Kinetic and Structural Differences between Cytochrome c Oxidases from Beef Liver and Heart

Abstract

1 The cytochrome content of beef liver mitochondria differs from that of beef heart mitochondria by an eight- fold lower cytochrome aa3 and a twofold lower cytochrome b and c + c₁ content. 2 The kinetic properties of cytochrome c oxidases from beef liver and heart were measured with intact cytochrome c-depleted membranes, deoxycholate-dissolved membranes, and with the isolated enzymes at various cytochrome c concentrations with an oxygen electrode. Under all conditions a higher V was found for the liver enzyme, both for the low-affinity and for the high-affinity binding site for cytochrome c. Differences were also found for the K_m of the two enzymes. 3 Isolated beef heart mitochondria contained about twice as much cardiolipin than beef liver mitochondria. The isolated enzymes contained one mole cardiolipin per mole of the heart enzyme, but 2 moles cardiolipin per mole of the liver enzyme. 4 By application of a high performance sodium dodecylsulfate gel electrophoretic system the two isolated enzymes could be separated into 13 different protein components, three of which (polypeptides VIa, VIIa and VIII) were found to differ in their apparent molecular weights. The functional meaning of cytochrome c oxidase iso- enzymes in liver and heart is discussed.