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THE SCANDINAVIAN SOCIETY FOR PERINATAL MEDICINE

The overall activity of the oxidative enzyme systems for drug metabolism is known to be very low in hepatic microsomes derived from fetal and newborn animals. The recent identification of the component members of the microsomal electron transport chain prompted the present study. The rate-limiting factor in the oxidative drug metabolic process appears to be not the amount of cytochrome-P450 (the last component of the chain) but rather the rate of the NADPH-mediated reduction of this cytochrome. Measurement of these molecular components may explain why activity is low in the neonate and may also elucidate the nature of the regulator($ of this enzyme system. Activity of the mixed-function oxidase system and its components has been measured in fetal, neonatal, and maternal liver and placenta both in control rabbits and following phenobarbital administration. The demethylation of aminopyrine and hydroxylation of triamcinolone and benzpyrene was minimal in fetal and neonatal liver and placenta. Activity could be induced by administration of phenobarbital to the pregnant doe. A much greater (10-fold) increase in cytochrome-P450 content and activity of the NADPH-mediated cytochrome reductase resulted from phenobarbital treatment. A possible explanation for these findings may be the oxygen-dependence of the cytochrome system.