Cyclic nucleotide phosphodiesterase from bone: characterization of the enzyme and studies of inhibition by thyroid hormones.

Studies were carried out to characterize the cyclic nucleotide phosphodiesterase from rat calvaria. 25-40% of the total enzyme activity was membrane-bound. pH, magnesium, and temperature requirements conformed closely to those established for phosphodiesterase from other tissues. Kinetic evidence was found for dual enzyme activities with different substrate affinities for both the particulate and soluble enzyme. Apparent Kms for the soluble enzyme (3.5 times 10-6 and 2.5 times 10-5M) approximated those for the particulate enzyme (5.7 times 10-6 and 2.5 times 10-5M). L-Thyroxine, 10-5M, inhibited competitively the low- and high-Km enzymes from both the particulate and soluble fractions (Ki equals 1.7 times 10-5M). T4 was more potent an inhibitor than T3 with all enzyme fractions, but this relationship could be altered by adding protein to the incubation mixtures. Tests of diverse thyroid hormone analogues showed that 1) T4 and its derivatives were more potent than T3 and its analogues; 2) acetic and propionic acid derivatives were more potent than the thyronines; 3) "reverse T3," an antagonist of some T3 actions, also inhibited phosphodiesterase. These effects were not attributable to chelation, and were not duplicated by iodide or by other physiologically inactive thyroid hormone analogues.