Investigation of the dependence of the intramitochondrial [ATP]/[ADP] ratio on the respiration rate

Abstract

The relationship between the respiration rate and the intra- and extramito-chondrial adenine nucleotides was investigated in isolated rat liver mitochondria. For the determination of adenine nucleotide patterns in both compartments a new procedure was developed, based on the evaluation of these metabolites from incubation of various amounts of mitochondria under identical stationary states of oxidative phosphorylation. These identical states were adjusted by addition of appropriate amounts of hexokinase to a glucose-containing incubation mixture. Adenine nucleotides were measured in aliquots of the total extract of the incubation mixture without any separation. The concentrations of the adenine nucleotides in both compartments were obtained from a plot of the total concentration of these species versus mitochondrial protein. Disturbances of this method by unspecific efflux of adenine nucleotides could be excluded. The results obtained for the total adenine nucleotide content (12 nmol · mg −1 protein) and the intramitochondrial [ATP]/[ADP] ratio (about 4 in the resting state) are in good agreement with data obtained by other methods. Strong evidence is provided for a decrease of the intramitochondrial [ATP]/[ADP] ratio with increasing rate of oxygen consumption. Therefore it is not necessary to assume a microcompartmentation of the intramitochondrial adenine nucleotide pool in respect to the ATPase reaction and the adenine nucleotide translocation.