Further characterization of Ca 2+-activated phospholipase A 2 in cat adrenal cortex

Abstract

When cat adrenocortical cells were incubated with exogenous phospholipid substrate (autoclaved E. coli ) in the presence of corticotropin, there was a Ca 2+-dependent increase in phospholipid breakdown activity, suggesting that a hormone-stimulated phospholipase is localized to the plasma membrane. Phospholipase activity in a particulate fraction from lysed cells at neutral pH was a function of the Ca 2+ concentration. The addition of increasing Ca 2+ concentrations to a subcellular fraction of lysed cells which had been prelabelled with [ 14C]arachidonic acid produced graded increases in fatty acid release. A depletion of label from phosphatidylcholine was observed, as well as a marked increase in radioactivity associated with phosphatidylethanolamine. The subcellular fraction of cells prelabelled with [ 14C]palmitic acid failed to release fatty acid in response to Ca 2+, although a loss of label from phosphatidylcholine and a modest gain in label by phosphatidylethanolamine was demonstrable. A Ca 2+-activated deacylation-reacylation reaction preferentially involving phosphatidylethanolamine was evident in cortical cells prelabelled with archidonic acid; whereas, other Ca 2+-stimulated lipolytic reactions also appeared to be operative in cells prelabelled with either arachidonic or palmitic acid. The Ca 2+-dependent mobilization of arachidonic acid from an endogenous phospholipid pool lends additional support to the idea that Ca 2+-mediated activation of phospholipase A 2 participates in the control of adrenocortical activity. However, since Ca 2+ also stimulated arachidonic acid liberation from cortical triglycerides, these lipid moieties may also contribute to the observed effects of Ca 2+ on fatty acid release.