Minor modifications of the C-terminal helix reschedule the favored chemical reactions catalyzed by theta class glutathione transferase T1-1

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Effects of Substrate-Binding Site Residues on the Biochemical Properties of a Tau Class Glutathione S-Transferase from Oryza sativa

Glutathione S-transferases (GSTs)—an especially plant-specific tau class of GSTs—are key enzymes involved in biotic and abiotic stress responses. To improve the stress resistance of crops via the genetic modification of GSTs, we predicted the amino acids present in the GSH binding site (G-site) and hydrophobic substrate-binding site (H-site) of OsGSTU17, a tau class GST in rice. We then examined the enzyme activity, substrate specificity, enzyme kinetics and thermodynamic stability of the mutant enzymes. Our results showed that the hydrogen bonds between Lys42, Val56, Glu68, and Ser69 of the G-site and glutathione were essential for enzyme activity and thermal stability. The hydrophobic side chains of amino acids of the H-site contributed to enzyme activity toward 4-nitrobenzyl chloride but had an inhibitory effect on enzyme activity toward 1-chloro-2,4-dinitrobenzene and cumene hydroperoxide. Different amino acids of the H-site had different effects on enzyme activity toward a different substrate, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole. Moreover, Leu112 and Phe162 were found to inhibit the catalytic efficiency of OsGSTU17 to 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, while Pro16, Leu112, and Trp165 contributed to structural stability. The results of this research enhance the understanding of the relationship between the structure and function of tau class GSTs to improve the abiotic stress resistance of crops.

Genetic Variants of Glutathione S-Transferase GSTT1 and GSTT2 in Cynomolgus Macaques: Identification of GSTT Substrates and Functionally Relevant Alleles

Glutathione S-transferase (GST) is a family of important drug-metabolizing enzymes, conjugating endogenous and exogenous compounds. Genetic polymorphisms result in the inter-individual variability of GST activity in humans. Especially, human GSTT1 and GSTT2 null alleles are associated with toxicity and various cancers derived from chemicals. Cynomolgus macaque, a nonhuman primate species widely used in drug metabolism studies, has molecular and enzymatic similarities of GSTs to the human orthologs; however, genetic polymorphisms have not been investigated in this species. In this study, resequencing of GSTT1 and GSTT2 in 64 cynomolgus and 32 rhesus macaques found 15 nonsynonymous variants and 1 nonsense variant for GSTT1 and 15 nonsynonymous variants for GSTT2. Some of these GSTT variants were distributed differently in Indochinese and Indonesian cynomolgus macaques and rhesus macaques. For analysis of functional relevance of the GSTT variants, 1-iodohexane and dibromomethane were determined to be suitable substrates for cynomolgus GSTT1 and GSTT2. However, the conjugation activities were roughly correlated with GSTT protein levels immunochemically quantified in cynomolgus liver samples with no statistical significances, implying the contributions of the GST genetic variants. Among the GSTT1 variants identified, the animals carrying R76C and D125G mutations showed lower conjugation activities toward dibromomethane than those of the wild-type in liver cytosolic fractions. Moreover, the recombinant R76C/D125G and D125G GSTT variant proteins showed significantly lower 1-iodohexane or dibromomethane conjugation activities than those of the wild-type protein. Therefore, inter-animal variability of GSTT-dependent drug metabolism is at least partly accounted for by GSTT1 and possibly GSTT2 variants in cynomolgus and rhesus macaques.

Exploring sequence-function space of a poplar glutathione transferase using designed information-rich gene variants

Exploring the vicinity around a locus of a protein in sequence space may identify homologs with enhanced properties, which could become valuable in biotechnical and other applications. A rational approach to this pursuit is the use of ‘infologs’, i.e. synthetic sequences with specific substitutions capturing maximal sequence information derived from the evolutionary history of the protein family. Ninety-five such infolog genes of poplar glutathione transferase were synthesized and expressed in Escherichia coli, and the catalytic activities of the proteins determined with alternative substrates. Sequence–activity relationships derived from the infologs were used to design a second set of 47 infologs in which 90% of the members exceeded wild-type properties. Two mutants, C2 (V55I/E95D/D108E/A160V) and G5 (F13L/C70A/G122E), were further functionally characterized. The activities of the infologs with the alternative substrates 1-chloro-2,4-dinitrobenzene and phenethyl isothiocyanate, subject to different chemistries, were positively correlated, indicating that the examined mutations were affecting the overall catalytic competence without major shift in substrate discrimination. By contrast, the enhanced protein expressivity observed in many of the mutants were not similarly correlated with the activities. In conclusion, small libraries of well-defined infologs can be used to systematically explore sequence space to optimize proteins in multidimensional functional space.

Recent advances in protein engineering and biotechnological applications of glutathione transferases

Glutathione transferases (GSTs, EC 2.5.1.18) are a widespread family of enzymes that play a central role in the detoxification, metabolism, and transport or sequestration of endogenous or xenobiotic compounds. During the last two decades, delineation of the important structural and catalytic features of GSTs has laid the groundwork for engineering GSTs, involving both rational and random approaches, aiming to create new variants with new or altered properties. These approaches have expanded the usefulness of native GSTs, not only for understanding the fundamentals of molecular detoxification mechanisms, but also for the development medical, analytical, environmental, and agricultural applications. This review article attempts to summarize successful examples and current developments on GST engineering, highlighting in parallel the recent knowledge gained on their phylogenetic relationships, structural/catalytic features, and biotechnological applications.

Characterization of lacrimal proline-rich protein 4 (PRR4) in human tear proteome

This study was initiated considering the lack of comprehensive characteristics profile of PRR4 in tears of healthy subjects. Therefore, detailed characterizations of PRR4 from basal tears employing in-gel and in-solution digestions for MS systems are presented herein. First, pooled tear samples (n = 10) were utilized to identify PRR4-rich region/spots in 1DE/2DE gels employing LC-MALDI-MS and 1DE-LC-ESI-LTQ-Orbitrap-MS systems. PRR4-rich region and ten spots with vast polymorphisms (Mr : 17-30 kDa, pI: 3.0-6.6) were identified in 1DE and 2DE gels, respectively. In addition, combinations of four types of PTMs, which are methylation, acetylation, oxidation, and pyroglutamate formation, were identified in these ten PRR4 spots. Furthermore, a targeted data-acquisition approach was utilized to identify PRR4 isoforms in individual tear samples (n = 61) by in-solution digestion combined with a LC-ESI-LTQ-Orbitrap-MS system. Importantly, a new PRR4 isoform designated as PRR4-N3 in addition to PRR4 (gi154448886) and pHL E1F1 (gi1050983) was identified. Moreover, different combinations of these three PRR4 isoforms identified in the individual tear samples could be categorized into six distinguished groups. Conclusively, these findings provide fundamental insight into the complex characteristics profile of PRR4 isoforms and their PTMs in tears of healthy individuals.

Glutathione-binding site of a bombyx mori theta-class glutathione transferase

The glutathione transferase (GST) superfamily plays key roles in the detoxification of various xenobiotics. Here, we report the isolation and characterization of a silkworm protein belonging to a previously reported theta-class GST family. The enzyme (bmGSTT) catalyzes the reaction of glutathione with 1-chloro-2,4-dinitrobenzene, 1,2-epoxy-3-(4-nitrophenoxy)-propane, and 4-nitrophenethyl bromide. Mutagenesis of highly conserved residues in the catalytic site revealed that Glu66 and Ser67 are important for enzymatic function. These results provide insights into the catalysis of glutathione conjugation in silkworm by bmGSTT and into the metabolism of exogenous chemical agents.

Molecular evolution and the role of oxidative stress in the expansion and functional diversification of cytosolic glutathione transferases

Background

Cytosolic glutathione transferases (cGST) are a large group of ubiquitous enzymes involved in detoxification and are well known for their undesired side effects during chemotherapy. In this work we have performed thorough phylogenetic analyses to understand the various aspects of the evolution and functional diversification of cGSTs. Furthermore, we assessed plausible correlations between gene duplication and substrate specificity of gene paralogs in humans and selected species, notably in mammalian enzymes and their natural substrates.

Results

We present a molecular phylogeny of cytosolic GSTs that shows that several classes of cGSTs are more ubiquitous and thus have an older ancestry than previously thought. Furthermore, we found that positive selection is implicated in the diversification of cGSTs. The number of duplicate genes per class is generally higher for groups of enzymes that metabolize products of oxidative damage.

Conclusions

1) Protection against oxidative stress seems to be the major driver of positive selection in mammalian cGSTs, explaining the overall expansion pattern of this subfamily;

2) Given the functional redundancy of GSTs that metabolize xenobiotic chemicals, we would expect the loss of gene duplicates, but by contrast we observed a gene expansion of this family, which likely has been favored by: i) the diversification of endogenous substrates; ii) differential tissue expression; and iii) increased specificity for a particular molecule;

3) The increased availability of sequence data from diversified taxa is likely to continue to improve our understanding of the early origin of the different cGST classes.

Metabolism of isothiocyanates in individuals with positive and null GSTT1 and M1 genotypes after drinking watercress juice

BACKGROUND & AIMS

Isothiocyanates (ITCs) derived from cruciferous vegetables have been shown to be promising agents against cancer in human cell culture, animal models, and in epidemiological studies. Several epidemiological studies have demonstrated an inverse relationship between intake of dietary isothiocyanates and the risk of cancers, particularly lung, colon, and breast. More importantly, the protective effects of dietary ITCs appear to be influenced by glutathione S-transferase (GST) genotype; specifically, individuals with glutathione S-transferase theta 1 (GSTT1) and glutathione S-transferase Mu 1 (GSTM1) null are better protected than those with GSTT1 and M1 positive. Although the majority of studies, especially those conducted in populations exposed to ITC rich diets, demonstrated such effects, there are a few studies that showed opposite or no association. While evidence for the interactions of dietary ITCs with GST genes is relatively strong, the reasons for the differential effects remain unclear. In this study, we examined one possible mechanism: whether subjects with null genotypes excrete ITCs at a slower rate than those with positive genotypes after drinking watercress juice, a rich source of ITCs.

METHODS

A total of 48 subjects, 28 GSTT1 and M1 positive and 20 null genotypes were enrolled in the study. The rates of excretion were determined using five urine samples collected over a period of 24 h after drinking watercress juice.

RESULTS

No statistically significant differences in the rates of isothiocyanate excretion and the time of peak excretion were observed between the two tested groups having positive and null genotypes.

CONCLUSIONS

GSTT1 and M1 genotypes are not likely to be involved in the rate of excretion of ITCs in watercress. The demonstrated differences in protection among subjects with the two genotypes are not likely due to differences in overall ITC excretion rates, however, excretion rates of ITCs other than PEITC need to be investigated. Other yet to be identified mechanism(s) may underlie the diet and gene interactions between dietary ITCs and GST genotypes in human cancer prevention. Further research is needed to evaluate the protective mechanisms of isothiocyanates against cancer.

Genetic variations in human glutathione transferase enzymes: significance for pharmacology and toxicology

Glutathione transferase enzymes (GSTs) catalyze reactions in which electrophiles are conjugated to the tripeptide thiol glutathione. While many GST-catalyzed transformations result in the detoxication of xenobiotics, a few substrates, such as dihaloalkanes, undergo bioactivation to reactive intermediates. Many molecular epidemiological studies have tested associations between polymorphisms (especially, deletions) of human GST genes and disease susceptibility or response to therapy. This review presents a discussion of the biochemistry of GSTs, the sources-both genetic and environmental-of interindividual variation in GST activities, and their implications for pharmaco- and toxicogenetics; particular attention is paid to the Theta class GSTs.