Diacylglycerol metabolism and properties of protein kinase C in aorta.

The proliferative response of arterial smooth muscle cells to growth factors may involve the stimulation of protein kinase C activity by diacylglycerols released from phosphoinositides. Therefore, kinase C has been partially purified from bovine aortas, and characterized with both histone and vinculin as in vitro substrates. Kinase C activity was dependent upon calcium and phosphatidylserine. Diolein stimulated enzyme activity by reducing the Ka for calcium from greater than 50 microM to 2 microM in assays with histone. When vinculin was the substrate, the Ka for Ca2+ was 4 microM and 0.8 microM in the absence and in the presence of diolein, indicating that the sensitivity to Ca2+ and the mechanism of the diolein activation can be influenced by the substrate. Enzyme activity determined with histone as substrate was six-fold greater than that measured in assays with vinculin. The metabolism of diacylglycerols was also determined in homogenates of smooth muscle cells isolated from rabbit aortas. Lipase activity (hydrolysis to release fatty acids) was greater than kinase activity (formation of phosphatidic acid). Metabolism of phosphoinositide-derived diacylglycerols by the lipase pathway could provide a portion of the arachidonic acid precursor necessary for the synthesis of prostaglandins by arterial smooth muscle cells.