Phosphorylation of cardiac troponin by guanosine 3':5'-monophosphate-dependent protein kinase

Homogeneous cGMP-dependent protein kinase catalyzes the rapid incorporation of phosphate, specifically into the inhibitory subunit of purified cardiac troponin with a maximal incorporation of 1 mol of phosphate/mol of troponin. When troponin was incubated in the presence of both cGMP- and cAMP-dependent protein kinases, a maximal incorporation of 1 mol of phosphate/mol of troponin was observed which suggested phosphorylation of the same site by the two kinases. Both cyclic nucleotide-dependent kinases had similar K(m) values for troponin, but the V(max) value for the phosphorylation reaction catalyzed by cAMP-dependent protein kinase was 12-fold greater than the value obtained for cGMP-dependent protein kinase.