Magnesium-Sensitive Guanylate Cyclase and Its Endogenous Activating Factor in Tetrahymena pyriformis1

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Abstract
Guanylate cyclase [EC 4.6.1.2] activity in Tetrahymena pyriformis cells was associated with particulate fractions, but not with soluble fractions. Mg2+was much more effective than Mn2+in activating the cyclase activity. Both specific and total cyclase activities with Mg2+in the particulate fraction were very much lower than those in the original homogenate. The addition of the soluble fraction resulted in a marked enhancement of the particulate-bound cyclase activity, while the adenylate cyclase [EC 4.6.1.1] activity was not enhanced. The enhancement was dependent on Ca2+, and the activating factor is suggested to be a protein.