NAD +-induced phosphate acceptor specificity in submitochondrial systems

Abstract

The nonspecific phosphorylation reaction in submitochondrial systems obtained by sonication became much more specific in the presence of pyridine nucleotides. Phosphorylation of IDP was inhibited by increasing concentrations of NAD +, an inhibition which became biphasic upon the addition of hexokinase (EC 2.7.1.1), whereas phosphorylation of ADP was totally unaffected. The inhibition by NAD + was of a noncompetitive type with K i = 1.5 · 10 −4 M. The nicotinamide part of the molecule was immediately responsible for the inhibitory effect. Although rotenone or antimycin A interfered with the specificity promoted by NAD +, the pyridine nucleotide effect was not due to the introduction of an energy-competing NAD + reduction. No evidence was found of an initiated selective nucleoside triphosphatase reaction, so the regulation was considered to take place at a primary event of the energy conservation mechanism and possibly exerted by an allosteric change. It is suggested that endogenous pyridine nucleotides which are lost upon sonic disintegration of mitochondria may constitute a factor for regulation of the nucleoside specificity during oxidative phosphorylation.