Heat‐resistant inhibitors of protein kinase C from bovine brain

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Abstract
Bovine brain cytosol is shown to contain two heat‐resistant inhibitors of protein kinase C, with the following characteristics: One protein kinase C inhibitor can be easily purified to homogeneity. Evidence is presented that this polypeptide of Mr 19 000 is calmodulin. It inhibits protein kinase C with an EC50 of about 2.5 μM and the inhibition is Ca2+‐independent. It inhibits only intact protein kinase C. Removal of the regulatory domain of protein kinase C, by limited proteolysis with trypsin, abolishes the inhibition. Another protein kinase C inhibitory activity has been partially purified. Its Mr 600–700, as estimated by gel chromatography). It is not digested by proteases, is hydrophilic, acid‐ and alkali‐resistant, acts Ca2+‐independently, and, in contrast to calmodulin, inhibits even the catalytic fragment of protein kinase C after removal of the regulatory domain by limited proteolysis. This inhibition is, at least partially, due to a competition with ATP. Besides protein kinase C, calcium/calmodulin‐dependent protein kinase II is inhibited to a similar extent. cAMP‐dependent protein kinase is not affected.

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