J. M. Mckenzie

Thyroid hydrolysis of cyclic AMP as influenced by thyroid gland activity

Abstract 1. 1. Cyclic AMP phosphodiesterase activity in rat thyroid homogenates was studied. Enzyme activity was maximal at pH 7.5–8.0 and was entirely localized to the 100 000 × g supernatant fluid. 2. 2. The apparent Km for cyclic AMP in whole homogenates and in the 100 000 × g supernatant fluid was 0.3–0.5 mM. 3. 3. Chronic thyroid stimulation, produced by feeding a low-iodine diet or 6-n-propylthiouracil, increased enzyme activity and thyroid suppression produced by adding thyroid U.S.P. to the diet, depressed the activity. 4. 4. 10 mM theophylline in the reaction mixture greatly inhibited cyclic AMP hydrolysis, but the same relative changes persisted. 5. 5. Phosphodiesterase activity, measured 15 min and 2 and 12 h after a single intravenous injection of 200 mU thyrotropin, was unchanged. 6. 6. It is suggested that hydrolysis of the high-energy 3′-bond of cyclic AMP may be related to its actions in mediating the effects of thyrotropin. 7. In addition to inhibiting cyclic AMP hydrolysis, theophylline is also a 5′-nucleotidase inhibitor. The major products of cyclic AMP hydrolysis were dephosphorylated compounds, but in the presence of theophylline, appreciable amounts of AMP accumulated. When AMP was used as substrate, theophylline inhibited its hydrolysis.