Divergent actions of protein kinase modulator in regulating mammalian cyclic GMP-dependent and cyclic AMP-dependent protein kinases. 
Protein kinase modulator can either augment or depress phosphorylation of substrate proteins catalyzed by cyclic GMP-dependent and cyclic AMP-dependent protein kinases prepared from mammalian and arthropod tissues.  Alteration by the modulator of the phosphorylating activity of the protein kinases is considered to be due to modification of protein substrate specificity subsequent to interaction of the modulator with the catalytic subunits of the enzymes.  It is likely that the physiologic role of the modulator is to monitor the opposing effects of cyclic GMP and cyclic AMP by regulating the activity of respective protein kinases.  Thus the modulator may function as a biologic "fine tuner" providing on additional mechanism by which the signals imparted to cells by physiologic stimuli can be correctly expressed.