Phospholipid modifications and adenylate cyclase in rat liver plasma membranes

Rat liver plasma membrane phospholipid headgroups or fatty acids were modified by enzymatic transmethylation or transacylation. To evaluate the effect of phospholipid modification on adenylate cyclase (AC), one of the enzymes of phospholipid metabolism as well as AC were measured in the same incubation medium. Methyl transfer from S-adenosyl-L-methionine (SAM) to phosphatidylcholine (PC) was the highest at pH 9.2. At low concentrations of SAM, synthesis of PC as well as synthesis of the monomethyl derivative were considerably decreased and Mg2+ had no effect at pH 9.2 or at pH 6.5. When phospholipid methylation was increased in relation to SAM concentration, there was no change in basal, NaF- and glucagon-stimulated AC. Methylation was not modified when AC was stimulated. On the other hand, there was an increase in the basal, NaF- and glucagon-stimulated AC when linoleate was incorporated into the membrane phospholipids. Other unsaturated fatty acids had no effect. The synthesis of linoleoyl-PC from added lysophosphatidylcholine (LPC) also stimulated AC and this in turn partly prevented the inhibitory effect of LPC. Thus in isolated plasma membranes transmethylation has no direct effect on AC, whereas synthesis of linoleoyl molecular species can modulate AC in a way which does not depend on the membrane fluidity.