cAMP-dependent protein kinases of rat pituitary GH3 cells.

Two isoenzymes (type I and type II) of cAMP-dependent protein kinases were found in GH3 cytosol, isozyme type II activity being the predominant form (approximately 90%). Photoaffinity labeling of GH3 cell extracts with 8-N3-[32P]cAMP revealed three cAMP-binding proteins exhibiting molecular weights of 53,000, 51,000 and 48,000, respectively. The latter represents the regulatory subunit of type I isoenzyme whereas the 53,000 and 51,000 cAMP-binding proteins correspond to two different molecular forms of the type II isoenzyme regulatory subunit which are phosphorylated by a cAMP-dependent mechanism.