Coupling of adenosine triphosphate formation in mitochondria to the formation of nucleoside triphosphates. Involvement of nucleoside diphosphokinase

Addition of the nucleoside diphosphates CDP, UDP, TDP, and IDP to intact, respiring, rat liver mitochondria immediately following ATP formation results in a marked stimulation of respiration at all concentrations of nucleotide tested. Addition of GDP stimulates respiration when added at concentrations below 150 μM, whereas at higher concentrations this nucleotide has much less effect on respiration and prevents stimulation given by other nucleoside diphosphates. Half maximal stimulation of respiration is provided by less than 150 μM of each nucleotide. Nucleoside triphosphate formation is associated directly with stimulation of respiration by nucleoside diphosphates. CTP, UTP, TTP, and ITP are formed in food yield at all concentrations of nucleoside diphosphate tested, whereas GTP is formed in best yield at concentrations of GDP below 150 μM. Concentrations of nucleoside diphosphates which give half maximal rates of nucleoside triphosphate formation correlate well with concentrations necessary to give half maximal stimulation of respiration. Concentrations of GDP higher than 150 μM inhibit both the formation of GTP and the formation of other nucleoside triphosphates. Oligomycin and 2,4 dinitrophenol, inhibitors of oxidative phosphorylation, also inhibit nucleoside triphosphate formation. Partial removal of nucleoside diphosphokinase localized in the outer mitochondrial compartment markedly reduces both nucleoside diphosphate stimulation of respiration and nucleoside triphosphate formation. Kinetic analysis of the solubilized preparation reveals typical Michaelis Menten kinetics for the substrate pairs ATP-CDP, ATP-UDP, and ATP-TDP, a slight deviation from normal kinetic behavior for the substrate pair ATP-IDP, and a marked deviation from normal behavior for the substrate pair ATP-GDP. GDP is a potent inhibitor of the enzyme at concentrations exceeding 150 μM and, as a result, biphasic kinetic response curves are obtained when the substrate pair is ATP-GDP.