1. A protein was demonstrated in mammalian islets of Langerhans that after purification appeared as a single component possessing both cyclic-AMP (adenosine 3′:5′-cyclic monophosphate)-binding and cyclic-AMP-dependent protein phosphokinase activities. 2. The protein had an intrinsic association constant for cyclic AMP of 1.15×10−8m, which was similar to the Km for cyclic AMP (1.11×10−8m) of the protein phosphokinase activity. 3. Incubation of the protein in the presence of cyclic AMP resulted in its dissociation into cyclic-AMP-independent protein phosphokinase (catalytic) and cyclic-AMP-binding (receptor) subunits, which could be separated on Sephadex G-200. 4. The cyclic-AMP-dependent protein phosphokinase was capable of phosphorylating a variety of proteins, the most readily phosphorylated being histone, casein and protein components of sub-cellular fractions prepared from islets of Langerhans. 5. The cyclic-AMP-dependent phosphorylation of histone had a Km for ATP of 1.1×10−5m. 6. The endogenous protein phosphokinase activity in rat islets incubated with agents that are known to alter the intracellular concentration of cyclic AMP was investigated. Theophylline and 3-isobutyl-1-methylxanthine, agents that raise cyclic AMP concentrations in islets, increased the activity of the protein phosphokinase, whereas adrenaline, which lowers islet cyclic AMP concentrations, decreased its activity. 7. It is suggested that cyclic AMP may exert its effects on insulin release by increasing the activity of a protein phosphokinase and may thereby promote the phosphorylation and activity of a rate-determining component of the secretory mechanism.

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