Calcium-and phospholipid-dependent protein kinase C and phosphatidylinositol kinase: Two major phosphorylation systems in the cornea

Abstract

The signaling messengers for the regulation of intracellular calcium-dependent functions involve a phospholipid-sensitive protein kinase (protein kinase C) modulated by diacylglycerol, which is a product of phosphoinositide degradation. We found that protein kinase C activity is two times higher in the epithelium than in the stroma-endothelial layers of the rabbit cornea. 12-0-tetradecanoylphorbol-13-acetate (TPA) and phosphati-dylserine stimulate protein kinase C at low Ca²⁺ concentrations. The TPA-phosphatidyl-serine stimulated activity of corneal epithelium was recovered in the soluble fraction; in the membrane-bound fraction, a very active phosphatidylinositol kinase accounted for half the basal phosphorylation of the corneal epithelium.