Differences between the ATP/ADP Ratios in the Mitochondrial Matrix and in the Extramitochondrial Space

Abstract

The relationship between the ATP/ADP ratios inside and outside the mitochondria was studied using silicon‐layer centrifugation. As predicted from the properties of the adenine nucleotide translocation, the ATP/ADP ratio in the extramitochondrial space was found to be higher than in the mitochondria when in the state of respiratory control. Taking into account the content of inorganic phosphate, the phosphorylation potential in the mitochondria is calculated to be 2.3 kcal lower than in the medium. This difference between the internal and external ATP/ADP ratios is abolished if uncoupler is added, When there is also no Mg2+ present in the medium, the internal ATP/ADP ratio may be even higher than the external ratio, provided that there was oligomycin added in order to inhibit ATP hydrolysis. It is shown that the ADP is transported against a gradient from the extramitochondrial space into the mitochondria. The discussion of a model suggests that the physiological role of the energy‐requiring transport of ADP into the mitochondria is to maintain a constant level of the ATP in the cytosol. Copyright © 1972, Wiley Blackwell. All rights reserved