Control of mitochondrial respiration: A quantitative evaluation of the roles of cytochrome c and oxygen

Abstract

The dependence of the mitochondrial respiratory rate on the reduction of cytochrome c has been measured as a function of the exogenous [ ATP] [ ADP][ P i ] ratio and pH. The respiratory rate at [ ADP] [ ADP][ P i ] values of less than 10 -1 m -1 is proportional to the reduction of cytochrome c and independent of pH from pH 6.5 to pH 8.O. The maximal turnover number (at 100% reduction) for cytochrome c is approximately 70 s −1. As the [ ATP] [ ADP][ P i ] ratio is increased from 10 −1 m −1 to 10 4 m −1, the respiration at any given level of reduction of cytochrome c is progressively inhibited. Greater inhibition is observed at more oxidized levels of cytochorme c with respiratory control values for oxidation of reduced cytochrome c exceeding 10. The behavior of mitochondrial respiratory control is shown to be quantitatively consistent with a proposed mechanism in which the regulation occurs in the reaction of oxygen with cytochrome oxidase. A steady-state rate expression is derived which fits the mitochondrial respiratory rate dependence on (i) the extramitochondrial [ ATP] [ ADP][ P i ] ratio; (ii) the level of reduction of cytochrome c (or the intramitochondrial [ NAD + ] [NADH]) at different [ ATP] [ ADP][ P i ] values; (iii) the pH of the suspending medium. This rate expression appears to correctly predict the relationships of the cytoplasmic [ ATP] [ ADP][ P i ] ratio, the mitochondrial [ NAD +] [ NADH] ratio, and the mitochondrial respiratory rate in intact cells as well as suspensions of isolated mitochondria.