Ca2+and Phospholipid-Dependent Protein Kinase Activity and Phosphorylation of Endogenous Proteins in Bovine Adrenal Medulla

Abstract

Soluble and membrane fractions of bovine adrenal medulla contain several substrates for the Ca2+/phospholipid-dependent and cAMP-dependent protein kinases. The phosphorylation of soluble proteins (36 and 17.7 kdaltons) and a membrane protein (22.5 kdaltons) showed an absolute requirement for the presence of both Ca2+ and phosphatidylserine; other substrates showed less stringent phosphorylation requirements and many of these proteins were specific for each of the protein kinases. The Ca2+/phospholipid-dependent phosphorylation was rapid, with effects seen as early as at 30 s of incubation. Measurement of enzyme activities with histone H1 as an exogenous substrate demonstrated that the Ca2+/phospholipid-dependent protein kinase was equally distributed between the soluble and membrane fractions; the cAMP-dependent enzyme was predominantly membrane-bound in adrenal medulla and chromaffin cells. The activity of the soluble Ca2+/phospholipid-dependent protein kinase of adrenal medulla was found to be about 50% of the enzyme level present in rat brain, a tissue previously shown to contain a very high enzyme activity. A prominent role for the Ca2+/phospholipid-dependent protein kinase in chromaffin cell function was suggested.