The influence of phenobarbital, 3-methylcholanthrene and 2,3,7,8-tetrachlorodibenzo- p-dioxin on glutathione s-transferase activity of rat liver cytosol

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Specific activities and apparent Michaelis-Menten kinetic parameters were determined for glutathione (GSH) S-transferase activity (E.C. 2.5.1.18) in rat liver cytosol, towards styrene oxide (STOX), 1,2-butylene oxide (BOX) and 1-chloro-2,4-dinitrobenzene (CDNB) as electrophilic substrates, before and after pretreatment with the drug-metabolizing enzyme inducers phenobarbital (PB), 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD). The measured GSH S-transferase activities appear to obey Michaelis-Menten kinetics. In non-induced animals the apparent K m values of the transferase activities were equal for STOX vs GSH, but they differed by a factor of 2 for CDNB vs GSH and by a factor of 14 for BOX vs GSH. The apparent V max values in each combination of GSH and electrophilic substrate were equal, but differed by one order of magnitude for the mutual substrate combinations. Pretreatment of the rats with MC resulted in enhancement of all measured activities expressed in terms of cytosol protein, while TCDD only enhanced the activities expressed as per gram body wt. PB enhanced both activities when STOX was employed as substrate, but when CDNB was used as the substrate, only the activity per gram body wt increased. All pretreatments increased the V max values using CDNB as the substrate, while PB and MC had an enhancing effect using STOX; the V max using BOX was enhanced after TCDD administration only. The K m values using BOX as the substrate was lowered after MC pretreatment; TCDD pretreatment decreased the K m using STOX, while it increased the K m using CDNB. It is concluded that the GSH S-transferase system is inducible, but in contrast to the induction of the mixed function oxidase system, qualitative differences between the inducing effects of PB and MC were not observed. Use of TCDD as inducing agent, however. resulted in a different induction pattern, which may indicate that during induction with this agent different types of GSH S-transferases are involved.