Control of respiration in isolated mitochondria: Quantitative evaluation of the dependence of respiratory rates on [ATP], [ADP], and [P_i]

Mitochondrial preparations with respiratory control ratios greater than 10 have been isolated from dog and pigeon heart muscle. Suspensions of these mitochondria have been tested for the qualitative and quantitative dependence of the respiratory rate on the extramitochondrial ATP, ADP, and orthophosphate (P_i) concentrations. Reciprocal dependence is observed for [ATP] and [P_i] (the [ADP] required for a given rate of oxygen consumption is proportional to [ATP] and inversely proportional to [P_i]) which indicates that these two reactants are of equal importance in regulating the mitochondrial respiratory rate. Analysis of the [ATP], [ADP], and [P_i] in suspensions of mitochondria at various respiratory rates shows that the respiratory rate is proportional to the logarithm of the [ATP]/[ADP][P_i] for values between 10³ and 10⁵, m^-1. Similar results are obtained for suspensions of mitochondria prepared from rat brain, rat kidney, and rat liver. These data demonstrate that, in mitochondria with high respiratory control, the rate of respiration is dependent on extramitochondrial [ATP]/[ADP][P_i] and not on [ATP]/[ADP]. It is concluded that [P_i]-independent adenine nucleotide translocation across the inner mitochondrial membrane is not a rate-limiting reaction in oxidative phosphorylation.