Liver Microsomal Glucose 6-phosphatase, Inorganic Pyrophosphatase, and Pyrophosphate-glucose Phosphotransferase. Ii. Kinetic Studies.

Recently, we (1) described experiments which strongly supported the common identity of glucose 6-phosphatase,’ inorganic pyrophosphatase, pyrophosphate-glucose phosphotransferase, and mannose B-phosphate-glucose phosphotransferase activities present in a preparation obtained by ammonium sulfate fractionation of deoxycholate-dispersed rat liver microsomes. Previously, Segal, Washko, and Lee (2, 3) and Hass and Byrne (4) concluded, from studies of the glucose-14C-glucose 6-phosphate exchange reaction and apparent glucose inhibition of sugar phosphate hydrolysis catalyzed by microsomal glucose 6-phosphatase, that the reaction mechanism involved formation of a binary enzyme-glucose 6-phosphate complex, followed by dissociation of glucose from the enzyme, leaving a phosphorylenzyme intermediate which then could transfer the phosphoryl group either to water (hydrolysis) or to glucose-14C (exchange). Their data differed, however, in that Segal et al. found that glucose appeared to be a noncompetitive inhibitor of the hydrolysis reaction while Hass and Byrne observed an apparent inhibition which was not of the “classical” type. The newly found phosphotransferase reactions afforded an independent, analytically highly sensitive means of supplementing previous kinetic studies (2-4) of glucose 6-phosphatase. Experiments indicating that the previously postulated glucose 6-phosphatase mechanism also applies to the phosphotransferase reactions, and confirming Segal’s (2) experimental observations, are described in this paper.