Studies on the Phospholipid Requirement of Glucose bPhosphatase*

The role of phospholipid in rat liver microsomal glucose 6phosphatase has been investigated with the use of phospholipase A and phospholipase C to alter the microsomal phospholipids. The phospholipase C-treated preparation lost a maximum of 80 to 90% of the original activity, and 70% of the microsomal phospholipid was hydrolyzed. Addition of phospholipid completely restored the original activity. The soluble and insoluble products of the phospholipase C treatment had no effect on the inactivation of glucose 6-phosphatase or the subsequent reactivation by phospholipid. The relative effectiveness of various single and mixed phospholipids, with respect to reactivation of the lipid-deficient, phospholipase C-treated glucose 6-phosphatase, was compared. Lecithin, which represents 53 % of the microsomal phospholipid, was essentially all hydrolyzed by phospholipase C, but it was ineffective in reactivation. Phosphatidyl ethanolamine, which represents 23% of the phospholipid, was 60% hydrolyzed by phospholipase C, and it was the most effective individual phospholipid in terms of maximal reactivation. Lysolecithin, a trace component in microsomes, was the most effective phospholipid in reactivation at low concentrations, but inhibited at higher levels. In contrast to the phospholipase C-treated preparation, the phospholipase A-treated samples were quite unstable; i.e. the ability to reactivate glucose 6-phosphatase with phospholipid was rapidly lost. In the presence of bovine serum albumin, a fatty acid-binding agent, there was essentially no loss of activity during incubation with phospholipase A. Lysolecithin, another product of the phospholipase A action, was shown to satisfy the phospholipid requirement of the lipid-deficient phospholipase C-treated preparation; thus, the phospholipase A products have contradictory effects on glucose 6-phosphatase activity.