BIOCHEMICAL-CHARACTERIZATION OF A PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE-SPECIFIC PHOSPHOLIPASE-C ACTIVITY IN GILLS AND DIGESTIVE GLAND OF THE MARINE MUSSEL MYTILUS-GALLOPROVINCIALIS LAM

1. Polyphosphoinositide-specific phosphodiesterase (phospholipase C, P LC) activity against phosphatidylinositol 4,5-bisphosphate, present in gill and digestive gland homogenates of mussel (Mytilus galloprovinci alis Lam.), has been biochemically characterized. 2. The enzyme was st rictly modulated by free calcium ion concentration in both tissues and maximally activated at 10(-5) M Ca2+ (19 +/- 4 and 11 +/- 2 nmol phos phatidylinositol 4,5-bisphosphate hydrolysed/min/mg of protein for gil l and digestive gland PLC, respectively, at 19-degrees-C). Optimum pH at 10(-5)M Ca2+ was around 7.0 in both cases. The Ca+-stimulated PLC a ctivity showed high specificity for PIP2; the K(Ma) for PIP2 were 150 and 170 muM for the gills and digestive gland, respectively. 3. Good s ubstrate dispersion was obtained in the presence of sodium deoxycholat e; the concentration routinely used in the assay (0.08%) produced a 9- fold activation of both gill and digestive gland PLC, consistent with previous reports. 4. The possible biochemical and physiological role o f the enzyme in mussel tissues is discussed.