Studies On Cyclic Nucleotide Metabolism In Tetrahymena Pyriformis: Partial Characterization of Cyclic Amp‐ and Cyclic Gmp‐Dependent Phosphodiesterases*

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Abstract
SYNOPSIS. Cyclic nucleotide phosphodiesterase [EC 3.1.4.17] was examined in Tetrahymena pyriformis strain NT‐1. Enzymic activity was associated with the soluble and the particulate fractions, whereas most of the cyclic GMP phosphodiesterase activity was localized in the soluble fraction: the activities were optimal at pH 8.0–9.0. Although very low activities were detected in the absence of divalent cations, they were significantly increased by the addition of either Mg2+ or Mn2‐. A kinetic analysis of the properties of the enzymes yielded 2 apparent KIII values ranging in concentration from 0.5 to 50 μM and from 0.1 to 62 μ M for cyclic AMP and GMP. respectively. A Ca2+‐dependent activating factor for cyclic nucleotide phosphodiesterase was extracted from Tetrahymena cells, but this factor did not stimulate guanylate cyclase [EC 4.6.1.2] activity in this organism. On the other hand, Tetrahymena also contained a protein activator which stimulated guanylate cyclase in the presence of Ca2+, although this activator did not stimulate the phosphodiesterase. the results suggested that Tetrahymena might contain 2 types of Ca2+‐dependent activators, one specific for phosphodiesterase and the other for guanylate cyclase.

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