Cyclic AMP-dependent protein kinase does not phosphorylate cyclic GMP-dependent protein kinase in vitro

The autophosphorylation reaction of purified cGMP-dependent protein kinase has been studied. Apparent initial rates of autophosphorylation in the absence of cyclic nucleotides and in the presence of cGMP and cAMP are 0.006, 0.04, 0.4 mol Pi incorp./min-1. mol cGMP-kinase subunit-1. In the presence of cGMP and cAMP approximately 1 and 2 mol Pi are incorporated/mol enzyme subunit. These values are independent of the enzyme concentration. Stimulation of autophosphorylation by cAMP is not due to activation of a contaminating cAMP-dependent protein kinase since: (a) addition of the heatstable inhibitor protein of cAMP-kinase does not inhibit autophosphorylation; and (b) catalytic subunit of cAMP-kinase added at a 10-fold excess over cGMP-kinase does not phosphorylate cGMP-kinase.