p-Nitroanisole O-demethylation in perfused hamster liver. High rates of pentose cycle-independent mixed-function oxidation.

Rates of p-nitroanisole O-demethylation in perfused livers from Syrian golden hamsters were three to four times greater than comparable rates measured in preparations from Sprague-Dawley rats. Hamsters also had greater microsomal p-nitroanisole O-demethylase activity and cytochrome P-450 contents than rats. In general, phenobarbital caused similar increases in these properties in both species. Fasting of hamsters for 24 hr increased p-nitroanisole O-demethylase activity in microsomes but did not affect rates in perfused livers. Rates were also unaffected in the perfused liver by pretreatment with 6-aminonicotinamide, an inhibitor of the pentose phosphate shunt. Hamster livers had low activities of pentose cycle enzymes but high activities of malic enzyme and isocitrate dehydrogenase compared to rats. In hamster livers, maximal rates of p-nitroanisole O-demethylation were not maintained but declined steadily over 40 min with prolonged p-nitroanisole infusion. The decreased rates of mixed-function oxidation in the non-recirculating perfusion system could not be explained by diminished tissue viability or degradation of cytochrome P-450 but were likely due to a decline in the formation of reduced cofactor. Hepatic concentrations of alpha-ketoglutarate and malate increased during p-nitroanisole infusion. Furthermore, rates of p-nitroanisole O-demethylation were inhibited by ethanol and aminooxyacetate, agents which inhibit the generation and/or movement of mitochondrial reducing equivalents into the cytosol. The infusion of pyruvate stimulated p-nitroanisole O-demethylation in perfused livers from fasted hamsters. This effect was maximal with 0.1 mM pyruvate, did not require gluconeogenesis, and was insensitive to 6-aminonicotinamide treatment. Thus, stimulation of p-nitroanisole metabolism by pyruvate in hamster livers is likely related to the mitochondrial oxidation of pyruvate, rather than to increased NADPH generation via the pentose phosphate cycle. These data indicate that mitochondrial sources of NADPH supply reducing equivalents for mixed-function oxidation in hamster liver.