Inducibility of cytochromes P-450 by dioxin in liver and extrahepatic tissues of the marmoset monkey ( Callithrix jacchus)

Abstract

Cytochrome P-450 induction was investigated in the marmoset monkey, a non-human primate, using dioxins as inducing agents. Animals received a single subcutaneous dose of 1.6 nmol tetrachlorodibenzo- p-dioxin or tetrabromodibenzo- p-dioxin/kg body weight. Microsomal fractions were prepared from liver, lung and kidney, and homogenates were prepared from gut and adrenal glands. Anti-peptide antibodies which bind to CYPIA1, CYPIA2, CYP2B6 and CYP3A4 in human were used to identify related forms in the marmoset. The results indicate that CYPIA2 is constitutively expressed in liver, but not in lung, kidney, gut or adrenal gland and that CYPIAI is not expressed in any of these tissues in untreated animals. Treatment with dioxin induced both CYPIAI and CYPIA2 in liver, but only CYPIA1 in lung. No induction of CYPIA1 or CYPIA2 was found in kidney, small intestine or adrenal glands. Methoxy-, ethoxy-, pentoxy- and benzoyloxyresorufin O-dealkylases and high affinity phenacetin O-deethylase activities were induced in the liver, whereas ethoxycoumarin O-deethylase and aryl hydrocarbon hydroxylase activities were not affected by dioxin treatment. High-affinity phenacetin O-deethylase and CYPIA2 apoprotein were detected only in liver, consistent with this activity being specifically catalysed by CYPIA2. Furafylline was found to be a competitive inhibitor of methoxyresorufin O-demethylase activity with a K i of 10 μM. In the lung the induction of CYPIAI was accompanied by 15- and 23-fold increases in ethoxyresorufin O-deethylase and methoxyresorufm O-demethylase activities, respectively, suggesting that both activities are catalysed by CYP1A1. In contrast, there was no induction of aryl hydrocarbon hydroxylase activity in lung or liver showing that, unlike in many other species, marmoset CYPIAI does not catalyse this reaction efficiently. The expression, distribution, induction and substrate specificities of marmoset monkey P-450 enzymes differ from the situation found in rodents and other species, demonstrating that caution has to be exercised when making cross-species extrapolations.