Critical controls in the evaluation of cAMP-dependent protein kinase activity ratios as indices of hormonal action.

Measurement of the cAMP-dependent protein kinase "activity ratio" was introduced by Corbin et al. (Corbin, J.D., Soderling, T.R., and Park, C.R. (1973) J. Biol. Chem. 248, 1813-1821) and has been used by a large number of investigators as an index of the activation of this enzyme in the intact cell. This communication reports that with the typical conditions used the dissociation of the protein kinase is not blocked throughout the extraction. This is demonstrated, with glucagon-stimulated rat liver as the example tissue, by the addition of exogenous protein kinase. These data call into question the meaning of results reported using this experimental approach.