Rates of pentose cycle flux in perfused rat liver. Evaluation of the role of reducing equivalents from the pentose cycle for mixed-function oxidation.
Rates of NADPH production via the pentose phosphate cycle were determined in perfused livers from phenobarbital-treated rats by measuring 14CO2 production from [1-14C]glucose infused in the presence and absence of p-nitroanisole (0.2 mM), a substrate for mixed-function oxidation. In the fed state, basal rates of NADPH generation were 34-44 mumol/g/hr. p-Nitroanisole, which was metabolized at rates of 8.9 mumol/g/hr, stimulated pentose cycle-dependent NADPH production by 21-24 mumol/g/hr. Fasting for 24 hr prior to perfusion diminished pentose cycle flux by 80% and largely abolished the stimulation of the pentose cycle by p-nitroanisole. In contrast, rates of p-nitroanisole O-demethylation were only diminished slightly, to 5.7 mumol/g/hr. Fasting decreased hepatic glucose, glucose 6-phosphate, and 6-phosphogluconate contents drastically as expected. Pretreatment of rats with 6-aminonicotinamide, which is metabolized to a potent inhibitor of 6-phosphogluconate dehydrogenase, decreased rates of NADPH generation via the pentose cycle to 6.9 mumol/g/hr but did not alter rates of p-nitroanisole metabolism (8.8 mumol/g/hr). Basal rates of NADPH generation decreased from 38 to 26 mumol/g/hr during infusion of potassium cyanide (2 mM), an inhibitor of mitochondrial energy metabolism. Cyanide also decreased rates of p-nitroanisole O-demethylation by over 60%; however, stimulation of NADPH generation via the pentose cycle by p-nitroanisole was as great in the presence (17-21 mumol/g/hr) as in the absence of cyanide. Since rates of mixed-function oxidation were unaffected after virtually complete inhibition of the pentose cycle with 6-amino-nicotinamide, it is concluded that reducing equivalents for the mixed-function oxidation of p-nitroanisole are not provided by the pentose cycle under these conditions.
D015120:6-Aminonicotinamide / Q000494:pharmacology; D000818:Animals; D005215:Fasting; D005260:Female; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009249:NADP / Q000378:metabolism*; D010084:Oxidation-Reduction; D010090:Oxidoreductases, O-Demethylating / Q000378:metabolism; D010429:Pentoses / Q000378:metabolism*; D011190:Potassium Cyanide / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains
D010429:Pentoses; D015120:6-Aminonicotinamide; D009249:NADP; C026048:4-nitroanisole O-demethylase; D006899:Mixed Function Oxygenases; D010090:Oxidoreductases, O-Demethylating; D011190:Potassium Cyanide