Reducing equivalents for mixed function oxidation in periportal and pericentral regions of the liver lobule in perfused livers from normal and phenobarbital-treated rats.
The supply of NADPH for cytochrome P-450-dependent mixed function oxidation from the pentose cycle and mitochondria in periportal and pericentral regions of the liver lobule was evaluated in perfused rat liver. Rates of 7-ethoxycoumarin O-deethylation in livers from fed, normal rats monitored with micro-light guides placed on periportal and pericentral regions were 1.2 mumol/g/hr in both regions of the liver lobule. In livers from fed, phenobarbital-treated rats, rates were 3.6 and 7.0 mumol/g/hr in periportal and pericentral regions, respectively. Following treatment of rats with 6-aminonicotinamide, an inhibitor of the pentose cycle, rates of 7-hydroxycoumarin production were approximately 0.9 mumol/g/hr in both regions of the lobule in livers from normal rats and 2.1 and 3.4 mumol/g/hr in periportal and pericentral regions, respectively, in livers from phenobarbital-treated rats. Based on the difference in rates of 7-hydroxycoumarin production in the presence and absence of 6-aminonicotinamide, we conclude that the pentose cycle supplies NADPH for 7-ethoxycoumarin metabolism at rates around 0.3 mumol/g/hr in both regions of the liver lobule in livers from normal rats and 1.5 and 3.6 mumol/g/hr in periportal and pericentral regions, respectively, in livers from phenobarbital-treated rats. Potassium cyanide, an inhibitor of mitochondrial oxidation, reduced rates of 7-ethoxycoumarin O-deethylation to approximately 0.6 mumol/g/hr in both regions of the liver lobule in livers from fed, normal rats and to around 0.2 mumol/g/hr after fasting or treatment with 6-aminonicotinamide. In livers from fasted, phenobarbital-treated rats, 7-hydroxycoumarin was produced at rates of 0.3 and 0.7 mumol/g/hr in periportal and pericentral regions, respectively, in the presence of KCN. Decreases in rates of 7-hydroxycoumarin production during KCN infusion indicate that the mitochondria supply about 0.7 mumol of NADPH/g/hr for 7-ethoxycoumarin metabolism in both regions in livers from normal rats and 1.3 and 2.7 mumol/g/hr in periportal and pericentral regions in livers from phenobarbital-treated rats. The sum of KCN and 6-aminonicotinamide-sensitive rates of 7-ethoxycoumarin metabolism closely approximated rates measured in the absence of the inhibitors. These data indicate that mitochondria supply 50 to 70% of the reducing equivalents for mixed function oxidation of 7-ethoxycoumarin in both regions of the liver lobule in livers from fed rats.
D015660:7-Alkoxycoumarin O-Dealkylase; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005215:Fasting; D005260:Female; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008930:Mitochondria, Liver / Q000378:metabolism; D009243:NAD / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D010105:Oxygenases / Q000378:metabolism*; D010477:Perfusion; D010634:Phenobarbital / Q000494:pharmacology*; D011190:Potassium Cyanide / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014018:Tissue Distribution
D009243:NAD; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; D010105:Oxygenases; D015660:7-Alkoxycoumarin O-Dealkylase; D011190:Potassium Cyanide; D010634:Phenobarbital