Effect of membrane phospholipid compositional changes on adenylate cyclase in LM cells.

Adenylate cyclase activities were examined in mouse LM cell membranes which had been supplemented with polar head groups and/or fatty acids. Basal, fluoride-, and PGE1-stimulated activities varied systematically with changes in phospholipid composition, and PGE1-stimulated activities correlated with the average degree of unsaturation of the phospholipid fatty acids or with the primary amino group character of the phospholipid polar head groups. In addition, the Km for ATP of basal adenylate cyclase was systematically changed by both polar head group and fatty acid supplementation. Alteration of the membrane lipid composition also changed the temperature dependence of the enzyme and the lag time between addition of PGE1 and the onset of a change in catalytic rate. However, none of the alterations in the enzyme activity could be correlated with the viscosities of supplemented membranes and, instead, seemed to be characteristic for a specific polar head group or fatty acid composition. The data suggest a specific interaction of the enzyme with phospholipids and indicate that structural features of phospholipids may play a role in regulating adenylate cyclase activity. It is proposed that adenylate cyclase can exist in several different conformations in the membrane depending upon the phospholipid composition.

Modification of adenylate cyclase activity in LM cells by manipulation of the membrane phospholipid composition in vivo.

Adenylate cyclase [atp pyrophosphate-lyase (cyclizing): EC 4.6.4.4] activities were examined in mouse LM cell (fibroblast) membranes that were supplemented with ethanolamine and/or fatty acids. The supplements were incorporated into the plasma membrane phospholipids in significant amounts. Fatty acid supplementations had distinct effects as compared to polar head group supplementations. All lipid supplementations increased basal adenylate cyclase activity relative to control cells grown in choline-containing medium. Double supplementation with ethanolamine and linoleate increased the specific activity of adenylate cyclase up to 4-fold. Activity in the presence of fluoride was unaffected by ethanolamine supplementation, but was increased by fatty acid supplementation. In contrast, prostaglandin E1 stimulation was 4.2-fold in controls and ethanolamine and/or elaidate supplements, 6-fold in choline plus linoleate supplements, and 3.1-fold in ethanolamine plus linoleate supplements. Differences in activity could not be ascribed to changes in membrane protein composition in supplemented cells, and could be abolished by detergent solublization. Fluidity of the supplemented membranes was monitored by fluorescence polarization, and no correlation was observed between membrane viscosity and adenylate cyclase activity or hormone stimulation. These results emphasize the importance of the membrane lipid phase for this enzyme.