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The contribution of individual cytochrome P-450 isozymes in the hydroxylation of the centrally acting skeletal muscle relaxant chlorzoxazone was determined in rat liver microsomes. The hydroxylation rate of chlorzoxazone was found to be 50% greater in male than female microsomes. Kinetic studies using control male microsomes showed that chlorzoxazone hydroxylation was biphasic with a calculated low Km (33 microM) and high Km (116 microM). Liver microsomes from isoniazid-, beta-naphthoflavone- or dexamethasone-treated male rats produced a Km of 93, 69 and 26 microM, respectively. When chlorzoxazone hydroxylation activity was measured at a high substrate concentration (200 microM), treatment of male rats with isoniazid, acetone, beta-naphthoflavone and dexamethasone produced increases in the activity rate of 124%, 117%, 81% and 32%, respectively. However, when the activity was measured at a low substrate concentration (2 microM), liver microsomes from dexamethasone-treated male and female rats produced 5- and 10-fold induction, respectively. In immunoinhibition studies at 200 microM of chlorzoxazone, antibodies specific for cytochrome P-450 2E1 inhibited the rate of chlorzoxazone hydroxylation in microsomes from control and isoniazid-treated male rats by 68% and 79%, respectively. A monoclonal antibody (C8) against P-450 1A1 inhibited 67% of the activity in microsomes from beta-naphthoflavone-treated male rats but was ineffective inhibiting chlorzoxazone hydroxylation in microsomes from control or dexamethasone-treated male rats. In liver from control female rats, antibodies against cytochrome P-450 2E1 inhibited 80% of chlorzoxazone hydroxylation, whereas it inhibited only 47% of the activity in dexamethasone-treated females.(ABSTRACT TRUNCATED AT 250 WORDS)
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