Differential effects of Mg 2+ on various hydrolytic and synthetic activities of multifunctional glucose-6-phosphatase

Abstract

The effects of added Mg 2+ on various hydrolytic and synthetic activities of rat hepatic d-glucose-6-phosphate phosphohydrolase (EC 3.1.3.9) have been investigated in detail under carefully controlled conditions, with four different types of enzyme preparations. In contrast with information in certain texts, supplemental Mg 2+ (0.1–15 m m) was found to exert little (<5%) or no stimulatory effect on glucose-6- P phosphohydrolase activity at any pH studied (pH 4.5–7). Mannose-6- P phosphohydrolase and mannose-6- P: glucose phosphotransferase activities of the enzyme were likewise unaffected. In contrast, carbamyl- P: glucose phosphotransferase was rather modestly inhibited by Mg 2+, while activities involving PP i, CTP, ATP, GTP, or ADP as substrates were extensively inhibited. Supplemental Mg 2+ failed to shift the pH-activity profile of ATP-glucose phosphotransferase toward alkalinity. Supplemental Mg 2+ did, however, ameliorate inhibition by ATP of glucose-6- P phosphohydrolase activity at pH 7.5. On the basis of various kinetic observations and a possible correlation between the binding constants of various phosphate substrates with Mg 2+ and observed extents of inhibition by Mg 2+ of enzymic activity with such compounds, it is concluded that inhibition (when observed) probably involves an initial chelation of added Mg 2+ with phosphate substrates which in turn precludes or significantly diminishes the ability of these substrates to bind to the metalloprotein enzyme. A variety of activity-discriminant directive metabolic implications, which are apparent from these observations, are discussed briefly. A possibly role for Mg 2+ in the control of blood sugar levels is suggested.