Differential response of cytosolic, microsomal, and mitochondrial glutathione S-transferases to xenobiotic inducers.

Subcellular levels of different isoenzymes of glutathione S-transferases (GSTs) and their catalytic activities in rat liver, lung and brain tissues were compared following treatment with phenobarbital (PB), -naphthoflavone (BNF) and dexamethasone (DEX). The constitutive expression of á and mu classes of GSTs, but not the GST , was maximum in the liver cytosol as compared to other tissues. Cytosolic GST activity using 1-chloro-2,4-dinitrobenzene (CDNB) as a substrate was 2-4 fold higher than that in the microsomal and mitochondrial fractions. Glutathione peroxidase activity with cumene hydroperoxide as a substrate was also highest in the rat liver cytosol. PB and BNF treatments markedly induced the amount of GST proteins in all the tissues studied with the maximum induction in the cytosol after 4 days of PB and 10 days of BNF treatments, respectively. The longer duration of treatments had a suppressive effect on the GST activity, particularly in the mitochondrial and microsomal fractions. DEX treatment, on the other hand, only marginally induced the cytosolic GST, while the mitochondrial GST and the membrane bound microsomal GST activities were mostly decreased. Northern blot analysis also showed an increase in the GST-á mRNA level indicating a possible upregulation of the GST gene expression by the xenobiotic agent. Differences between the subcellular GSTs were studied by the in vitro addition of N-ethylmaleimide (NEM), a selective activator of the microsomal GST. The cytosolic GST activity, both in livers of uninduced and PB-treated, was inhibited to about 50% of the control levels by NEM. The mitochondrial activity, on the other hand, was significantly activated by the addition of NEM, similar to that reported for the microsomal GST. These results suggest selectivity in the effects of different xenobiotics on the expression and catalytic activity of GST isoenzymes from different subcellular compartments of tissues. More importantly, these observations are also relevant in studies on xenobiotic induced organ-specific toxicity and carcinogenicity.