Sheep liver 6-phosphogluconate dehydrogenase. Inhibition by nucleoside phosphates and by other metabolic intermediates.

A variety of metabolic intermediates of the glycolytic pathway and tricarboxylic acid cycle have been found to be inhibitors of 6-phosphogluconate dehydrogenase isolated from sheep liver. All nucleoside 5’-triphosphates and 5’diphosphates are inhibitors with Ki values averaging 0.8 mM, nucleoside 5’-monophosphates are also inhibitory with Ki values averaging 6 mu. The 2’and 3’monophosphates of adenosine and guanosine are also inhibitory, the 2’-isomers being especially potent inhibitors. The following compounds are also inhibitory with Ki values averaging 6 fIwI: oxalacetate, oxalate, fructose-l-P, fructose-6-P, glucose-6-P, Pi, PPi, citrate, and sulfate. Fructose-1,6-Pz is a potent inhibitor of the enzyme with a Ki value of 70 PM (DYSON, J. E. D., AND D’ORAZIO, R. E. (1971) Biochem. Biophys. Res. Commun. 43, 183). The presence of EDTA prevents inhibition by fructose-1,6-Pz, but not by the other inhibitory compounds. All the nucleoside phosphates, with the exception of 2’AMP, inhibit competitively with respect to both 6-phosphogluconate and NADP+, as do citrate, PPi, Pi, and SUMfate. 2’-AMP inhibits competitively with respect to NADP+ and noncompetitively with respect to 6-phosphogluconate. The hexose monophosphates inhibit competitively with respect to 6-phosphogluconate and noncompetitively with respect to NADP+. Total inhibition caused by combinations of two or three nucleoside phosphates indicates that binding to the enzyme is mutually exclusive for all nucleoside phosphates except 2’-AMP, indicating that 2’-AMP binds at a different site from the other nucleoside phosphates. Binding of citrate and the hexose monophosphates is also mutually exclusive with the nucleoside phosphates, except 2’-AMP, indicating overlap of the binding site of citrate and the hexose monophosphates with that of the nucleoside phosphates.