Soluble cyclic AMP-dependent protein kinases from chick kidney. Effects of dilution and non-protein inhibitors

Cyclic AMP-dependent protein kinases prepared from crude cytosols of chick kidney, rat kidney and rat liver were found on dilution to exhibit complex kinetics. Dilution of the cytosols appears to increase the state of activation of the enzymes. This effect was due to the presence of inhibitory agents in the cytosol which had a greater inhibitory effect on the cyclic AMP-dependent than on the cyclic AMP-independent enzyme. Two types of inhibitory activity were found by column chromatography, one resistant to trichloroacetic acid precipitation and boiling but affected by trypsin digestion and the other resistant to boiling and trypsin digestion but precipitated by trichloroacetic acid. Inhibitory activity corresponding to the former characteristics has been described previously but the presence of additional soluble inhibitory agents in the cytosol has not been documented. The complete characterisation of this previously undescribed inhibitory activity requires further investigation. The relevance of such cytosolic inhibitory activity to the interpretation of states of activation of protein kinase enzymes is discussed.