Studies on the regulation of mitochondrial ATP formation in rabbit reticulocytes.

Respiration and phosphorylation in functionally intact isolated reticulocyte mitochondria are controlled by the extramitochondrial ATP/ADP ratio. Mitochondrially bound hexokinase exhibits an apparent preference against mitochondrially formed ATP, as indicated by the lower apparent Km values under phosphorylating conditions. Thereby the control characteristics are shifted to higher ATP/ADP ratios compared to hexokinase free mitochondria. Respiration of intact reticulocytes was estimated to be smaller than had to be expected from the ATP/ADP ratios of total cells roughly corresponding to the cytosolic values. Inhibition of respiration leads to small drop of total adenine nucleotides and lowers the phosphorylation potential directly determined or calculated from metabolite indicators. Compartmentation of ADP by protein binding is discussed as a possible reason for the difference of determined and calculated phosphorylation potentials and the apparent discrepancy between the regulation of mitochondrial ATP-formation in isolated mitochondria and intact reticulocytes.