title: Guanine nucleotides stimulate soluble phosphoinositide-specific phospholipase C in the absence of membranes.
abstract: The effect of guanine nucleotides on platelet and calf brain cytosolic phospholipase C was examined in the absence of membranes or detergents in an assay using labeled lipid vesicles. Guanine nucleotides stimulate hydrolysis of [3H]phosphatidylinositol 4,5-bisphosphate [( 3H]PtdIns-4,5-P2) catalyzed both by enzyme from human platelets and by partially purified enzyme from calf brain. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) was the most potent guanine nucleotide with a half-maximal stimulation at 1-10 microM, followed by guanosine 5'-(beta, gamma-imido)triphosphate greater than GTP greater than GDP = guanosine 5'-O-(2-thiodiphosphate). Guanosine 5'-O-(2-thiodiphosphate) was able to reverse the GTP gamma S-mediated stimulation. NaF also stimulated phospholipase C activity, further implying a role for a guanine nucleotide-binding protein. In the presence of GTP gamma S, the enzyme cleaved PtdIns-4,5-P2 at higher pH values, and the need for calcium ions was reduced 100-fold. The stimulation of PtdIns-4,5-P2 hydrolysis by GTP gamma S ranged from 2 to 25-fold under various conditions, whereas hydrolysis of [3H]phosphatidylinositol was only slightly affected by guanine nucleotides. We propose that a soluble guanine nucleotide-dependent protein activates phospholipase C to hydrolyze its initial substrate in the sequence of phosphoinositide-derived messenger generation.
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000201:enzymology; D001921:Brain / Q000201:enzymology*; D002417:Cattle; D002462:Cell Membrane / Q000201:enzymology; D006150:Guanine Nucleotides / Q000494:pharmacology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols / Q000378:metabolism*; D012969:Sodium Fluoride / Q000494:pharmacology; D012995:Solubility; D013329:Structure-Activity Relationship; D010738:Type C Phospholipases / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D006150:Guanine Nucleotides; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols; D012969:Sodium Fluoride; D010738:Type C Phospholipases