Respiratory Enzymes in Oxidative Phosphorylation

In a complex enzymatic sequence the site of action of substrates and inhibitors is clearly marked by the way they affect the steady state concentrations of the components of the system (1). Antimycin A, for example, inhibits respiration in the succinic oxidase system (2) and at the same time increases the steady state reduction of cytochrome b and decreases that of cytochromes c, a, and aa (3). In the oxidative phosphorylation system of liver mitochondria, phosphate and phosphate acceptors cause a considerable activation of respiration and may do so by a reversal of inhibitory reactions along the respiratory chain (4-6). Thus measurements of changes in the steady state of the members of the respiratory chain upon initiation and cessation of oxidative phosphorylation of ADP’ may identify sites in the chain where the phosphorylation reactions occur. It has been possible to study six members of the respiratory chain of intact mitochondria by spectrophotometric methods. Their probable sequence of action (7) and the appropriate pairs of wave-lengths (in millimicrons) used in their measurement are as follows: