EFFECTS OF D-LIMONENE ON HEPATIC-MICROSOMAL MONOOXYGENASE ACTIVITY AND PARACETAMOL-INDUCED GLUTATHIONE DEPLETION IN MOUSE

1. D-Limonene, a monoterpenoid constituent of citrus fruit oil, blocks tumour induction by chemical carcinogens in laboratory animals, appar ently by preventing bioactivation of procarcinogens and by enhancing c onjugation of proximal carcinogenic metabolites. 2. Inhibitory effects of D-limonene were measured in vitro using cytochrome P450 isoform-sp ecific substrates. D-Limonene inhibited p-nitrophenol hydroxylase (pNP ) activity in vitro in liver microsomes from acetone-, phenobarbital ( PB)- and beta-naphthoflavone (BNF)-treated mouse, and 7-ethoxyresorufi n O-deethylase (EROD) activity in microsomes from PB- and BNF-treated mouse. p-Nitrophenol and ethoxyresorufin are substrates for cytochrome s P2E1 and P1A1, respectively. No inhibition of benzphetamine (BNZP) o r aminopyrine (AP) demethylases by D-limonene was observed. 3. EROD, B NZP and AP activities in liver microsomes were increased 18 h after i. p. administration of D-limonene to acetone-induced mouse, while pNP ac tivity was unchanged. The immunodetectable protein level of cytochrome P2B1 in non-acetone treated mouse was increased 18 h after D-limonene , with no differences in P2E1 or P1A1. 4. Acute D-limonene did not pro tect against paracetamol (acetaminophen)-induced depletion of liver re duced glutathione (GSH). A prolonged paracetamol challenge (0.6% diet for 10 days) elevated liver cytosolic GSH-S-transferase activity (GST) two-fold and decreased liver GSH to 46% of control values. Dietary D- limonene (1.0% diet for 10 days) maintained liver GSH concentrations a t 92% of control values in the paracetamol-challenged mouse without al tering GST activity. D-Limonene also increased liver GSH concentration (23%) in mouse fed 1.0% D-limonene alone.