S-transferase Inhibitor Research Articles

Naturally occurring enzyme inhibitors and their pharmaceutical applications

Enzyme inhibitors play a significant role in the drug discovery process. For instance, acetylcholinesterase (AChE) inhibitors have applications in curing Alzheimer’s disease (AD), senile dementia, ataxia, myasthenia gravis, and Parkinson’s disease. Glutathione S-transferase (GST) inhibitors have applications as adjuvants to overcome anticancer and antiparasitic drug resistance problems. Compounds inhibiting the activity of α-glucosidase are used to treat type 2 diabetes mellitus and obesity problems. This article describes the identification of natural products exhibiting AChE, GST, and α-glucosidase inhibitory activities from medicinally important plants. Additionally, structure–activity relationship (SAR) studies of these newly discovered enzyme inhibitors are also discussed.

Antioxidant properties of Prangos ferulacea (L.) Lindl., Chaerophyllum macropodum Boiss. and Heracleum persicum Desf. from Apiaceae family used as food in Eastern Anatolia and their inhibitory effects on glutathione- S-transferase

Therapeutic effects of several medicinal plants and vegetables, which are commonly used as food and in folk medicine against many disease, are well known. Antioxidant capacities of Heracleum persicum Desf., Prangos ferulacea (L.) Lindl., Chaerophyllum macropodum Boiss. species from Apiaceae family were evaluated by determining their effects on DPPH radical scavenging, and lipid peroxidation inhibition, as well as their total phenolic contents. Potential natural glutathione- S-transferase inhibitors have gained great importance in the last decade especially because of the role of glutathione- S-transferases in developing resistance to chemotherapy. Selected plants were therefore further investigated for their influence on the activity of glutathione-S-transferase enzyme. In comparing antioxidant capacities, P. ferulacea was found to be a better antioxidant than the other two plants in the family, with 50% inhibitory concentration values as 0.242 and 0.152 mg/ml for DPPH radical scavenging and lipid peroxidation inhibition, respectively. The same plants were also evaluated for their effects on GST activities and among the studied plants, P. ferulacea was the most efficient inhibitor with IC 50 value of 79.25 μg/ml for GST. C. macropodum and H. persicum were less effective antioxidants than P. ferulacea with IC 50 values of 0.623 and 0.438 mg/ml for DPPH radical scavenging, respectively, however, they could also be considered as potential GST inhibitors.

Effect of ethacrynic acid, a glutathione- S-transferase inhibitor, on nitroglycerin-mediated cGMP elevation and vasorelaxation of rabbit aortic strips

The effects of ethacrynic acid (ECA), an inhibitor of glutathione- S-transferase, on both the pharmacologic and biochemical responses of aortic tissue to nitroglycerin (GTN) were evaluated. Using the rabbit aortic strip model, relaxation responses to 0.6 μM GTN were measured with and without ECA (0.2 mM) pretreatment. These same strips were frozen, and the concentrations of cGMP in the strips were measured using a 3H-labeled radioimmunoassay. Both the relaxation response and the increase in cGMP upon GTN treatment were reduced significantly by pretreatment of the strips with ECA. A correlation was observed between the decreases in the pharmacodynamic and biochemical responses upon ECA pretreatment. cGMP levels in strips treated with sodium nitroprusside, which generates nitric oxide by mechanisms distinct from that for organic nitrates, were not decreased by ECA pretreatment. These observations suggest that the mechanism of GTN action involves a glutathione- S-transferase-mediated metabolic step for GTN and that the isozyme(s) involved in this activation process may be inhibited by ECA.

Hepatic and extra hepatic glutathione depletion and glutathione- S-transferase inhibition by monocrotophos and its two thiol analogues

Effect of monocrotophos (MCP) and its thiol analogues (coded as RPR-2 and RPR-5) on hepatic and extra-hepatic glutathione (GSH) depletion and glutathione- S-transferase (GST) inhibition was studied at 0.96, 1.23 and 3.0 mg/kg respectively 24 h after medication in rats. All the three compounds caused tissue specific depletion of GSH from hepatic and extra-hepatic tissues. Cytosolic GST activity was significantly inhibited in all the tissues, MCP being the most potent inhibitor. Both in vitro and in vivo data indicate that hepatic GST inhibiting potential of the three compounds lies in the order MCP > RPR-5 > RPR-2. In vitro effect of 3 compounds on GSH activation kinetics of GST demonstrate competitive inhibition by MCP and non-competitive inhibition by the two analogues. However, CDNB activation kinetics of the enzymes revealed mixed inhibition by all 3 compounds. The present study suggests that monocrotophos and its thiol analogues may bring about physiological upsets by altering GSH and GST dependent events in different tissues of exposed organisms.

The mechanism of biliary excretion of methyl mercury: studies with methylthiols.

The S-methylated derivatives of N-acetylpenicillamine, thiola and cysteine as well as methyl iodide decreased biliary excretion of methyl mercury markedly. Excretion of sulfhydryl in bile was not influenced by S-methyl-cysteine, S-methylthiola, S-methyl-N-acetylpenicillamine or a low dose of methyliodide (0.5 mmol/kg body weight). This seems to indicate that coupling of methyl mercury to glutathione in the liver before biliary excretion is a glutathione S-transferase dependent reaction. It also indicates that the methylthiols tested, or metabolites of these compounds are likely to be inhibitors of S-transferase. The effect of S-methylcysteine and low doses of methyl iodide probably reflects glutathione S-transferase inhibition as both compounds are metabolized to the S-transferase inhibitor S-methylglutathione in the liver. A higher dose of methyl iodide (1 mmol/kg body weight) seems to deplete the liver of reduced glutathione through S-methylation as illustrated by decreased biliary excretion of sulfhydryl. S-methylthiola and S-methyl-N-acetylpenicillamine are metabolized in the liver to unknown components which are excreted in bile. Whether S-methylthiola and S-methyl-N-acetylpenicillamine are inhibitors of S-transferase themselves or cause inhibition through metabolites cannot be stated from the present investigation.

Formation of 3-(glutathion-S-yl)-N-methyl-4-aminoazobenzene and inhibition of aminoazo dye-nucleic acid binding in vitro by reaction of glutathione with metabolically-generated N-methyl-4-aminoazobenzene- N-sulfate

The reaction of glutathione (GSH) with metabolically-formed N-methyl-4-aminoazobenzene- N-sulfate (MAB- N-sulfate), a presumed ultimate carcinogenic metabolite of N, N-dimethyl-4-aminoazobenzene (DAB), was investigated using a hepatic sulfotransferase incubation mixture containing GSH and the proximate carcinogen, N-hydroxy- N-methyl-4-aminoazobenzene ( N-HO-MAB). Under these conditions, 6–16% of the MAB- N-sulfate formed could be trapped as an aminoazo dye-GSH adduct. Upon subsequent purification, the adduct was shown to be chromatographically and spectrally identical to 3-(glutathion- S-yl)- N-methyl-4-aminoazobenzene (3-GS-MAB), a known biliary metabolite of DAB and a product of the reaction of the synthetic ultimate carcinogen, N-benzoyloxy- N-methyl-4-aminoazobenzene( N-BzO-MAB), with GSH. Neither 2′- nor 4′-GS-MAB, both products of the latter reaction, were detected in the sulfotransferase incubation mixture. GSH- S-transferases did not appear to be involved in the reaction of MAB- N-sulfate or N-BzO-MAB with GSH. The addition of triethyltin, a potent GSH- S-transferase inhibitor, had no effect on the yield of 3-GS-MAB in (N-HO-MAB sulfotransferase)-GSH incubations; and the addition of cytosol or purified GSH transferases A and B to a (N-BzO-MAB)-GSH reaction mixture did not increase the amount of 3-GS-MAB formed. GSH was shown to inhibit only partially the covalent binding of [ 3H]-MAB- N-sulfate to DNA and rRNA. At 10 and 100 mM GSH, the sulfotransferase-mediated binding of [ 3H] N-HO-MAB to both nucleic acids was reduced by 30% and 70%, respectively. The role of GSH in the detoxification of chemical carcinogens is discussed.