Phosphodiesterase and Its Ca2+-Dependent Activating Factor in Bovine Thyroid*

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Abstract
Properties of cyclic 3'',5''-nucleotide phosphodiesterase in the 100,500 .times. g supernatant of the bovine thyroid were investigated. The enzyme activity was measured by a radioisotopic method using an anionic-exchange resin, and the activity was stimulated by Mg2+. Sephadex G-200 gel filtration separated the supernatant into an activating factor, which required the presence of Ca2+, and an enzyme form dependent on the factor. The MW were estimated to be 25,000 and 130,000, respectively. There appeared to be another enzyme form of c[cyclic]AMP phosphodiesterase with different dependence on the activating factor as suggested by gel filtration, but this enzyme form could not be clearly separated. cGMP phosphodiesterase purified by gel-filtration showed biphasic kinetic behavior in the absence of Ca2+ and the activating factor, whereas, in their presence, the Lineweaver-Burk plot gave a single Km. The activating mechanism of phosphodiesterase may play a role in the control of concentrations of intracellular cyclic 3'',5''-nucleotides in the bovine thyroid.