Regulation of multiple forms of cyclic nucleotide phosphodiesterase from bovine hypothalamus: New factors modulating enzyme activity

Studies of bovine hypothalamic cyclic nucleotide phosphodiesterase (PDE) indicate the presence of several peaks of PDE activity, distinguishable by DEAE-cellulose column chromatography, displaying different substrate specificities, kinetic behavior, and regulatory properties. Evidence is presented that chromatographically separated forms of PDE activity are subject to control by Ca²⁺-calmodulin, cyclic nucleotides, limited proteolysis, reagents affecting sulfhydryl groups, and neurohormone “C”—one of several new cardioactive compounds isolated from hypothalamic magnocellular nuclei of animals—in a complex substrate-specific and concentration-dependent manner. Of particular interest is the finding that each of the forms of cGMP PDE, being Ca²⁺/calmodulin-dependent, possesses sensitivity to activation by cAMP, especially under conditions favoring the oxidation of thiol groups of PDE, resulting in a loss in responsiveness of the enzyme to the activation by calmodulin. This effect appears to be relatively stable but readily reversible by sulfhydryl reducing reagents, which restore both the cGMP PDE sensitivity to competitive inhibition by cAMP and the responsiveness of the enzyme to activation by calmodulin. A reinterpretation of the regulatory properties of multiple forms of PDE is proposed. Special Issue dedicated to Dr. Eugene Kreps.