Two protein phosphokinases (EC 2.7.1.37) were found to be present in rabbit reticulocytes. The two enzymes were separated by DEAE-cellulose chromatography and called kinases I and II. Adenosien 3′:5′-cyclic monophosphate stimulated the activity of both enzymes. However, the degree of stimulation was different and depended on the protein acceptor used. In the presence of adenosine 3′:5′-cyclic monophosphate, protein kinase I dissociated into two subunits: a subunit binding adenosine 3′:5′-cyclic monophosphate, and a catalytic subunit. The component binding the cyclic nucleotide appeared to act as an inhibitory protein, regulating the activity of the catalytic subunit. The mechanism of action of the cyclic nucleotide on kinase II appeared to be different from that of kinase I.

Tao, M., Salas, M. L., & Lipmann, F. (1970). Mechanism of Activation by Adenosine 3′:5′-Cyclic Monophosphate of a Protein Phosphokinase from Rabbit Reticulocytes. Proceedings of the National Academy of Sciences, 67(1), 408–414.