Role of C-5 chiral center in R-(+)-pulegone-mediated hepatotoxicity: Metabolic disposition and toxicity of 5,5-dimethyl-2-(1-methylethylidene)-cyclohexanone in rats

Metabolic disposition of 5,5-dimethyl-2-(1-methylethylidene)-cyclohexanone (I) was examined in rats. Compound (I) was administered orally (250 mg/kg o f body weight/day) to rats for 5 days. The following urinary metabolites we re isolated and identified: 4,5,6,7-tetrahydro-3,6,6-trimethylbenzofuran (I II), 3,3-dimethylcyclohexanone (VI), 5,5-dimethyl-3-hydroxy-2-(1-methylethy lidene)-cyclohexanone (X), 5,5-dimethyl-2-(1-hydroxymethylethyl)-cyclohexan one (IX), 3-hydroxy-5-hydroxymethyl-5-methyl-2-(1-methylethylidene)-cyclohe xanone (XI), 5,6-dihydro-3,6,6-trimethyl-2(4H)-benzofuranone (VIII), and 5, 5-dimethyl-3-hydroxy-2-(1-carboxy ethylidene)-cyclohexanone (XIII). Incubat ion of compound (I) with phenobarbital (PB)-induced rat liver microsomes in the presence of NADPH resulted in the formation of a metabolite, tentative ly identified as a furanoterpene (III) based on proton magnetic resonance, gas chromatography, and gas chromatography-mass spectroscopy analyses. The formation of III was inhibited to a significant extent by carbon monoxide, metyrapone, SKF 525-A, and cytochrome c, suggesting the participation of PB -induced microsomal cytochrome P-450 system in the conversion of I to III. Compound I gave type I spectral change in the PB-induced liver microsomes a nd the dissociation constant (Ks) for I was 38.5 mu M. Intraperitoneal admi nistration of a single dose (250 mg/kg) of I to rats resulted in 26, 23, an d 41% decreases in the levels of cytochrome P-450, glucose-6-phosphatase, a nd aminopyrine N-demethylase, respectively, at the end of 24 h. During this period, a 11-fold increase in serum glutamate pyruvate transaminase level was also observed. However, a decrease in the level of cytochrome P-450 and glucose-6-phosphatase, and an increase in serum glutamate pyruvate transam inase values were comparatively more pronounced when R-(+)-pulegone (250 mg /kg) or CCl4 (0.6 ml/kg) was administered to rats. Pretreatment of rats wit h PB potentiated the hepatotoxicity caused by I, whereas pretreatment with 3-methylcholanthrene protected from it. This suggests that PB-induced cytoc hrome P-450-catalyzed reactive metabolites may be responsible for the toxic effects caused by I.