Mixed function oxidation in perfused rat liver. The effect of aminopyrine on oxygen uptake.

Abstract

Oxygen uptake was monitored in perfused livers from normal and phenobarbital treated rats. Basal rates (200–250 μatoms × g−1× h−1), as well as antimycin A (80–100 μatoms × g−1× h−1) and cyanide (30–35 μatoms × g−1× h−1) insensitive respirations were similar in both groups. The addition of aminopyrine to livers from fed, phenobarbital treated rats caused a maximal stimulation of oxygen uptake of 60 μatoms × g−1× h−1. Half-maximal effect was observed with 0.1–0.2 mM aminopyrine. Prior addition of antimycin A or cyanide did not diminish the stimulation by aminopyrine. In livers from fasted, phenobarbital treated rats, the maximal stimulation was smaller (30–35 μatoms × g−1× h−1), and no increase in oxygen uptake was observed in the presence of antimycin A. The data support the hypothesis that extramitochondrial NADPH generation controls mixed function oxidase activity in the intact cell. Since highest rates were observed under conditions of enhanced glycogenolysis, it is suggested that the main source of NADPH generation is via dehydrogenases of the hexosemonophosphate shunt. In the glycogen depleted state, however, it is predicted that reducing equivalents from mitochondrial oxidations are transferred into the extramitochondrial space. A substrate shuttle mechanism is proposed which provides these reducing equivalents in the form of malate for the extramitochondrial generation of NADPH via malic enzyme.