Factors affecting the respiratory control ratio of rabbit heart mitochondria.

Factors affecting the respiratory control ratio of isolated rabbit heart mitochondria have been studied. The magnitude of the control ratio was shown to be affected by the presence of endogenous substrates, the nature of exogenous substrate, and the concentrations of mitochondria and of adenosine triphosphate. Higher control ratios were obtained in the presence of endogenous substrates and at a high mitochondrial concentration while lower control ratios usually resulted from an increase in ATP concentration. The endogenous substrates appear to act by maintaining a high capacity for activated respiration. An increase in mitochondrial concentration tended to increase the State 3, and to decrease the State 4 Qoz. An increase in ATP concentration tended to increase the State 4 rate. The respiratory control ratio of a single preparation may vary as a result of alterations in any one or several of these factors. The occurrence of “anomalous” State 3 to 4 transitions has been noted and possible origins of the effect are discussed.