Pyridine nucleotide involvement in rat hepatic microsomal drug metabolism--III. The influence of the 1,4,5,6-tetrahydronicotinamide analogue of NADP on the NADPH kinetics of aminopyrine-N-demethylation.

1,4,5,6-Tetrahydronicotinamide adenine dinucleotide (NADH₃), a structural analogue of NADH, was unable to support the demethylation of aminopyrine or the reduction of the cytochrome P450-aminopyrine complex. However, the combination of NADH₃ with NADPH stimulated the NADPH dependent reduction of the cytochrome P450-aminopyrine complex. There was no significant alteration in the apparent K_m (NADPH) value, but there was an 80 per cent increase in apparent V of NADPH for NADPH-cytochrome P450-reductase (plus aminopyrine) when the kinetic constants were determined in the presence of 100 μM NADH₃. The inclusion of NADH₃ in the medium for aminopyrine demethylation also resulted in a significant increase in apparent V compared to the value obtained in the absence of NADH₃. The results suggest that the structure of NADH, as well as its capacity to donate the electron, is responsible for the NADH mediated increase in aminopyrine metabolism.