Comparative effect of the induction of the subunits of rat liver glutathione S-transferases by butylated hydroxytoluene

Selective expression of the three classes of glutathione S-transferase isoenzymes in mouse tissues

The suitability of mouse as an animal model for studying the glutathione S-transferase (GST)-mediated detoxification mechanisms has been studied by analyzing the expression of the alpha, mu, and pi classes of glutathione S-transferase isoenzymes in mouse brain, heart, kidney, spleen, liver, and muscle. Individual isoenzymes from each of these tissues have been purified, characterized, and classified into the three known classes of GST. These studies demonstrate that GST isoenzymes are variably expressed in different mouse tissues, suggesting that their expression is tissue specific. A major isoenzyme, belonging to the pi class, with a pI value in the range of 8.6-9.1 and an approximate subunit Mr value of 22,500 was detected in each tissue investigated in this study. A variable number of mu class isoenzymes with subunit Mr values of 26,500 were expressed in all mouse tissues studied, except spleen and muscle. Only liver and kidney showed the expression of an alpha class isoenzyme, each having a basic pI value and subunit Mr of approximately 25,000. Another minor acidic alpha class isoenzyme, also with a subunit Mr value of 25,000, was detected in liver, kidney, and brain. While multiple GST isoenzymes were detected in all other tissues studied, only spleen showed the presence of a single isoenzyme, which belonged to the pi class. These results reveal considerable differences in the GST isoenzyme composition of mouse tissues as compared to rat and human tissues. However, several apparent similarities in mouse and human tissues exist, suggesting that the mouse model can be used to analyze the GST-mediated detoxification mechanisms in humans.

Positive correlation exists between glutathione S-transferase activity and aflatoxin formation in Aspergillus flavus

The presence of glutathione (GSH) S-transferase activity, using 1-chloro-2, 4-dinitrobenzene (CDNB) as a substrate, has been established in the cytosolic fraction of the toxigenic (aflatoxin producing) and nontoxigenic strains of Aspergillus flavus. Significant differences in the GSH S-transferase activity were observed between the toxigenic and non-toxigenic strains. A positive correlation has been demonstrated for the first time between aflatoxin formation and a biochemical parameter, namely GSH S-transferase activity. The evidence in support of A. flavus GSH S-transferase induction by endogenous aflatoxins is as follows: (i) the age-related production of aflatoxin follows the same pattern as the cytosolic GSH S-transferase activity profile; (ii) significantly higher enzyme activity was associated with mycelia of a toxigenic strain grown in medium supporting high aflatoxin production (sucrose-low-salts medium) while the enzyme activity was low in medium producing less aflatoxin (glucose-ammonium nitrate medium). The GSH S-transferase activity of the non-toxigenic strain was hardly affected by a change in the medium as it produces no aflatoxins; and (iii) the toxigenic strain demonstrated significantly higher apparent Vmax. with no change in Km as compared with the non-toxigenic strain. This indicates that the enzyme induction by endogenous aflatoxins is similar to the action of phenobarbitol and other inducing drugs (Kaplowitz et al., 1975).

Glutathione S-transferases of mouse lung. Selective binding of benzo[a]pyrene metabolites by the subunits which are preferentially induced by t-butylated hydroxyanisole

Six isoenzymes of glutathione S-transferase (GST) present in mouse lung have been purified and characterized. GST I (pI 9.8) is a dimer of Mr-26,500 subunits and GST II is a heterodimer of Mr-26,500 and -22,000 subunits, and GST III (pI 7.9) and IV (pI 6.4) are dimers of Mr-24,500 subunits. GST V (pI 5.7) is a heterodimer of Mr-24,500 and -23,000 subunits, whereas GST VI (pI 4.9) is a dimer of Mr-23,000 subunits. Immunological studies indicate that the Mr-24,500 subunits present in GST III (pI 7.9) are distinct from those present in GST IV (pI 6.4) and V (pI 5.7). Structural and immunological studies provide evidence that at least five distinct types of subunits in their different binary combinations give rise to various GST isoenzymes of mouse lung. These isoenzymes express varying degrees of catalytic activities towards a wide range of electrophilic substrates including benzo[a]pyrene 7,8-oxide and benzo[a]pyrene 4,5-oxide. The dietary antioxidant t-butylated hydroxyanisole (BHA) preferentially induces GST II and III. Also, these two isoenzymes selectively bind benzo[a]pyrene (B[a]P) metabolites, indicating that they play an important physiological role in the detoxification of B[a]P metabolites. The preferential induction of the GST isoenzymes involved in the detoxification of activated B[a]P metabolites indicates that the anti-neoplastic activity of BHA against B[a]P-induced neoplasia in mouse lung [Wattenberg (1973) J. Natl. Cancer Inst. 50, 1541-1544] may be due to the enhanced detoxification of B[a]P metabolites.