| Abstract: | Gastric mucosal membranes derived primarily from parietal cells were found to contain endogenous protein kinase systems as well as several phosphate-accepting substrates. One specific membrane protein with a molecular weight of 88 000 was phosphorylated only in the presence of calcium, while the degree of phosphorylation of three other membrane proteins was similarly increased. The activity of the calcium-dependent protein kinase was found to be totally inhibited in the presence of trifluoperazine, a phenothiazine known to specifically inactivate calmodulin. These results suggest that a calmodulin- and calcium-dependent phosphorylation system may be a component of the parietal cell membrane. Phosphorylation of the membrane proteins was not affected by either cyclic AMP or cyclic GMP. The heat-stable inhibitor protein of cyclic AMP-dependent protein kinase did not inhibit the endogenous protein kinase activity suggesting that the membrane enzyme is not similar to the cytosolic protein kinase. However, the catalytic subunit of the soluble enzyme was capable of phosphorylating a number of membrane proteins indicating that after maximal autophosphorylation of the gastric membranes, phosphate-acceptor sites are still available to the cytosolic cyclic AMP-dependent protein kinase. |
