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 Ca2+. This inhibition was reversed by EGTA or EDTA in molar excess of the Ca2+ concentration. Calmodulin was not involved in the Ca2+ inhibition, nor was Ca2+ inhibition of the heat‐stable PDE due to cleavage inactivation of the enzyme by a Ca2+‐stimulated protease. In addition to Ca2+, 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 Ca2+.

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