Studies on cyclic nucleotide metabolism in Tetrahymena pyriformis: partial characterization of cyclic AMP- and cyclic GMP-dependent phosphodiesterases.

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 K(m) values ranging in concentration from 0.5 to 50 micron and from 0.1 to 62 micron 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.