Carbamyl phosphate glucose transferase activity of glucose-6-phosphatase in the isolated rat hepatocyte

Abstract

Glucose-6-phosphatase (G-6-Pase) will phosphorylate glucose using many compounds as phosphate donors. This activity has been manifested when microsomes are disrupted by exposure to detergent, high pH, or other similar treatment. Others have suggested that this reaction has physiological significance and of the possible phosphoryl donors, carbamyl phosphate is the most likely to be involved in the cell. In our study, rates of glucose phosphorylation were determined in isolated intact hepatocytes incubated under conditions where intracellular CP formation was favored (glucose plus NH4Cl) or diminished (glucose, NH4Cl, and ornithine, with and without malate). Results showed that when CP formation was favored, less glucose was phosphorylated than under control conditions (glucose alone). When CP was diminished by the addition of ornithine or ornithine plus malate, no decrease in glucose phosphorylation occurred. The lack of a decrease under these conditions shows that NH4Cl addition did not mask a stimulation of phosphorylation by CP. Further studies with homogenates of liver cells isolated from these animals did not show an inhibition of glucokinase by NH4Cl or CP. Thus we have shown that when CP formation was stimulated, glucose phosphorylation was not. If CPGT activity were important in the cell, then under these conditions this should be observed. Since it was not, we conclude that the carbamyl phosphate glucose transferase (CPGT) activity of G-6-Pase is unlikely to have physiological significance.