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

Abstract

1. Added Ca2+stimulates the translocation of ATP by isolated rat liver mitochondria. 2. The apparent Km for added Ca2+in stimulating the translocation of 200μm-ATP is approx. 160μm (75μm ‘free’ Ca2+). 3. The greatest stimulation of ATP translocation by Ca2+occurs at the lower concentrations of ATP. 4. Sr2+(and to a lesser extent Ba2+) can replace Ca2+whereas Mg2+and Mn2+have only little ability to stimulate ATP translocation. 5. Translocation of dATP is also stimulated by Ca2+whereas that of ADP is stimulated to only a relatively small degree. 6. Studies with metabolic inhibitors and uncouplers provide evidence that stimulation by Ca2+and by uncouplers is additive and that the mechanism of Ca2+stimulation does not seem to involve the high-energy intermediate of oxidative phosphorylation. 7. In the presence of Ca2+, ATP is able to effectively compete with ADP for translocation. 8. Added K+further enhances the ability of Ca2+to stimulate ATP translocation. 9. These findings are discussed in relation to the potential involvement of Ca2+in modifying enzymic reactions involved in the regulation of cell metabolism.