Further studies on the interaction of protein kinase C with phospholipid, diacylglycerol, and phorbol ester
Go, M.
Kobe Journal of Medical Sciences 33(5): 179-196
1987
ISSN/ISBN: 0023-2513 PMID: 3437730 Document Number: 296731
It is generally accepted that the receptor-mediated hydrolysis of inositol phospholipids produces diacylglycerol for the activation of protein kinase C to modulate many Ca2+-dependent processes. Phospholipids and Ca2+ are essential for the activation of protein kinase C, and diacylglycerol increases the affinity of this enzyme for phospholipid and this divalent cation. Tumor-promoting phorbol esters substitute for diacylglycerol to activate this protein kinase, and pleiotropic actions of the phorbol ester are, if not all, believed to be mediated through the protein phosphorylation by this protein kinase. It has been also clarified that protein kinase C is activated by 1,3-sn-diacylglycerol but not by 2,3-sn- nor 1,3-diacylglycerol. However, the precise biochemical mechanism in which this enzyme is activated by these substances has not well been elucidated. In this study, interaction of protein kinase C with phospholipid was examined using a phospholipid monolayer technique as a model of the cell membrane. Protein kinase C was activated by phorbol ester in the presence of a monolayer of phosphatidylserine, and the potency of the phospholipid for the activation of protein kinase C was proportional to the surface pressure of the monolayer film. Even in the presence of phorbol ester, protein kinase C did not form a rigid complex with phospholipid monolayer, and a part of the enzyme activity was shown to remain in the solution beneath the monolayer. Kinetic studies for the specificity of diacylglycerol was also made using 1,2-diacylglycerol free from 1,3-isomer in the liposome of phosphatidylserine. 1-Stearoyl-2-arachidonylglycerol, a main species of diacylglycerol generated from the receptor-linked hydrolysis of inositol phospholipids, was most active, although many other diacylglycerols, having at least one unsaturated fatty acyl moiety were almost equally active. 1,2-Diacylglycerols having saturated short carbon chains, which are permeable to intact cell membranes to induce the protein phosphorylation by protein kinase C, showed the Ka values for this enzyme activation slightly larger than those of unsatured diacylglycerols. These results suggest the specific interaction of protein kinase C with phospholipid, diacylglycerol, and phorbol ester.