Cyclic nucleotide-dependent protein kinases of the rat pancreas

A cyclic GMP-dependent protein kinase, which catalyzes the phosphorylation of histones and protamine by ATP, was present together with a cyclic AMP-dependent protein kinase and a readily active protein kinase in the rat pancreas. These three protein kinases were separated by chromatography on DEAE-cellulose. The cyclic GMP-dependent protein kinase was relatively cationic and fragile. Upon activation by cyclic GMP, this kinase dissociated into a light catalytic subunit and a somewhat heavier cyclic GMP binding subunit. A crude 27,000 × g pancreas supernatant had two apparent K_a values for cyclic GMP of 2.10^?8 M and 3.10^?7 M. The possible relationships between protein kinases and enzyme secretion are discussed.