Pyridine nucleotide involvement in rat hepatic drug metabolism. The influence of NADH on the NADPH kinetics of phenobarbital pre-treated rat microsomes.
Apparent kinetic constants for NADPH have been calculated for NADPH-cytochrome P450 reductase and demethylase activity in the presence of aminopyrine and ethylmorphine with hepatic microsomal preparations from phenobarbital pre-treated rats. The presence of NADH changed Km values but not the V values for cytochrome P450 reductase. In addition, NADH reduced Km values for demethylase activity and had a biphasic effect on V values; elevating them at high and depressing them at low concentrations. The V values at different NADH concentrations appeared to correlate with the concentration of reduced cytochrome b5 that the added NADH could sustain during the incubation period. Tetrahydronicotinamide adenine dinucleotide (NADH3), a structural analogue of NADH, reduced demethylase activity without affecting NADPH-cytochrome P450 reductase activity or the concentration of reduced cytochrome b5. As this analogue cannot reduce biological systems, the depressed V values for demethylation suggest that in the presence of NADH3 and NADPH there exists an alternative mechanism for the supply of electrons from NADPH to the partially reduced oxygenated cytochrome P450-substrate complex.
D000818:Animals; D003573:Cytochrome b Group; D003580:Cytochromes / Q000378:metabolism; D003640:Dealkylation; D004579:Electron Transport; D004790:Enzyme Induction; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism; D009243:NAD / Q000494:pharmacology*; D009249:NADP / Q000378:metabolism*; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D004364:Pharmaceutical Preparations / Q000378:metabolism*; D010634:Phenobarbital / Q000494:pharmacology*; D051381:Rats
D003573:Cytochrome b Group; D003580:Cytochromes; D004364:Pharmaceutical Preparations; D009243:NAD; D009249:NADP; D009251:NADPH-Ferrihemoprotein Reductase; D010634:Phenobarbital