Properties and cellular distribution of cyclic AMP‐dependent protein kinase from thymus

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Abstract
A protein kinase that catalyzes the phosphorylation of histone was partially purified from rat thymus, and the rate of histone phosphorylation was stimulated three‐ to fourfold by 1 × 10−6 M adenosine 3′,5′‐monophosphate (cyclic AMP). Thymic protein kinase was more active than the enzyme from spleen. Histone fractions f1, f2a, f2b, and f3 were all capable of serving as phosphate acceptors for the thymic protein kinase, and the rate of phosphorylation of each fraction was stimulated by cyclic AMP. The ability of various 3′,5′‐mononucleotides to stimulate protein kinase activity was compared. Inosine 3′,5′‐monophosphate (cyclic IMP) was the most effective substitute for cyclic AMP.The cellular distribution of cyclic AMP‐dependent protein kinase and adenylate cyclase activities in the thymus was determined. Cyclic AMP‐dependent protein kinase activity is present in both small thymocytes and residual thymic tissue. The specific activity of protein kinase from residual tissue, both for basal and cyclic AMP‐stimulated enzyme, was greater than that of enzyme from small thymocytes. In contrast to this, adenylate cyclase activity is predominately localized in the thymocytes.

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