Competitive Inhibition of Beef Heart Cyclic AMP Phosphodiesterase by Cytokinins and Related Compounds

Two cytokinins and four related analogs, none of which is a cyclic ribonucleotide, have been shown to act as competitive inhibitors of the high K _m cyclic-AMP phosphodiesterase (3′:5′-cyclic-AMP 5′-nucleotidohydrolase, EC 3.1.4.17) activity from beef heart. Weak inhibition of the low K _m cyclic AMP phosphodiesterase activity was also observed, suggesting a possible mechanism for regulation of intracellular cyclic AMP levels by the exogenously added compounds. In addition to the kinetic data, obtained on the six inhibitors in four different heterocyclic series, 15 other cytokinins and related compounds have been shown to inhibit the high K _m cyclic AMP phosphodiesterase activity at single concentrations of substrate and inhibitor. Heterocycles such as adenosine and 7-amino-3-methylpyrazolo[4,3- d ]pyrimidine, which lack the N -substituent, were inactive as cyclic AMP phosphodiesterase inhibitors. The observed inhibition of cyclic AMP phophodiesterase supports prior observations which implicate exogenously added cytokinins in cyclic AMP metabolism.