Modification by calcium ions of adenine nucleotide translocation in rat liver mitochondria

1. Added Ca(2+) stimulates the translocation of ATP by isolated rat liver mitochondria. 2. The apparent K(m) for added Ca(2+) in stimulating the translocation of 200mum-ATP is approx. 160mum (75mum ;free' Ca(2+)). 3. The greatest stimulation of ATP translocation by Ca(2+) occurs at the lower concentrations of ATP. 4. Sr(2+) (and to a lesser extent Ba(2+)) can replace Ca(2+) whereas Mg(2+) and Mn(2+) have only little ability to stimulate ATP translocation. 5. Translocation of dATP is also stimulated by Ca(2+) whereas that of ADP is stimulated to only a relatively small degree. 6. Studies with metabolic inhibitors and uncouplers provide evidence that stimulation by Ca(2+) and by uncouplers is additive and that the mechanism of Ca(2+) stimulation does not seem to involve the high-energy intermediate of oxidative phosphorylation. 7. In the presence of Ca(2+), ATP is able to effectively compete with ADP for translocation. 8. Added K(+) further enhances the ability of Ca(2+) to stimulate ATP translocation. 9. These findings are discussed in relation to the potential involvement of Ca(2+) in modifying enzymic reactions involved in the regulation of cell metabolism.