The “malate shuttle” and control of steroid hydroxylation in the adrenal cortex

Studies on the distribution of malic enzyme in bovine adrenal cortex have shown that this tissue contains two forms of the enzyme, one in the cytosol, and the other in the mitochondria. The mitochondrial malic enzyme activity has been shown to be a source of NADPH for mitochondrial mixed-function oxidations in this tissue. A study of the properties of these enzymes has indicated the kinetic reversibility of the reaction catalyzed by the cytosol malic enzyme. The reaction catalyzed by the mitochondrial enzyme, however, was not readily reversible. On this basis, a “malate shuttle” has been postulated, effecting the net transfer of NADPH reducing equivalents into the mitochondria, by the operation of the cytosol malic enzyme to produce malate from pyruvate and carbon dioxide.