Phospholipase C in rat liver plasma membranes. Phosphoinositide specificity and regulation by guanine nucleotides and calcium.

Phospholipase C activity against phosphoinositides in isolated rat liver plasma membranes has been examined using exogenous substrates. The enzyme hydrolyzed phosphatidylinositol 4,5-bisphosphate 30-40-times faster than phosphatidylinositol 4-monophosphate, while phosphatidylinositol was not a substrate. Maximum activity was observed with 1.1 mM phosphatidylinositol 4,5-bisphosphate at pH 5.0. The enzyme was stimulated by micromolar concentrations of Ca2+. The GTP analogue guanylyl (beta,gamma-methylene)diphosphonate enhanced phospholipase C activity at and above 0.3 microM Ca2+, but was inhibitory at 0.1 microM Ca2+. This supports the suggestion that plasma membrane phospholipase C is regulated by guanine nucleotide-binding protein, but indicates a regulatory mechanism different from that of other enzymes regulated by such proteins.