Use of Protein‐Mediated Lipid Exchange in the Study of Membrane‐Bound Enzymes

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Abstract
The ability of liver lipid-exchange proteins to introduce foreign phospholipids into microsomes was used in study of the lipid dependence of glucose-6-phosphatase. Supplementation of intact rat liver and hepatoma microsomes with exoexogeneous aminophospholipids prevents the decline of glucose-6-phosphatase activity during incubation, whereas the introduction of exogeneous phosphatidylcholine has no protective effect. With deoxycholate-disrupted hepatoma microsomes, introduction of additional phosphatidylcholine causes activation while phosphatidylethanolamine has only little effect. Assuming that the transport unit and the catalytic moiety of the glucose-6-phosphatase system have different lipid requirements, the activity of the former protein depending mainly on phosphatidylethanolamine and phosphatidylserine and that of the catalytic protein depending on phosphatidylcholine. In deoxycholate-disrupted liver microsomes (in which both the glucose-6-phosphatase activity and the phosphatidylcholine content are much higher than in hepatoma microsomes) incubation with phosphatidylcholine and lipid-exchange proteins alters neither the phospholipid composition nor the enzyme activity. The diminished activity of glucose-6-phosphatase in hepatomas may be partly due to a low level of phosphatidylcholine.