This basic science study establishes the regulatory role of G-proteins for membrane-bound phospholipase C using endogenous substrates, while highlighting that exogenous substrate cleavage by GTP analogues may be due to increased substrate accessibility rather than G-protein activation.
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Establishes G-protein regulation of membrane phospholipase C via endogenous substrates; leaves open relevance to cardiovascular signaling pathways.
The influence of nucleotides and pyrophosphate on phospholipase C from rabbit thymocytes was investigated by using two different methods for the determination of phospholipase C activity. In a first approach the release of radiolabeled inositol phosphates from [3H]inositol-labeled membranes was examined. By a second type of experiment the cleavage of exogenously added radiolabeled phosphatidylinositol 4,5-bisphosphate (PtdIns-4,5-P2) was measured. Using internally labeled membranes only guanosine 5'-O-(thiotriphosphate) exhibited a stimulatory effect on the phospholipase C suggesting the involvement of a G-protein. When exogenous [3H]PtdIns-4,5-P2 was used as substrate, cleavage of PtdIns-4,5-P2 was stimulated by all nucleotides investigated; in addition pyrophosphate showed a stimulatory effect. From these data we conclude that the increased cleavage of exogenous PtdIns-4,5-P2 induced by GTP analogues is not conclusive in terms of the involvement of a G-protein. Rather than induced by a G-protein this activation may be caused by an increased substrate accessibility. Our experiments with endogenous substrate clearly established the regulatory role of G-proteins for membrane-bound phospholipase C.
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Sommermeyer et al. (1989) studied this question.
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