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Thyroxine 5'-Deiodinase in Human Anterior Pituitary Tumors^*

Second Department of Internal Medicine, the Departments of Clinical Biology and Hormonal Regulation (K.Y.) and Neurosurgery (H.I.), Tohoku University School of Medicine Sendai 980, Japan

Address requests for reprints to: Dr. Katsumi Yoshida, Department of Clinical Biology and Hormonal Regulation, Tohoku University School of Medicine, 1–1 Seiryocho, Sendai, Japan.

The activity of T₄ 5'-monodeiodinase (5'D) in the pituitary was measured in 12 patients with pituitary adenoma (3 patients with acromegaly, 2 with prolactinoma, 1 with Cushing’s disease, 1 with TSH-producing tumor, and 5 with nonfunctioning tumor) and, as a control, in a patient who died of parotid cancer.

The pituitaries, obtained at operation or autopsy, were homogenized in 0.1 mol/L potassium phosphate buffer, pH 7.0, and centrifuged at 800 x g. Supernatants were incubated with [^l25I]T₄ and 20 mmol/L dithiothreitol (DTT) at 37 C for 90 min. T₄ 5'-D was measured by the release of ¹²⁵I^– with the ion exchange method.

The activity of T₄ 5'-D in the pituitaries from patients with prolactinoma and parotid cancer was dependent on protein concentration, incubation time, incubation temperature, and T₄ concentration, and was labible to prior heating at 70 C for 30 min. T₄ 5'-D was not inhibited by 1 mmol/L propylthiouracil, but was inhibited 95% by 0.1 mmol/L iopanoic acid. The apparent K_m and maximum velocity for T₄ 5'-D in homogenates of prolactinoma at 20 mmol/L DTT were 11 nmol/L and 1.54 pmol/mg protein · h, respectively. This reaction followed sequential-type reaction kinetics when the DTT concentration was varied. All other homogenates of pituitary tumors, except two nonfunctioning tumors, also demonstrated T₄5'-D activity.

These results indicate that 1) the human pituitary express a low K_m and PTU-insensitive T₄ 5'-D activity which is very similar to the type II enzyme activity in the rat pituitary; and 2) various types of pituitary tumor cells contain T₄5'-D activity.

^* Presented in part at the Fourth Asia and Oceania Thyroid Association Meeting, Seoul, Korea, 1989.

Received August 16, 1989.

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