The metabolism of pyruvate by bovine-adrenal-cortex mitochondria.

Abstract

1 The metabolism of pyruvate by isolated bovine-adrenal-cortex mitochondria has been studied. The rate of pyruvate oxidation depends on the availability of oxaloacetate. Oxalo-acetate is formed from malate but malate cannot function in a catalytic manner in the Krebs cycle, because some is oxidised to pyruvate via malic enzyme activity with resulting diminution of the oxaloacetate pool. 2 An additional source of oxaloacetate is provided by low levels of pyruvate carboxylase. In the presence of ATP and bicarbonate, the principal initial product of pyruvate metabolism is citrate. Reducing equivalents for 11β-hydroxylation of deoxycorticosterone may be provided by oxidation of citrate. 3 When oxaloacetate is supplied by malate oxidation, the rate of citrate formation in the presence of fluorocitrate is controlled by the rate of electron flux through either the respiratory chain or the hydroxylation chain, and is increased by addition of ADP on the one hand, or deoxy-corticosterone on the other. This appears to be due to reoxidation of the pyridine nucleotides resulting in increased availability of oxaloacetate for citrate formation. Equilibration of NAD+/NADH and NADP+/NADPH is achieved by transhydrogenase activity but this activity is not energy-linked under the conditions of these experiments. 4 In the absence of fluorocitrate, the stimulation of citrate formation brought about by initiation of 11β-hydroxylation results in increased Krebs cycle activity. In the transition from state 4 to state 3 respiration, there is an additional activation of citrate oxidation, which is interpreted to be due to activation of NAD+-linked isocitrate dehydrogenase on addition of ADP. 5 Addition of both deoxycorticosterone and ADP together to the mitochondria results in inhibition of state 3 electron flux through the respiratory chain, with concomitant inhibition of ATP formation. The rate of 11β-hydroxylation is not affected. Inhibition of ATP formation can be achieved by a variety of steroids and correlates with inhibition of NADH oxidase.