Calcium inhibition of a heat-stable cyclic nucleotide phosphodiesterase from Neurospora crassa

Abstract Neurospora crassa had a heat-stable (up to 95°C), soluble cyclic nucleotide phosphodiesterase (PDE). Both unheated and heat-stable PDE activities were inhibited by micromolar concentrations of Ca 2+ . This inhibition was reversed by EGTA or EDTA in molar excess of the Ca 2+ concentration. Calmodulin was not involved in the Ca 2+ inhibition, nor was Ca 2+ inhibition of the heat-stable PDE due to cleavage inactivation of the enzyme by a Ca 2+ -stimulated protease. In addition to Ca 2+ , several other cations inhibited the activity of the heat-stable enzyme. Cyclic AMP and cGMP, but not 2′3′ cAMP were substrates for both unheated and heat-stable PDEs. This is the first report of a PDE which is inhibited by micromolar concentrations of Ca 2+ .