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Evidence for the involvement of Ca(2+) and calmodulin in the regulation of phospholipid breakdown by microsomal membranes from bean cotyledons has been obtained by following the formation of radiolabeled degradation products from [U-(14)C]phosphatidylcholine. Three membrane-associated enzymes were found to mediate the breakdown of [U-(14)C] phosphatidylcholine, viz. phospholipase D (EC 3.1.4.4), phosphatidic acid phosphatase (EC 3.1.3.4), and lipolytic acyl hydrolase. Phospholipase D and phosphatidic acid phosphatase were both stimulated by physiological levels of free Ca(2+), whereas lipolytic acyl hydrolase proved to be insensitive to Ca(2+). Phospholipase D was unaffected by calmodulin, but the activity of phosphatidic acid phosphatase was additionally stimulated by nanomolar levels of calmodulin in the presence of 15 micromolar free Ca(2+). Calmidazolium, a calmodulin antagonist, inhibited phosphatidic acid phosphatase activity at IC(50) values ranging from 10 to 15 micromolar. Thus the Ca(2+)-induced stimulation of phosphatidic acid phosphatase appears to be mediated through calmodulin, whereas the effect of Ca(2+) on phospholipase D is independent of calmodulin. The role of Ca(2+) as a second messenger in the initiation of membrane lipid degradation is discussed.

Calcium- and Calmodulin-Regulated Breakdown of Phospholipid by Microsomal Membranes from Bean Cotyledons