High levels of phosphorylation in minor phospholipids of Discopyge tschudii electrocyte membranes

Phospholipid phosphate group metabolism has been studied in the electrocyte of Discopyge tschudii (Torpedinidae), following for several hours the incorporation and distribution of [³²P] in living electrocyte stacks and acetylcholine receptor membranes prepared therefrom. Conditions are reported for the incubation of electrocyte columns in vitro with [³²P]O₄Na₂H resulting in active and sustained incorporation of the precursor in minority phospholipids. Phosphatidic acid, phosphatidylinositol, phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-bisphosphate, representing 2.3, 2.1, 0.3 and 0.2% respectively of the total phospholipids were almost exclusively labelled. Relative specific activities of 13, 7, 56 and 140 respectively were attained. These values were much higher than those of major phospholipids like phosphatidylcholine (0.03) or phosphatidylethanolamine (0.01). Subcellular fractionation of the metabolically active electrocyte stacks yielded various membrane fractions differing in their degree of [³²P]-labelling, specially in phosphatidic acid and polyphosphoinositides. The fraction richest in nicotinic acetylcholine receptor protein showed the highest levels of [³²P] incorporation. The results indicate that phosphorylation reactions are actively operative in certain phospholipid classes of the electrocyte and in membranes obtained from these cells after in vitro labelling with [³²P]. In particular, phosphomonoester groups in polyphosphoinositides and the phosphate group in phosphatidic acid display a high metabolic activity.