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. 1. Polyphosphoinositide-specific phosphodiesterase (phospholipase C, PLC) activity against phosphatidylinositol 4,5-bisphosphate, present in gill and digestive gland homogenates of mussel (Mytilus galloprovincialis Lam.), has been biochemically characterized. 2. 2. The enzyme was strictly modulated by free calcium ion concentration in both tissues and maximally activated at 10^-5 M Ca²⁺ (19 ± 4 and 11 ± 2 nmol phosphatidylinositol 4,5-bisphosphate hydrolysed/min/mg of protein for gill and digestive gland PLC, respectively, at 19°C). Optimum pH at 10^-5 M Ca²⁺ was around 7.0 in both cases. The Ca²⁺-stimulated PLC activity showed high specificity for PIP₂; the K_Ma for PIP₂ were 150 and 170 μM for the gills and digestive gland, respectively. 3. 3. Good substrate dispersion was obtained in the presence of sodium deoxycholate; 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. 4. The possible biochemical and physiological role of the enzyme in mussel tissues is discussed.