished from the control enzyme, modified enzyme does not adsorb to a column of an agarose-linked Cibacron Blue derivative, indicating that it has lost its ability to bind a hydrophobic substrate analogue. These results are supported by kinetic characteristics of modified and control enzymes: upon modification of the enzyme with 4-FSB, the apparent Km for glutathione is unchanged, while the apparent Km for CDNB increases dramatically from 193 to 1690 microM. When the reaction of 4-FSB with enzyme is monitored, the final percent residual activity is found to be dependent on the substrate used in the assay: 11% for CDNB, 20% for ethacrynic acid, 2.5% for trans-stilbene oxide, and 2% for trans-4-phenyl-3-butene-2-one. Analysis of the kinetics of modified enzyme suggests that Tyr115 of glutathione S-transferase, isozyme 4-4, contributes to xenobiotic substrate binding and, when certain types of substrates are employed, is involved in catalysis.

Glutathione S-transferases in the organ of Corti of the rat: enzymatic activity, subunit composition and immunohistochemical localization

Glutathione S-transferases (GSTs), a family of ubiquitous cytosolic isozymes, catalyze the detoxification of electrophilic substrates with reduced glutathione and participate in intracellular binding and transport of lipophilic substances. This study measured GST activity biochemically in the inner ear of the rat; determined the isozyme profile by Western blotting; and identified, immunohistochemically, the distribution of the mu and pi class GSTs in the organ of Corti. GST enzymatic activity in inner ear tissues ranged from 117 to 348 nmoles glutathione converted/min/mg protein, values somewhat higher than those found in brain (130) and much lower than in liver (1011). Of the GST isoforms, the pi class (identified by antibodies against the Yp subunit) was most prominent, the mu class (Yb1 subunit) clearly evident while the alpha class (Y(a) subunit) was barely detectable on Western blots. Immunocytochemical analysis showed differential distribution of the Yb1 and Yp subunits. The Yb1 subunit was present in the sensory cells, while supporting cells were not specifically stained. At the subcellular level, the isozyme was localized in the apical zones of inner (IHCs) and outer hair cells (OHCs) close to the cuticular plate. The extent of staining, however, varied between OHCs and IHCs. In the OHCs, staining appeared in discrete spots in the apical areas only, whereas in IHCs staining extended further towards the center of the cells. The Yp subunit was mainly localized to Deiters cell processes and pillar cells. Both Yb1 and Yp colocalized with tubulin-specific antibody. The functional significance of GST in the cochlear receptor cells is speculative. However, a role analogous to that in other tissues (detoxification, prostaglandin synthesis) can be assumed. In addition, an association of GST with the microtubule system is possible based on immunohistochemical colocalization with tubulin.

Fluorescence characterization of Trp 21 in rat glutathione S-transferase 1-1: microconformational changes induced by S-hexyl glutathione

The glutathione S-transferase (GST) isoenzyme A1-1 from rat contains a single tryptophan, Trp 21, which is expected to lie within alpha-helix 1 based on comparison with the X-ray crystal structures of the pi- and mu-class enzymes. Steady-state and multifrequency phase/modulation fluorescence studies have been performed in order to characterize the fluorescence parameters of this tryptophan and to document ligand-induced conformational changes in this region of the protein. Addition of S-hexyl glutathione to GST isoenzyme A1-1 causes an increase in the steady-state fluorescence intensity, whereas addition of the substrate glutathione has no effect. Frequency-domain excited-state lifetime measurements indicate that Trp 21 exhibits three exponential decays in substrate-free GST. In the presence of S-hexyl glutathione, the data are also best described by the sum of three exponential decays, but the recovered lifetime values change. For the substrate-free protein, the short lifetime component contributes 9-16% of the total intensity at four wavelengths spanning the emission. The fractional intensity of this lifetime component is decreased to less than 3% in the presence of S-hexyl glutathione. Steady-state quenching experiments indicate that Trp 21 is insensitive to quenching by iodide, but it is readily quenched by acrylamide. Acrylamide-quenching experiments at several emission wavelengths indicate that the long-wavelength components become quenched more easily in the presence of S-hexyl glutathione. Differential fluorescence polarization measurements also have been performed, and the data describe the sum of two anisotropy decay rates. The recovered rotational correlation times for this model are 26 ns and 0.81 ns, which can be attributed to global motion of the protein dimer, and fast local motion of the tryptophan side chain. These results demonstrate that regions of GST that are not in direct contact with bound substrates are mobile and undergo microconformational rearrangement when the "H-site" is occupied.

Site-directed mutagenesis of glutathione S-transferase YaYa. Mapping the glutathione-binding site

Previous studies from our laboratory have shown that aspartic acid 101 plays an important role in glutathione interaction to rat glutathione S-transferase YaYa, while tyrosine 9 is directly involved in catalysis. Based on the available structural information, site-directed mutagenesis was conducted to examine the function of arginine, lysine, glutamine, and proline residues surrounding the GSH binding pocket. Arginine mutants R13K, R15K, R20K, and R20I retained partial enzymatic activities, while R13I and R15I lost most of their activities. Kinetic studies showed a marked increase in Km toward GSH for R15I suggesting that arginine 15 contributes significantly to the binding of GSH in the active site of glutathione S-transferase YaYa. A drastic decrease in enzymatic activities for R13I suggested the importance of the charged group of arginine 13 either in maintaining the structural integrity of the enzyme or in serving a vital role in enzymatic function. Replacement of glutamine 54 and 67 with glutamic acid or asparagine resulted in decreased enzymatic activities. Moreover, an 11-, 17-, and 9-fold increase in Km values toward GSH for mutant Q54E, Q54N, and Q67N was observed, respectively. These results suggested that glutamine 54 and 67 also contributed significantly to the binding of GSH. Proline at position 56 appears to be important for maintaining the structural integrity of the enzyme since mutants P56A and P56F were much less active and extremely less stable than that of the wild type enzyme. Both lysine mutants, K45R and K45I, exhibited substantially higher catalytic efficiencies toward both 1-chloro-2,4-dinitrobenzene and GSH than the wild type enzyme. Our data clearly show that lysine 45 is not an essential residue for catalysis nor for GSH binding in glutathione S-transferase YaYa.

Human Mu-class glutathione S-transferases present in liver, skeletal muscle and testicular tissue

The major human Mu-class glutathione S-transferases (GST) have been purified to allow comparisons of their catalytic, physicochemical and immunochemical properties. GST isoenzymes, purified from hepatic, testicular and skeletal muscle tissue were found to comprise three distinct subunits (M1, M2 and M3) which may combine to form both homodimeric and heterodimeric proteins. Two distinct subunits, M1a and M1b, which represent allelic charge variants have been isolated but no polymorphic forms encoded at the GST M2 and M3 loci have been observed. Three GST isoenzymes (M1a-1a, M1a-1b and M1b-1b) have been purified from a single liver specimen. In addition, GST M1a-2, M1b-2, M2-2 and M2-3 have been isolated from muscle, whilst the M3-3 homodimer has been purified from human testis. The homodimeric enzymes GST M1a-1a, M1b-1b, M2-2 and M3-3 have pI values of 6.1, 5.5, 5.3 and 5.0, whilst SDS-PAGE indicated that M1a, M1b, M2 and M3 have molecular masses of 26.7, 26.6, 26.0 and 26.3 kDa, respectively. The M1, M2 and M3 subunits isolated from either liver, skeletal muscle or testis, are catalytically distinct. Both M1-type subunits (M1a and M1b) possess a high activity for trans-4-phenyl-3-buten-2-one, whereas, the skeletal muscle subunit M2 has a high activity towards 1,2-dichloro-4-nitrobenzene. By contrast, the testicular GST subunit M3 has no detectable activity towards either of these substrates. However, all three Mu-class subunits are active towards the compounds 4-hydroxynonenal and 4-hydroxydecinal, possible endogenous substrates which are produced by lipid peroxidation. The human Mu-class subunits can be distinguished immunochemically; antisera raised against the testicular GST M3-3 showed no reactivity towards either the M1 or M2 subunits. The M3 subunit has a blocked N-terminus but automated amino-acid sequencing of a CNBr-derived peptide allowed 14 residues of the M3 subunit to be identified. These data indicated that testicular GST M3-3 is likely to correspond to the brain/testis Mu-class GST cDNA described by Campbell et al. (Campbell E., Takahashi Y., Abramovitz M., Peretz M., & Listowsky I. (1990) J. Biol. Chem. 265, 9188-9193).

Cooperative self-assembly of SH2 domain fragments restores phosphopeptide binding

Multifunctional proteins frequently can be subdivided into discrete functional domains. Selected cytoplasmic proteins involved in signal transduction contain catalytic domains in addition to protein binding modules termed Src homology (SH) domains; SH2 domains bind phosphotyrosyl peptide sequences. Even as isolated modules, SH2 domains have the intrinsic capacity to fold properly and retain sequence selectivity for binding. Following limited digestion with trypsin, the 14-kDa SH2 domains of Src and PI 3-kinase p85 were split at a lysine within the flexible, phosphotyrosine-binding (BC) loop into 5- and 9-kDa fragments. Whereas the purified fragments did not exhibit cooperative unfolding or phosphopeptide binding, when combined they spontaneously reassembled to restore specific phosphopeptide binding and the unique spectroscopic signatures of bound and free intact SH2 domains. Like fragments of intact proteins, we now show that fragments of SH2 domains, and therefore protein modules, possess the intrinsic capacity for self-assembly with restoration of function. Analyses of fragment structures may provide insights into pathways of module folding, which will facilitate a more global understanding of how complex, multifunctional proteins fold.

Expression of glutathione S-transferase P-form in primary cultured rat liver parenchymal cells by coplanar polychlorinated biphenyl congeners is suppressed by protein kinase inhibitors and dexamethasone

Glutathione S-transferase P-form (GST-P, EC 2.5.1.18) mRNA was expressed by epidermal growth factor as well as by 3,4,5,3',4'-penta-chlorinated biphenyl (PenCB) in primary cultured rat liver parenchymal cells. The expression of GST-P was suppressed by inhibitors of protein kinase C and dexamethasone, an antagonist of AP-1 transcription factor activity, whereas expression of cytochrome P450IA2 by PenCB was not affected by these reagents. The AP-1 related transcription factor may be essential for the expression of GST-P by PenCB as also may be a protein kinase C type enzyme.

Differential induction of glutathione S-transferase subunits by phenobarbital, 3-methylcholanthrene and ethoxyquin in rat liver and kidney

The inducibility of Glutathione S-transferase (GST) in male Sprague-Dawley rats, treated with phenobarbital (PB), 3-methyl-cholanthrene (MC) and ethoxyquin (ETQ), was examined in detail. The subunit compositions of hepatic and renal GST were determined by using a reverse-phase HPLC technique. In liver, PB was found to induce the Yb1, Yb2, Ya1, Ya2 and Yk subunits by about 2.1-, 1.8-, 1.8-, 4.4- and 2-fold, respectively, while MC induced the Yb2, Yc, Ya2 and Yk subunits by about 1.5-, 1.5-, 6- and 1.7-fold, respectively, and ETQ increased the levels of Yb1, Yb2, Yc, Ya2 and Yk subunits by about 2.1-, 1.7-, 1.9-, 14.9- and 1.8-fold, respectively. In contrast, kidney cytosolic GSTs were induced only by treatment with ETQ and PB and MC had little or no effect. The Pi class subunit Yp in the rat kidney was increased about 4-fold and the Mu class Yb2 was induced by about 2-fold, by the ETQ treatment.

The catalytic mechanism of glutathione S-transferase (GST). Spectroscopic determination of the pKa of Tyr-9 in rat alpha 1-1 GST

The rat alpha 1-1 glutathione S-transferase (GST) contains a single, non-essential tryptophan and only 8 tyrosines in each subunit. One of these tyrosines, Tyr-9, hydrogen bonds to the substrate glutathione and stabilizes the nucleophilic thiolate anion. Two mutant proteins that allow for the spectrocopic determination of the pKa of this catalytic residue have been constructed. The W21F mutant provides a fully active GST with no tryptophans, and the double mutant W21F/Y9F lacks both tryptophan and the active site tyrosine. The intrinsic fluorescence and absorbance properties of these mutants are dominated by tyrosine. Fluorescence emission, fluorescence excitation, and absorbance spectral changes of samples containing the W21F mutant at several pH values in the range 6.8-9.0 reveal a pH-dependent increase in the contribution of tyrosinate. No spectral changes are observed with the W21F/Y9F protein in this pH range. At pH 12.5, both proteins exhibit complete deprotonation of all tyrosines. The pKa of Tyr-9 determined from these spectroscopic changes is 8.3-8.5. The changes in absorbance at 250 and 295 nm correspond to titration of 0.95 +/- 0.29 tyrosines/subunit in the W21F protein between pH 6.9 and 9.3. Moreover, addition of the inhibitor S-hexylglutathione results in an apparent increase in the pKa of Tyr-9. Together, these results indicate that the catalytically active Tyr of GSTs has a pKa value that is 1.8-2.0 pKa units below tyrosine in solution. It is likely that this decrease in the pKa of Tyr-9 contributes to catalysis by altering the equilibrium position of the proton shared between Tyr-9 and GSH, and this active site residue may function as a general base catalyst in addition to a hydrogen bond donor.

Peculiar spectroscopic and kinetic properties of Cys-47 in human placental glutathione transferase. Evidence for an atypical thiolate ion pair near the active site

Cys-47, the most reactive cysteine in the homodimeric glutathione transferase (EC 2.5.1.18) from human placenta (class Pi), displays peculiar acid base and spectroscopic properties. The thiolate form of this residue is characterized by a sharp UV absorption spectrum centered at 229 nm with an epsilon = 7,500 M-1 cm-1. The dependence of the apparent extinction coefficient on pH indicates that the sulfhydryl group of Cys-47 has a pKa value of 4.2. Moreover the dependence of the reactivity of Cys-47 toward bromopyruvate and iodoacetamide with pH resembles that found for the functional sulfhydryls of thiol proteases, which have very low pKa values and exist mainly as a mercaptide-imidazole ion pair. The apparent pKa value for Cys-47, calculated by this kinetic approach, is in good agreement with that determined spectroscopically. X-ray crystallographic data indicate that the protonated amino group of Lys-54, 4.9 A from the sulfur atom, is probably involved in the deprotonation of Cys-47. Calculation of the electrostatic potential on the sulfur atom of Cys-47 gives a theoretical pKa value of 3.5 for the sulfhydryl group. The simulated neutralization of Lys-54 shifts the pKa value of Cys-47 to a normal value of 9.5. These findings suggest that at physiological pH values, Cys-47 exists as the thiolate ion stabilized by an ion pair formation with the protonated amino group of Lys-54, and this probably accounts for its high reactivity.

Molecular cloning and heterologous expression of an alternatively spliced human Mu class glutathione S-transferase transcript

Two cDNA clones encoding a new Mu class glutathione S-transferase (GST) have been isolated from a human testis cDNA library. Both clones are incomplete and appear to result from alternative splicing. One clone is missing the sequence encoding exon 4 and the other is missing exon 8. The complete sequence of the previously undescribed isoenzyme can be deduced from the two cDNA clones. This is the first report of alternative splicing in a GST transcript and may represent either a novel form of regulation in this multigene family or illegitimate transcription and experimental alternative splicing as part of the evolutionary process. By combining components from each clone a complete cDNA has been constructed and the encoded protein expressed in Escherichia coli. In general, the recombinant enzyme has relatively low activity when compared with all the previously described human Mu class GST isoenzymes.

Identification of ionizable groups essential for the enzyme catalysis on glutathione S-transferase P

The pH-Vmax/KmGSH plot of glutathione S-transferase P (GST-P) showed a bell-shaped profile, indicating bifunctional catalysis for glutathione (GSH) conjugation. The ionization constant (Ke) and the heat of ionization (delta He) of the essential ionizable group in the GSH binding site were measured and the value of pKe1 was 5.9 and that of pKe2, 8.4, while the delta He1 and delta He2 were -0.2 and 7.9 kcal/mole, respectively. In a solvent containing 25% ethanol, pKe1 and pKe2 shifted to the alkaline side by 0.47 and 0.2, respectively. These kinetic results indicated that carboxyl and phenolic groups were ionizable groups essential for the GSH conjugation. Chemical modifications using aminomethane sulfonic acid and N-acetylimidazole supported the results of the kinetic studies.

Inactivation of mouse liver glutathione S-transferase YfYf (Pi class) by ethacrynic acid and 5,5'-dithiobis-(2-nitrobenzoic acid)

Mouse liver glutathione S-transferase YfYf (Pi class) reacts with [14C]ethacrynic acid to form a covalent adduct with a stoichiometry of 1 mol per mol of subunit. Proteolytic digestion of the enzyme-[14C]ethacrynic acid adduct with V8 protease produced an 11 kDa fragment containing radioactivity. Sequencing revealed this to be an N-terminal peptide (minus the first 15 residues, terminating at Glu-112) which contains only one cysteine residue (Cys-47). This is tentatively identified as the site of ethacrynic attachment. Kinetic studies reveal that glutathione S-conjugates protect against inactivation by ethacrynic acid, but the level of protection is not consistent with their potency as product inhibitors. A model is proposed in which glutathione S-conjugates and ethacrynic acid compete for the free enzyme, and a second molecule of ethacrynic acid reacts covalently with the enzyme-ethacrynic acid complex. The native protein contains one thiol reactive with 5,5'-dithiobis-(2-nitrobenzoic acid) at neutral pH. The resultant mixed disulphide, like the ethacrynic acid adduct, is inactive, but treatment with cyanide (which incorporates on a mol for mol basis) restores activity to 35% of that of the native enzyme.

Characterization of glutathione transferase from Xanthomonas campestris

A single form of glutathione transferase (Xc-GST-4.5) having an isoelectric point at pH 4.5 was resolved from Xanthomonas campestris cytosol by affinity chromatography and isoelectric focusing. HPLC,N-terminal amino acid sequence, and SDS-PAGE analyses indicate that Xc-GST-4.5 is composed of two identical subunits, each with a molecular mass of 22 kDa. As indicated by its substrate specificity, immunological reactivity, and CD spectra, as well as by its N-terminal amino acid sequence, Xc-GST-4.5 appears to be distinct from the other bacterial glutathione transferases, Pm-GST-6.0 and Sm-GST-7.3, previously purified from the cytosolic fraction of Proteus mirabilis and Serratia marcescens. Xc-GST-4.5 also appears to be distinct from the GST so far purified from other sources.

Electrostatic evidence for the activation of the glutathione thiol by Tyr7 in pi-class glutathione transferases

A number of spectrophotometric studies [Graminski, G.F., Kubo, Y. & Armstrong, R.N. (1989) Biochemistry 28, 3562-3568; Liu, S., Zhang, P., Ji, X., Johnson, W.W., Gilliland, G.L. & Armstrong, R.N. (1992) J. Biol. Chem. 267, 4296-4299] have recently shown that the glutathione (GSH) thiol is deprotonated when it is in complex with glutathione S-transferase. Different models have been proposed for the activation of the glutathione S gamma, all pointing out the key role of active-site residue Tyr7. It remains unclear, however, how Tyr7 is actually involved in this process. In this paper we present an analysis of the electrostatic potential in the region of the active site of a pi-class GSH transferase. This analysis provides evidence that the titration behaviour of the absorption band of the E.GSH complex with a pK between 6 and 7 [Liu, S., Zhang, P., Ji, X., Johnson, W.W., Gilliland, G.L. & Armstrong, R.N. (1992) J. Biol. Chem. 267, 4296-4299] should rather be explained by the protonation/deprotonation equilibrium of Tyr7 than by the protonation/deprotonation equilibrium of the GSH thiol group itself. On the basis of this conclusion, a mechanism for activation of GSH is proposed: the Tyr7 OH group is deprotonated by the influence of the protein charge constellation and the peptide dipoles. Thus it acts as a general base, promotes proton abstraction from the GSH thiol and creates a thiolate anion with high nucleophilic reactivity.

Multiphasic denaturation of glutathione transferase B1-1 by guanidinium chloride. Role of the dimeric structure on the flexibility of the active site

The unfolding and refolding mechanisms of dimeric glutathione transferase GSTB1-1 from Proteus mirabilis, using guanidinium chloride as a denaturant, have been investigated. The protein transitions were monitored by enzyme activity, intrinsic fluorescence, far ultraviolet circular dichroism and gel-filtration chromatography. The non coincidence of denaturation curves at equilibrium indicates that the unfolding of GSTB1-1 is a multistep process, i. e. inactivation of the structured dimer, dissociation into partially structured monomers followed by complete unfolding. In the 50% inactivated enzyme the Km for glutathione increases threefold, while the kcat appears almost the same, indicating that the initial phase of the denaturation involves the binding site of glutathione. The rapid recovery of the folded dimer precedes the complete enzyme reactivation. This indicates that the reconstitution of the native structure of GSTB1-1 is the result of folding and association of compact monomers followed by subtle rearrangements of assembled monomers that build up the active site.

Conformational states of human placental glutathione transferase as probed by limited proteolysis

Limited proteolysis experiments have been carried out on human placental glutathione transferase in its different forms. The reduced enzyme, as well as the oxidized form and that inactivated with cystamine were all sensitive to 10% (w/w) trypsin, under nondenaturing conditions. The proteolytic cleavage was accompanied by a concomitant loss of enzymatic activity. On the contrary, the presence of glutathione or glutathione conjugates strongly protected the reduced enzyme against inactivation and from the proteolytic attack. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis and peptide sequence analysis showed that only the peptide bond between Lys 44 and Ala 45 was cleaved. Since Lys 44 has been demonstrated to be involved in the glutathione binding, it is suggested that the region surrounding this amino acid residue (alpha B helix) could be more exposed to the solvent, in the absence of glutathione. Crystallographic data also indicated that this region is flexible, supporting the idea that it may be involved in the observed conformational change upon glutathione binding.

Chemical modification of GSH transferase P1-1 confirms the presence of Arg-13, Lys-44 and one carboxylate group in the GSH-binding domain of the active site

GSH transferase P1-1 (GSTP1-1) was modified with group-specific reagents. Kinetic experiments demonstrated that inactivation of GSTP1-1 occurred upon reaction of one arginine residue per subunit with diacetyl, one lysine residue per subunit with 2,4,6-trinitrobenzene sulphonate, or one carboxylate group per subunit with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. All three inactivation reactions were inhibited by compounds known to bind at the GSH site of the enzyme but were unaffected by the electrophile 1-chloro-2,4-dinitrobenzene. N-terminal sequence analysis showed that Arg-13 was modified by diacetyl and that this modification was inhibited by GSH. Arg-11 was not modified. The lysine residue modified by 2,4,6-trinitrobenzene sulphonate and protected by S-octylglutathione was identified as Lys-44 by sequencing of tryptic peptides. The findings are in agreement with the involvement of Arg-13 and Lys-44 in binding of GSH, as determined from the crystal structure [Reinemer, Dirr, Ladenstein, Huber, Lo Bello, Frederici and Parker (1992) J. Mol. Biol. 227, 214-226]. The present data also implicate a single carboxylate in GSH binding, consistent with the involvement of Asp-98 of subunit B determined from the crystallographic study. The GSH-binding determinants of GSTP1-1 are compared using sequence similarity with those of GSTs of Alpha, Mu and Theta classes.

Purification and characterization of a glutathione S-transferase from benoxacor-treated maize (Zea mays)

A glutathione S-transferase (GST) isozyme from maize (Zea mays Pioneer hybrid 3906) treated with the dichloroacetamide herbicide safener benoxacor (CGA-154281) was purified to homogeneity and partially characterized. The enzyme, assayed with metolachlor as a substrate, was purified approximately 200-fold by ammonium sulfate precipitation, anion-exchange chromatography on Mono Q resins, and affinity chromatography on S-hexylglutathione agarose from total GST activity present in etiolated shoots. The purified protein migrated during sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) as a single band with a molecular mass of 27 kD. Using nondenaturing PAGE, we determined that the native protein has a molecular mass of about 57 kD and that the protein exists as a dimer. Two-dimensional electrophoresis revealed only a single protein with an isoelectric point of 5.75 and molecular mass of 27 kD. These results further suggest that the protein exists as a homodimer of two identical 27-kD subunits. The enzyme was most active with substrates possessing a chloroacetamide structure. trans-Cinnamic acid and 1-chloro-2,4-dinitrobenzene were not effective substrates. Apparent Km values for the enzyme were 10.8 microM for the chloroacetamide metolachlor and 292 microM for glutathione. The enzyme was active from pH 6 to 9, with a pH optimum between 7.5 and 8. An apparently blocked amino terminus of the intact protein prevented direct amino acid sequencing. The enzyme was digested with trypsin, and the amino acid sequences of several peptide fragments were obtained. The sequence information for the isolated GST we have designated "GST IV" indicates that the enzyme is a unique maize GST but shares some homology with maize GSTs I and III.

Identification and characterization of two enzymes involved in the intracellular metabolism of cobalamin. Cyanocobalamin beta-ligand transferase and microsomal cob(III)alamin reductase

Two enzymes involved in the intracellular metabolism of cobalamin have been identified and characterized: cyanocobalamin beta-ligand transferase and microsomal cob(III)alamin reductase. The beta-ligand transferase is a cytosolic enzyme utilizing FAD, NADPH and reduced glutathione. The product of the reaction has been identified as glutathionyl-cobalamin. NADH-linked cob(III)alamin reductase has been found in two subcellular fractions: microsomal and inner mitochondrial membrane. The product of the reduction catalyzed by the microsomal enzyme has been identified as cob(II)alamin. In cbl C mutant fibroblasts, the specific activities of cyanocobalamin beta-ligand transferase and cob(III)alamin reductase were markedly decreased and have varied from 3%-30% and 36%-42% of normal, respectively. The specific activity of mitochondrial cob(III)alamin reductase was only 30% of normal in two cbl C mutants and normal in remaining mutant cell lines. In the cbl D cells, the specific activities were 33% and 55%. Mitochondrial cob(III)alamin reductase was not affected by cbl D mutation. Methionine synthase, L-methylmalonyl-CoA mutase and microsomal cytochrome c and b5 reductases are not affected by both mutations. The cbl E mutation affects only the activity of methionine synthase. These results support the hypothesis that the early enzymatic steps of intracellular metabolism of cobalamin are similar in the synthesis of both methylcobalamin and adenosylcobalamin and these steps are altered by the cbl C and cbl D mutations.

1H and 15N assignments and secondary structure of the Src SH3 domain

The 1H and 15N sequential assignments of the Src SH3 domain have been determined through a combination of 2D and 3D Nuclear Magnetic Resonance (NMR) methods. The secondary structure of the protein has been identified based on long-range NOE patterns. The SH3 domain of Src consists largely of six beta-strands that form two anti-parallel beta-sheets.

Tyrosine 115 participates both in chemical and physical steps of the catalytic mechanism of a glutathione S-transferase

The participation of the hydroxyl group of tyrosine 115 in the catalytic mechanism of isoenzyme 3-3 of rat glutathione (GSH) S-transferase is implicated by x-ray crystallographic analysis of a product complex and confirmed by comparison of the catalytic properties of the native enzyme and the Y115F mutant. Tyrosine 115 is located in domain II of the protein (the xenobiotic substrate binding domain) and is the first residue in this domain to be shown to play a direct role in catalysis. The 1.8-A structure of isoenzyme 3-3 in complex with (9S,10S)-9-(S-glutathionyl)-10-hydroxy-9,10-dihydrophenanthrene, one of the diastereomeric products of the reaction of GSH with phenanthrene 9,10-oxide, indicates that the hydroxyl group of Tyr115 is within hydrogen-bonding distance of the 10-hydroxyl group of the bound product and, by implication, is proximal to the oxirane oxygen of the substrate in the Michaelis complex. Site-specific replacement of Tyr115 with phenylalanine has profoundly different effects on catalysis depending on the type of reaction and whether the rate-limiting step in catalysis is a chemical step or a physical step. Stopped flow measurements of the rate constants for product release and viscosity effects on the steady-state kinetics establish that the rate-limiting step in catalysis with phenanthrene 9,10-oxide (kcat = 0.4 s-1) is probably a chemical one, whereas the physical step of product dissociation (koff) is rate-limiting in the reaction of 1-chloro-2,4-dinitrobenzene (kcat = 20 s-1). The Y115F mutant is severely impaired in catalyzing the addition of GSH to phenanthrene 9,10-oxide (kcat = 0.0044 s-1), evidence that the -OH of Tyr115 provides electrophilic assistance in the epoxide ring opening. In contrast, the Y115F mutant is a better catalyst toward 1-chloro-2,4-dinitrobenzene (kcat = 72 s-1) than is the native enzyme. The enhanced rates of product release in the mutant are ascribed to the loss of hydrogen bonds between the -OH of Tyr115 and the side chain -OH and main chain NH of serine 209, interactions that block the channel to the active site or inhibit the segmental motion of the protein.

Anionic glutathione S-transferases in shrimp eyes

1. Two anionic isoenzymes of glutathione transferases (EC 2.5.1.18), QI and QII, have been purified from the eyes of the shrimp Penaeus japonicus by using a combination of S-hexylglutathione affinity column chromatography and Mono-Q fast protein liquid chromatography (f.p.l.c.). 2. Both QI and II glutathione S-transferases are homodimers. They show similarity in substrate specificities and pH optima, but not in isoelectric points. 3. QI is distinct from QII by anion-exchange f.p.l.c., reverse-phase h.p.l.c. chromatography and amino acid sequencing analysis. QI has N-terminal amino acid sequences homologous to mu glutathione S-transferase, whereas QII is homologous to theta glutathione S-transferases.

Constitutive and inducible profile of glutathione S-transferase subunits in biliary epithelial cells and hepatocytes isolated from rat liver

The constitutive and inducible cytosolic glutathione S-transferase (EC 2.5.1.18) subunit compositions of parenchymal cells (hepatocytes) and biliary epithelial cells (BEC) from rat liver have been quantitatively analysed using reverse-phase h.p.l.c. Hepatocytes, analysed in the absence of non-parenchymal cells, expressed constitutively the following subunits, in order of their concentration: 3, 4, 2, 1a, 1b, 8, 6 and 10. BEC express constitutively only four of the GST subunits expressed by hepatocytes and these are, in order of their concentration: subunits 2, 7, 4 and 3. Notable differences from hepatocytes are that BEC completely lack the Alpha-class subunits 1a and 1b that are major subunits in hepatocytes, Mu-class subunits make up a very low proportion of the total, and the Pi-class subunit 7 is a major subunit in BEC, whereas it is essentially absent from hepatocytes. For the first time, the effects of the inducing agents phenobarbitone (PB), beta-naphthoflavone (beta-NF) and ethoxyquin (EQ) have been characterized in a comprehensive and quantitative manner in both cell types. PB, beta-NF and EQ increased total GST protein in hepatocytes by approx. 2-fold, 3-fold and 4-fold respectively. Subunits significantly induced in hepatocytes were (in order of fold-induction): by PB, 1b > 8 > 3 > 2 > 4; by beta-NF, 1b > 8 > 2 > 3 > 4; and by EQ, 7 > 1b > 10 > 8 > 3 > 2 > 1a > 4. In BEC, neither PB nor beta-NF had significant effects on the total amount of GST protein, although PB did significantly induce subunit 3 at the expense of other subunits. EQ increased total GST protein nearly 5-fold in BEC, subunits 7 and 3 being induced dramatically above constitutive levels.

Glutathione S-transferase mu polymorphism does not explain variation in nitroglycerin responsiveness

OBJECTIVE

To determine whether the considerable interindividual variability in nitroglycerin-induced venodilation in humans is related to the polymorphic expression of the mu class of glutathione S-transferase (GST mu). Recently vascular glutathione S-transferase (EC 2.5.1.18) of the mu-class (GST mu), a polymorphic group of enzymes present in only about 60% of the population, have been identified and shown in vitro to possess high metabolic activity toward nitroglycerin. Their clinical relevance is unknown.

DESIGN

Dose-response relationships to nitroglycerin were constructed in vivo measuring changes in compliance of dorsal hand veins in 26 healthy volunteers during local infusion of small amounts of nitroglycerin. Polymerase chain reaction was applied to detect the deoxyribonucleic acid sequence that codes GST mu in whole blood samples.

RESULTS

The GST mu isozyme was present in 15 subjects (58%) and deficient in 11 subjects. Values for mean maximum venodilation (Emax) and dose rates producing 50% of Emax (ED50) were not significantly different between the groups with or without GST mu. The respective values were 98% and 103% dilation for Emax and 9 and 16 ng/min for ED50. There was no gender difference in the venodilatory response to nitroglycerin.

CONCLUSIONS

Subjects lacking GST mu can clearly respond normally to nitroglycerin, and the large interindividual variability in nitroglycerin potency is not related to the expression of this polymorphic enzyme. Intersubject variability is therefore more likely to be the result of differences in the presence or activity of other vascular enzymes or in steps further distal in the venodilatory cascade.

Identification of the hydrophobic ligand-binding region in recombinant glutathione S-transferase P and its binding effect on the conformational state of the enzyme

Recombinant glutathione S-transferase P (GST-P) was purified in a homogeneous state. Fatty acid analysis of the enzyme revealed that the final enzyme preparation endogenously bound fatty acids, mostly palmitic acid or stearic acid, which were difficult to dissociate from the complex. Temperature-dependent analysis by 1H NMR indicated that the molecular motion of fatty acids was strongly restrained under physiological conditions, which was significantly different from that of serum albumin. On the other hand, there existed another hydrophobic ligand-binding region in GST-P, to which 1-amino-8-naphthalenesulfonic acid and bilirubin would bind with relatively lower affinity than the endogenously bound fatty acid. The hydrophobic ligand-binding region was determined to be around 141-156 residues from the N-terminus by procedures including association of the enzyme to fatty acid-linked Sepharose and affinity labeling with fluorescent fatty acid. Furthermore, circular dichroism analysis showed that the binding of hydrophobic ligand to GST-P produced a remarkable conformational change of the enzyme, which led to states devoid of transferase activity. In addition, the hydrophobic ligand binding caused a significant fluorescence quenching of tryptophan 38, which was assumed to be located at the active center of GST-P. It could be the result of a conformational change of the active center of the enzyme.

Development of type-specific and cross-reactive serological probes for the minor capsid protein of human papillomavirus type 33

Human papillomavirus type 33 (HPV33) is associated with malignant tumors of the cervix. In an attempt to develop immunological probes for HPV33 infections, antisera against various bacterial fusion proteins carrying sequences of the minor capsid protein encoded by L2 were raised in animals. Antigenic determinants on the HPV33 L2 protein were identified by using truncated fusion proteins and were classified as type specific or cross-reactive with respect to HPV1, -8, -11, -16, and -18. Cross-reactive epitopes map to amino acids 98 to 107 or to amino acids 102 to 112 and 107 to 117, respectively, depending on the fusion protein used for immunization. Antibodies directed toward these epitopes detect L2 proteins of HPV11, -16, and -18, but not of HPV1 and -8, in Western immunoblots and enzyme-linked immunosorbent assays. HPV33 L2 amino acids 82 to 94 and 117 to 130 induce type-specific antibodies, with the major response directed to amino acids 117 to 130. By using a synthetic peptide corresponding to L2 amino acids 117 to 130, high-titered, type-specific antisera were obtained. These antisera should be useful as immunological probes for HPV33 infection.

Deduced amino acid sequence, gene structure and chromosomal location of a novel human class Mu glutathione S-transferase, GSTM4

The Mu-Class glutathione S-transferases (GSTs) are subject to marked inter-individual variation in man, owing to the fact that 40-50% of the population fail to express M1 subunits. Mu-Class GST from two lymphoblastoid cell lines (expressing M1 subunits and the other 'nulled' for M1) have been studied. Both cell lines were found to express a Mu-Class GST that has not been described previously. The cDNA encoding this novel transferase, designated 'GSTM4' has been isolated and the enzyme shown to be comprised of 218 amino acids (including the initiator methionine residue) with an M(r) of approx. 25.5 kDa. Molecular cloning demonstrated that the lymphoblastoid cell line which expressed GSTM1 possessed the b allelic variant (i.e. that with an asparagine residue at position 173). The genes for GSTM4 and GSTM1b have been cloned and found to contain seven introns and eight exons. The coding region of the GSTM4 gene, including the seven introns, encompasses 5.0 kb, whereas the same region of GSTM1b is 5.5 kb; the difference in the size of the two genes is due to the length of intron 7. DNA sequencing allowed a GSTM4-gene-specific oligo-primer to be designed which has been utilized in a PCR-based assay to determine that the GSTM4 gene is located on chromosome 1.

Inhibition of glutathione S-transferases from rat liver by basic triphenylmethane dyes

(1) Basic triphenylmethane dyes related to pararosanilin inhibit class alpha glutathione S-transferases (GSTs) from rat liver. The inhibitory potency of each dye correlates with its octanol-water partition coefficient. Values of Ki determined at pH 6.5 ranged from about 1 x 10(-7) M for Ethyl violet to 7 x 10(-5) M for Methyl green. GST 3-3, a class mu isoenzyme, was an order of magnitude less sensitive to inhibition by Ethyl violet. (2) All of the dyes tested were bleached to varying degrees by glutathione. The bleaching appears to result from the formation of an adduct between the dye and glutathione. At pH 6.5, adduct formation is significant only for Malachite green and Methyl green. There is kinetic evidence that for these dyes the adduct contributes significantly to the overall inhibition. It is probable that at physiological pH, all of the dyes would exist to a significant extent in the adduct form. (3) The dyes are excreted extensively in the bile, at least partly as the glutathione adduct. The free dye is regenerated on standing, it is assumed as a result of removal of glutathione by oxidation.

Sequence analysis of a Fasciola hepatica glutathione S-transferase cDNA clone

A lambda gt11 Fasciola hepatica cDNA library was previously constructed from poly(A)+ RNA extracted from adult worms. A cDNA encoding F. hepatica glutathione S-transferase (FhGST) (GenBank Accession Number M93434) was cloned by screening the library with a rabbit anti-FhGST antiserum. The FhGSTs are suspected to be early developmentally expressed-proteins that act as potent immunogens and may be useful as candidate vaccines against the parasite. The cDNA was sequenced and contains 627 translated bases encoding a 24,211-Dalton polypeptide with 209 amino acids and an isoelectric point (pI) of 5.324. This polypeptide has significant homology with 26-kD GSTs from Schistosoma mansoni (57.42%) and S. japonicum (57.14%). In addition, we have identified the predicted T cell epitopes in the FhGST protein molecule.

Purification to homogeneity and the N-terminal sequence of human leukotriene C4 synthase: a homodimeric glutathione S-transferase composed of 18-kDa subunits

Human leukotriene C4 (LTC4) synthase was purified > 25,000-fold to homogeneity from the monocytic leukemia cell line THP-1. Beginning with taurocholate-solubilized microsomal membranes, LTC4 synthase was chromatographically resolved by (i) anion exchange, (ii) affinity chromatography (through a resin of biotinylated LTC2 immobilized on streptavidin-agarose), and then (iii) gel filtration. The final preparation contained only an 18-kDa polypeptide. The molecular mass of the pure polypeptide was consistent with an 18-kDa polypeptide from THP-1 cell membranes that was specifically photolabeled by an LTC4 photoaffinity probe, 125I-labeled azido-LTC4. On calibrated gel-filtration columns, purified LTC4 synthase activity eluted at a volume corresponding to 39.2 +/- 3.3 kDa (n = 12). The sequence of the N-terminal 35 amino acids was determined and found to be a unique sequence composed predominantly of hydrophobic amino acids and containing a consensus sequence for protein kinase C phosphorylation. We therefore conclude that human LTC4 synthase is a glutathione S-transferase composed of an 18-kDa polypeptide that is enzymatically active as a homodimer and may be phosphoregulated in vivo.

Squid glutathione S-transferase. Relationships with other glutathione S-transferases and S-crystallins of cephalopods

Glutathione S-transferase (GST, EC 2.5.1.18) was purified from the digestive gland of the squid Ommastrephes sloani pacificus. It had high enzymatic activity for the 1-chloro-2,4-dinitrobenzene substrate and was composed of a major and a minor polypeptide band, both with molecular masses near 25 kDa on SDS-polyacrylamide gels. GST cDNA clones were derived from the digestive gland mRNA. The deduced GSTs of the longest cDNAs (pGST5 and pGST11) containing the entire coding sequence have a molecular mass near 23 kDa. Sequence comparisons showed that the squid GST is 42-44% identical to both squid and octopus S-crystallins (the major proteins of the lens), 32-34% identical to class pi and 29-32% identical to class alpha GSTs of vertebrates, and 19-23% identical to other GSTs of vertebrates and insects. Northern blot hybridization revealed that GST mRNAs were much more abundant in the digestive gland than in the testis, mantle, or lens. Analysis of a squid GST gene indicated that it has an exon-intron structure similar to that of the vertebrate class pi GST gene. An apparently novel repetitive element was identified in the 5'-flanking sequence of the squid GST gene. Our results suggest that multiple duplications of an ancestral GST gene gave rise to a family of enzymatically inactive crystallins specialized for lens refraction and one (or two) active GST enzyme expressed preferentially, but not exclusively, in the digestive gland in squids. This differs from the innovation of refractive function from a metabolic enzyme by increased expression in the lens with minimal or no gene duplication, as occurred among the enzyme-crystallins of vertebrates.

Purification and characterization of glutathione S-transferase of murine ovary and testis

Recent studies have indicated that sex hormones may regulate expression of murine glutathione S-transferase (GST) isozymes. Therefore, we have purified and compared GST isozymes of murine ovary and testis, two tissues with markedly different hormonal milieu. Isoelectric profiles of the GST isozymes of both these tissues were found to be closely similar. Both expressed one alpha-class GST (pI9.8), one pi-class GST (pI8.9), and three mu-class GSTs (pI8.5, 7.9, and 6.7). In addition, an isozyme (pI5.8) corresponding to the rat GST 8-8 was also expressed in both these tissues. Total GST protein/g tissue was about 1.7-fold more abundant in testis. The specific activities of the cationic isozymes of testis were 1.2- to 2.4-fold higher as compared to those of ovaries. On the other hand, the specific activities of the anionic testicular isozymes were 6.4- to 10-fold higher compared to the corresponding ovarian isozymes. Structural properties including the N-terminal sequences of the testicular isozymes were indistinguishable from those of their ovarian counterparts. The N-terminal sequence of the pi-class GST of both tissues was similar to that of mouse liver GST pi. The three mu-class GSTs of testis and ovary arise from the dimeric combinations of two subunits whose N-terminal sequences determined up to 24 residues were similar to those of mouse liver GST subunits mu 1 and mu 2. Although testicular and ovarian isozymes were structurally similar, Kcat values of some of the testicular isozymes were up to 10-fold higher than those of the corresponding ovarian isozymes. The substrate specificities were also significantly different for the corresponding isozymes of testis and ovary.

Photoaffinity labelling of human leukotriene C4 synthase in THP-1 cell membranes

Human leukotriene C4 synthase specific activity in the human monocytic leukemia cell line THP-1 (0.302 +/- 0.062 nmol LTC4 formed.min-1 x mg-1) was 7.6-fold higher than in U937 cells (0.040 +/- 0.017 nmol LTC4 formed.min-1 x mg-1) and comparable to dimethylsulfoxide-differentiated U937 cells (0.399 +/- 0.084 nmol LTC4 formed.min-1 x mg-1). Using the photoaffinity probe, azido[125I]-LTC4, a single polypeptide with a molecular mass of 18 kDa was specifically labelled in THP-1 microsomal membranes. The rank order of potencies for competition of azido[125I]-LTC4 photolabelling of the 18 kDa protein by glutathione, leukotrienes and their analogs was found to be LTC2 > (azido[127I]-LTC4 approximately LTC4) > (LTD4 approximately LTE4) > (LTA4 approximately LTB4) > S-hexyl glutathione > glutathione, corresponded with the rank order of potencies for inhibition of LTC4 synthase activity but not inhibition of microsomal glutathione S-transferase activity. The 18 kDa protein specifically labelled by azido[125I]-LTC4 had high specificity for LTC4 and closely related leukotrienes and was separable from microsomal glutathione S-transferase. We conclude that azido[125I]-LTC4 specifically photolabels LTC4 synthase which is an 18 kDa polypeptide or contains an 18 kDa subunit.

Identification of the fatty acid binding site on glutathione S-transferase P by immobilization to fatty acid-linked sepharose

The association of glutathione S-transferase P (GST-P) with various fatty acids was confirmed by gas-liquid chromatography and mass-spectrometry. Palmitic and stearic acids were the major fatty acids bound to the enzyme in which the molar ratio was 1:0.8 (GST-P:fatty acid). To evaluate the association with respect to the fatty acid carbon chain length and identify the binding site, we prepared a series of fatty acid-linked Sepharoses. GST-P tightly bound the fatty acid-linked Sepharoses (CH3(CH2)nCOOH, n = 4 approximately 16), and the site was determined to be residues 121-156 from the amino terminus by tryptic digestion of GST-P bound to a fatty acid-linked Sepharose. By fatty acid affinity labeling, we identified the binding site as residues 141-188 (Biochem. Biophys. Res. Commun., in press). The overlapping hydrophobic residues (residue 141-156) is expected to be essential for the ligand binding site.

Characterization of two novel subunits of the alpha-class glutathione S-transferases of human liver

More than 85% of the complete amino-acid sequence of the alpha-class glutathione S-transferase omega (GST omega) of human liver, described for the first time in this communication, show that GST omega is a heterodimer of two closely related novel alpha-class GST subunits. The sequences of these subunits, omega 1 and omega 2, have over 97% homology between them and are also highly homologous to the two alpha-class subunits characterized previously. Characterization of these two novel alpha-class subunits described in this report would explain the molecular basis for high degree of heterogeneity observed among the alpha-class human GSTs.

The amino acid sequence of glutathione transferase from Proteus mirabilis, a prototype of a new class of enzymes

The complete amino acid sequence of glutathione transferase from Proteus mirabilis was determined. The sequence was reconstructed by analysis of peptides obtained after cleavage by trypsin, Glu-C and Asp-N endoproteinases. The enzyme subunit is composed of 203 amino acid residues corresponding to a molecular mass of 22856 Da. Comparison of this sequence with other known primary structures of the corresponding enzyme from different sources shows a low level of identity (17-26%) with only seven conserved residues in all the sequences considered. This novel glutathione transferase could represent the prototype of a new class, possibly including other bacterial enzymes.

Schistosoma mansoni 28-kDa glutathione S-transferase and immunity against parasite fecundity and egg viability. Role of the amino- and carboxyl-terminal domains

We have previously shown that a mAb that inhibits the enzymatic activity of the Schistosoma mansoni 28-kDa glutathione S-transferase (Sm28 GST) also reduces female worm fecundity and egg viability in vivo and in vitro. By peptidic epitope mapping and an activity reconstitution assay, the carboxyl terminus (CT) amino acid residues 190-211 and to a lesser extent the truncated amino terminus (NT) residues 10-43 of the enzyme were identified as mAb recognition sites. Sera from rats immunized with the NT (10-43) and CT (190-211) peptides showed a partial inhibitory effect on Sm28 GST activity in a late phase (6 to 7 wk) but not in an early phase (2 to 4 wk) after immunization. Passive transfer of Sm28 GST-inhibiting anti-N- and C-terminal sera, but not of the noninhibitory sera, protected the infected mice by reducing tissue egg deposition and the ability of eggs to hatch. In active immunization experiments, the CT peptide significantly decreased the worm burden (37 to 40%) in mice as did the rSm28 GST (28 to 52%). In terms of tissue egg deposition and egg-hatching ability, immunization with both the NT and CT peptides reproduced the reduction observed after immunization with rSm28 GST. A constant reduction in egg numbers was noted in the small intestines and the livers of the immunized mice. A clear reduction in the ability of intestinal or hepatic eggs to hatch was observed. The results are discussed in terms of the conformational participation of the NT and CT of Sm28 in the expression of GST activity.

Site-directed mutagenesis and chemical modification of histidine residues on an alpha-class chick liver glutathione S-transferase CL 3-3. Histidines are not needed for the activity of the enzyme and diethylpyrocarbonate modifies both histidine and lysine residues

Each chick liver glutathione S-transferase CL 3 subunit contains three histidine residues: His142, His158 and His228. CL 3-3 can be inactivated by treating with diethylpyrocarbonate. The inactivation process is pH dependent and the pKa of the modified residue is 6.4. The second-order inhibition rate constant is 741 M-1min-1 at pH 7.0. Based on difference-spectrum and kinetic analysis, inactivation coincides with the modification of one histidine residue. However, hydroxylamine treatment of the diethylpyrocarbonate-modified enzyme only partially restored the activity (30-50%) of CL 3-3. By tryptic mapping and amino acid sequence analysis, His228 and Lys14 have been identified as the modified residues. Mutants with histidine to serine replacement (H142S and H158S) or C-terminal histidine deletion (des-H228) were constructed and over-expressed in Spodoptera frugiperda cells using a baculovirus system. The mutants are enzymically active. Furthermore, the des-H228 mutant can be inactivated by diethylpyrocarbonate. These results support the conclusion that histidines are not involved in the enzymic mechanism of CL 3-3.

Purification and characterization of the major glutathione transferase from adult toad (Bufo bufo) liver

Five forms of glutathione transferase (GST) were resolved from the cytosol of adult common toad (Bufo bufo) liver by GSH-affinity chromatography followed by isoelectric focusing. The major enzyme (GST-7.64; 55% of total activity bound to the column) has a pI value of 7.64, is composed of two subunits each with a molecular mass of 23 kDa, and has the N-terminal amino acid residue blocked. GST-7.64 has also been characterized with respect to amino acid composition, substrate specificity, inhibition characteristics, c.d. spectra and immunological reactivity. The N-terminal sequence of some peptides obtained after tryptic digestion has also been determined. All together the results obtained suggest that the major toad liver GST is distinct from any known GST, including microbial, plant and mammalian GSTs.

[Identification and characterization of hydrophobic ligand binding region in glutathione S-transferase P]

Recombinant glutathione S-transferase P (GST-P) was purified in a homogeneous state. Fatty acid analysis by gas-liquid chromatography-mass spectrometry (GC-MS) revealed that GST-P forms 1:1 complex with fatty acids, mostly palmitic acid or stearic acid, which were hardly isolated from the complex even through Lipidex 1,000 column chromatography at 37 degrees C. Temperature dependent analysis of 1H-NMR on the association between GST-P and fatty acids indicated that molecular motion of fatty acids were strongly restrained in a hydrophobic 'pocket' below the temperature of protein denaturation. On the other hand, there existed another hydrophobic ligand binding region, to which fatty acid and bilirubin would bind with relatively lower affinity. The binding region was determined to be at around 142-157 residues from amino terminus by the studies of GST-P binding to fatty acid-linked Sepharose and affinity labelings with either fluorescent fatty acid or bilirubin. The binding to this region noncompetitively inhibited the enzyme activity. Furthermore, circular dichroism (CD) analysis showed that the binding of hydrophobic ligands changed the secondary structure of GST-P, which suggested that the enzyme activity was regulated through conformational changes. As tryptophan 38 was assumed to locate at the active center from the study of site-directed mutagenesis, conformation of the active center was investigated by measuring the intrinsic tryptophan fluorescence. It showed that hydrophobic ligand binding caused the drastic conformational change, of which would be referred to the regulation of the enzyme activity.

Isozyme specificity of novel glutathione-S-transferase inhibitors

A systematically diversified set of peptide analogs of the reaction product of glutathione with an electrophilic substrate have been tested as isozyme-specific inhibitors of human glutathione-S-transferase (GST). The potency of the best of the inhibitors is in the 0.5 to 20 micromolar range, with kinetics indicative of competitive inhibition with glutathione at the active site. The specificity observed among three recombinant-derived GST isozymes at both low and high potency ranged from negligible to high (at least 20-fold over the next most sensitive isozyme). These results define a novel strategy for the design of drugs targeting cells with elevated levels of particular GST isozymes, such as tumor cells for which elevated levels of GST are believed to be an important cause of chemotherapeutic drug resistance.

Expression and lipoylation in Escherichia coli of the inner lipoyl domain of the E2 component of the human pyruvate dehydrogenase complex

The dihydrolipoamide acetyltransferase subunit (E2p) of mammalian pyruvate dehydrogenase complex has two highly conserved lipoyl domains each modified with a lipoyl cofactor bound in amide linkage to a specific lysine residue. A sub-gene encoding the inner lipoyl domain of human E2p has been over-expressed in Escherichia coli. Two forms of the domain have been purified, corresponding to lipoylated and non-lipoylated species. The apo-domain can be lipoylated in vitro with partially purified E. coli lipoate protein ligase, and the lipoylated domain can be reductively acetylated by human E1p (pyruvate dehydrogenase). Availability of the two forms will now allow detailed biochemical and structural studies of the human lipoyl domains.

Immunocytochemical localization of the Yf subunit of glutathione S-transferase P shows regional variation in the staining of epithelial cells of the testis, efferent ducts, and epididymis of the male rat

Glutathione S-transferases (GSTs) are a family of isozymes that catalyze the conjugation of glutathione (GSH), a tripeptide found in all mammalian cells; this function plays a protective role, as the addition of GSH to an electrophile generally forms a less toxic product. The pi class of GSTs contains homodimers of the Yf subunit, also known as Yp or rat subunit 7; this subunit is found in high concentrations in the testis and epididymis. The objective of the present study was to localize immunocytochemically the Yf subunit in the testis and in the various regions of the epididymis using light, electron, and confocal microscopy. In the testis, immunoperoxidase staining was localized exclusively to Sertoli and Leydig cells. The low cuboidal epithelial cells of the rete testis and the sparse ciliated cells of the ductuli efferents were also immunoreactive. A distinct pattern of immunostaining for the Yf subunit was observed in the different regions of the epididymis. The proximal area of the initial segment showed intense reactivity localized to epithelial basal cells. Basal cells in the middle area of the initial segment were also reactive, as were a second unidentified population of cells located in the apical region of the epithelium. The epithelium, including both principal and basal cells, in the distal initial segment, intermediate zone, and proximal caput epididymidis showed a weak, moderate, or strong degree of reactivity, respectively. In the distal caput epididymidis, however, principal cells showed a checkerboard-like pattern of immunoreactivity, with some cells being intensely stained or faintly stained, whereas others were unreactive. Strikingly, in the corpus and proximal cauda epididymidis, intense immunostaining was localized exclusively over the epithelial basal cells. As viewed in the light and confocal microscope, the intensely stained basal cells showed extensive processes that covered most of the base of the epididymal tubule. Upon quantitation of the immunogold labeling density (the number of gold particles/microns2) in principal and basal cells of the different regions of the epididymis, we observed a sharp decline in immunogold labeling of principal cells coupled with a dramatic increase in labeling of basal cells as we progressed along the tissue, particularly in the transition from the caput to the corpus epididymidis. This study constitutes the first demonstration of a protein that is selectively expressed in epithelial basal cells of the corpus and proximal cauda epididymidis.(ABSTRACT TRUNCATED AT 400 WORDS)

Circular dichroic evidence for regulation of enzymatic activity by nonsubstrate hydrophobic ligand on glutathione S-transferase P

1-Anilinonaphthalene-8-sulfonic acid (ANS) noncompetitively inhibited enzyme activity of glutathione S-transferase P for both glutathione and 1-chloro-2,4-dinitrobenzene (Ki = 30 microM). Dissociation constant for ANS.GST-P complex calculated from the binding study was 15 microM. From the similar values of the inhibition constant and the dissociation constant, it was concluded that specific ANS binding caused the loss of enzyme activity. In the protein structural analysis by circular dichroism, the secondary structures remarkably changed by ANS binding in accordance with the decrease of enzymatic activities. The conformational change of the protein and the decrease in enzymatic activity were reversed by dissociation of ANS. This fact strongly suggested that the enzymatic activity was regulated by a nonsubstrate hydrophobic ligand.