Regulation of Microsomal Enzymes by Phospholipids

Incubation of bovine liver microsomes with partially purified phospholipase A from Naja naja venom inactivates the phosphohydrolase activities of glucose 6-phosphatase, but there is no quantitative correlation between hydrolysis of phospholipids and loss of enzyme activity. Furthermore, addition of EDTA to an incubating mixture of microsomes and phospholipase A completely stops hydrolysis of phospholipids, but does not halt the decline of glucose 6-phosphatase activity. These results indicate that hydrolysis of phospholipids by phospholipase A does not per se inactivate glucose 6-phosphatase. Instead, hydrolysis of phospholipids by phospholipase A produces an unstable form of the enzyme. The phospholipase A-treated form of glucose 6-phosphatase can be stabilized by asolectin and albumin, and at pH 5.75 by glucose-6-P or PPi. In contrast to its effects on phosphohydrolase activities, conditions of phospholipase A treatment which lead to a 50 to 60% loss of these activities activate slightly the phosphotransferase. It was found, however, that the phospholipase A treatment led to a pH-dependent instability of phosphotransferase which could be separated from the instability of phosphohydrolase activities. Phospholipids, in the form of asolectin, inhibit the phospholydrolase activities of untreated microsomes whereas they activate after phospholipase A treatment. Albumin, which has no effect on the untreated enzyme, is a more effective activating agent of phospholipase A-treated glucose 6-phosphatase than asolectin. Asolectin activates, but albumin inhibits, the phosphotransferase activity of phospholipase A-treated microsomes. Since either albumin or asolectin can increase the phosphohydrolase activity of phospholipase A-treated microsomes to levels greater than those of untreated ones, their activating action cannot result from reversal of phospholipase A effects. Phospholipase C treatment inactivates both the phosphohydrolase and phosphotransferase activities of glucose 6-phosphatase. This action does not seem to result from phospholipase C-induced instability of the enzyme. However, inactivation cannot be interpreted as indicating a phospholipid requirement for the enzyme since albumin, in addition to asolectin, activates phospholipase C-treated glucose 6-phosphatase, and the combined action of phospholipase C plus asolectin increases phosphotransferase activity to 50% greater than that of untreated microsomes.