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

The dependence of the mitochondrial respiratory rate on the reduction of cytochrome c has been measured as a function of the exogenous $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ ratio and pH. The respiratory rate at $\text{[}\text{ADP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ values of less than 10^-1m^-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 $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ ratio is increased from 10^?1m^?1 to 10⁴m^?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 $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ ratio; (ii) the level of reduction of cytochrome c (or the intramitochondrial $\text{[}\text{NAD}{}^{\mathrm{+}}\text{]}\text{[NADH])}$ at different $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ values; (iii) the pH of the suspending medium. This rate expression appears to correctly predict the relationships of the cytoplasmic $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ ratio, the mitochondrial $\text{[}\text{NAD}{}^{\mathrm{+}}\text{]}\text{[}\text{NADH}\text{]}$ ratio, and the mitochondrial respiratory rate in intact cells as well as suspensions of isolated mitochondria.