Protein kinase activities in rat pancreatic islets of Langerhans

1. Protein kinase activities in homogenates of rat islets of Langerhans were studied. 2. On incubation of homogenates with [gamma-32P]ATP, incorporation of 32P into protein occurred: this phosphorylation was neither increased by cyclic AMP nor decreased by the cyclic AMP-dependent protein kinase inhibitor described by Ashby & Walsh [(1972) J. Biol. Chem. 247, 6637–6642]. 3. On incubation of homogenates with [gamma-32P]ATP and histone as exogenous substrate for phosphorylation, incorporation of 32P into protein was stimulated by cyclic AMP (approx. 2.5-fold) and was inhibited by the cyclic AMP-dependent protein kinase inhibitor. In contrast, when casein was used as exogenous substrate, incorporation of 32P into protein was not stimulated by cyclic AMP, nor was it inhibited by the cyclic AMP-dependent protein kinase inhibitor. 4. DEAE-cellulose ion-exchange chromatography resolved four peaks of protein kinase activity. One species was the free catalytic subunit of cyclic AMP-dependent protein kinase, two species corresponded to ‘Type I’ and ‘Type II’ cyclic AMP-dependent protein kinase holoenzymes [see Corbin, Keely & Park (1975) J. Biol. Chem. 250, 218–225], and the fourth species was a cyclic AMP-independent protein kinase. 5. Determination of physical and kinetic properties of the protein kinases showed that the properties of the cyclic AMP-dependent activities were similar to those described in other tissues and were clearly distinct from those of the cyclic AMP-independent protein kinase. 6. The cyclic AMP-independent protein kinase had an s20.w of 5.2S, phosphorylated a serine residue(s) in casein and was not inhibited by the cyclic AMP-dependent protein kinase inhibitor. 7. These studies demonstrate the existence in rat islets of Langerhans of multiple forms of cyclic AMP-dependent protein kinase and also the presence of a cyclic AMP-independent protein kinase distinct from the free catalytic subunit of cyclic AMP-dependent protein kinase. The presence of the cyclic AMP-independent protein kinase may account for the observed characteristics of 32P incorporation into endogenous protein in homogenates of rat islets.