Cytochrome P-450 2E1 is not the sole catalyst of chlorzoxazone hydroxylation in rat liver microsomes. off.

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)

Induction of peroxisomal enzymes by a tetrazole-substituted 2-quinolinylmethoxy leukotriene D4 antagonist.

The induction of hepatic peroxisomal beta-oxidation and the peroxisomal bifunctional enzyme (PBE) by the tetrazole-substituted leukotriene D4 receptor antagonist RG 7152 was evaluated in vivo following subchronic treatment in the mouse, rat, guinea pig, dog, and rhesus monkey. The ability of RG 7152 to induce this enzyme system in rat extrahepatic tissues reported to respond to peroxisome proliferators and in vitro in primary rat hepatocytes was also investigated. Western blot analysis for PBE and beta-oxidation assays revealed significant induction by RG 7152 in liver homogenates from rats and mice with a lesser effect in guinea pigs and monkeys and no effect in dogs. The degree of induction in rat liver was less than that observed in a positive control group treated with clofibrate (CF). There was slight induction of PBE in rat kidney and small intestine by CF, whereas RG 7152 elicited a minimal response in the kidney and no effect in the small intestine. In vitro, RG 7152 produced a response that was greater than that produced by diethylhexyl phthalate, approximately equivalent to that produced by clofibric acid, but less than that produced by bezafibrate. Dose-response comparison of RG 7152 with the tetrazole-substituted leukotriene D4 antagonist LY 171883 to be slightly more potent than RG 7152. Thus, RG 7152 represents a second chemical class of tetrazole-substituted leukotriene D4 antagonist that causes peroxisomal enzyme induction in rodents.