Abstract
Studies were carried out on the developmental profile of PB-cytochrome P-450 and cytochrome P-448-dependent mixed-function oxidase activities in the foetal and neonatal rat. PB-Cytochrome P-450 activity, as examplified by benzphetamine N-demethylase was low at birth but increased with age. In contrast, cytochrome P-448 activity, as exemplified by ethoxyresorufin 0-deethylase (EROD) and biphenyl 2-hydroxylase, was higher in the neonate, reaching maximum levels at about two weeks postpartum and decreased with age. Cytochrome P-448 inducibility by 3-methylcholanthrene was low at birth and increased with age. Investigations were also carried out on the induction of cytochrome P-448, as measured by the 0-deethylation of ethoxyresorufin, by carcinogens and several other xenobiotics in hepatic and extrahepatic tissues of the adult male rat. EROD activity was highest in the liver, followed by the kidney > lung, in the untreated animal; activity was not detectable in the heart and the brain. Treatment with carcinogens enhanced EROD activity most markedly in the liver and to a smaller extent, in the kidney and lung. In addition, induction of EROD activity was achieved with very low doses of the inducing agent. A single intraperitoneal dose of 50 ug/kg of benzo(a)pyrene increased hepatic EROD activity by 2-fold. Activity was also significantly enhanced by single intraperitoneal doses of 0.5 mg/kg of 2-acetamidofluorene or safrole. Maximum induction was achieved at dose levels of 2-5 mg/kg, with peak activity occurring 24 hours after dosing. In addition, the O-dealkylations of a series of alkoxyresorufins (1C to 8C) and the debenzylation of benzyloxyresorufin were induced by 3-MC only in the responsive C57BL/6 mice but not in the non-responsive DBA/2 mice, indicating that these activities are associated with the Ah locus. The present study showed that carcinogens induced the O-dealkylation of the short-chain alkoxyresorufins (methoxy- to butoxy-) to a greater extent than did the long-chain derivatives. In addition, heptoxyresorufin dealkylation was most markedly enhanced by DDT while benzyloxyresorufin debenzylation was induced to similar extents by carcinogens and non-carcinogens. Although cimetidine has been shown to be an inhibitor of hepatic drug metabolism by binding to cytochrome P-450 through its imidazole ring structure, the present study demonstrated that neither cimetidine nor its nitrosated derivative, nitrosocimetidine exhibited any significant inhibition towards cytochrome P-448 activity in the rat, as measured by the EROD assay, even when administered at five times the oral therapeutic dose in man. Results of the present study indicate that the induction of cytochrome P-448 is associated with chemical toxicity/carcinogenicity. In addition, the 0-deethylation of ethoxyresorufin is a very sensitive and highly specific marker for cytochrome P-448 activity.