Phospholipase C activity and substrate specificity in frog photoreceptors.

The photoreceptor, like many other cells, undergoes receptor-mediated breakdown of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. The lack of phosphatidylinositol (PdtIns) breakdown during receptor stimulation suggests the existence of a phospholipase C specific for polyphosphoinositides. Phospholipase C activity in frog rod outer segments was assayed with several substrates. The activity was selective for the polyphosphoinositides, phosphatidylinositol 4-phosphate [PtdIns(4)P] and PtdIns(4,5)P2. PtdIns was hydrolysed at 2% of the rate of hydrolysis of PtdIns(4,5)P2. No activity was detected when phosphatidylcholine, phosphatidylserine, or phosphatidylethanolamine were used as substrates. The enzymatic activity was optimal at neutral pH. These findings suggest, but do not prove, that this activity might contain the light-regulated phospholipase C of the photoreceptor.