Developmental aspects of xenobiotic transformation

Area of Science:

• Pharmacology
• Biochemistry
• Developmental Biology

Background:

• Monooxygenases, particularly cytochrome P-450 (CYP450) enzymes, are crucial for drug metabolism.
• In most laboratory animals, fetal organ monooxygenase activity is minimal until late gestation.
• Understanding perinatal enzyme activity is vital for assessing drug safety and efficacy in newborns.

Purpose of the Study:

• To investigate the developmental patterns of hepatic cytochrome P-450-dependent reactions in perinatal rats.
• To characterize the basic kinetics (Vmax, Km) and inducibility of specific CYP450-mediated reactions.
• To elucidate the reasons for low monooxygenase activity in the perinatal period.

Main Methods:

• Kinetic analysis of various CYP450-dependent reactions (O-demethylation, N-demethylation, hydroxylation) using liver preparations from newborn and adult rats.
• Measurement of Vmax, Km, and phenobarbital inducibility.
• Assessment of NADPH/NADH cytochrome P-450 reductase activity and metyrapone binding.

Main Results:

• Significant differences in developmental patterns were observed for O-demethylation of codeine, phenazone hydroxylation, N-demethylation of amidopyrine, and N-demethylation of ethylmorphine.
• Postnatal development of Km and phenobarbital inducibility varied markedly between reactions.
• Low perinatal monooxygenase activity was not attributed to cofactor availability, reductase activity, or mitochondria-endoplasmic reticulum interaction, but to a lower percentage of active CYP450.

Conclusions:

• The distinct developmental profiles of various CYP450-dependent reactions suggest the involvement of different CYP450 isoforms.
• The low perinatal hepatic monooxygenase activity in rats is primarily due to a reduced proportion of active cytochrome P-450.
• These findings highlight the complex developmental regulation of drug-metabolizing enzymes.