Maintenance of nicotinamide dinucleotide phosphate content and oxidation-reduction state during mixed-function oxidation of p-nitroanisole in isolated perfused livers of various species.
The influence of p-nitroanisole, a substrate for mixed-function oxidation, on total NADP+ and NADPH and NADP+/NADPH ratios was examined in perfused livers from three different species. Studies were performed using livers from Sprague-Dawley rats, Syrian golden hamsters and C57BL/6J mice. Although rates of p-nitroanisole O-demethylation varied more than 16-fold in perfused livers from these species, NADP+/NADPH ratios calculated from measured concentrations of NADP+ and NADPH and from ratios calculated from substrate pairs assumed to be in near equilibrium with NADP+-dependent dehydrogenases remained remarkably constant under most conditions. Thus, rates of NADPH utilization and generation must be tightly coupled in perfused livers during high rates of mixed-function oxidation. Exceptions to the general pattern noted above occurred in livers of fasted, phenobarbital-treated rats where carbohydrate reserves were depleted and in livers from 3-methyl-cholanthrene-treated mice where rates of p-nitroanisole O-demethylation were exceptionally high. Livers from fed phenobarbital-treated rats displayed a paradoxical decrease in NADP+/NADPH ratios reflecting reduction calculated from substrates assumed to be in near equilibrium with 6-phosphogluconate dehydrogenase during mixed-function oxidation, suggesting that NADPH generation exceeded NADPH utilization in the rat in the fed state. In contrast, the NADP+/NADPH ratio calculated from measured pyridine nucleotides increased in livers of 3-methylcholanthrene-treated mice perfused with p-nitroanisole, reflecting oxidation. Moreover, the NADP+/NADPH ratio calculated from substrates assumed to be near equilibrium with 6-phosphogluconate dehydrogenase increased in livers of fasted rats, suggesting that utilization of NADPH exceeded generation. Thus, adequate carbohydrate reserves appear essential for maintenance of NADPH during high rates of mixed-function oxidation.
D000818:Animals; D000840:Anisoles / Q000378:metabolism*; D006224:Cricetinae; D005260:Female; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008647:Mesocricetus; D008748:Methylcholanthrene / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009249:NADP / Q000378:metabolism*; D010084:Oxidation-Reduction; D010477:Perfusion; D010634:Phenobarbital / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013045:Species Specificity
D000840:Anisoles; D009249:NADP; D008748:Methylcholanthrene; D006899:Mixed Function Oxygenases; C005306:4-nitroanisole; D010634:Phenobarbital