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YOICHI ITAGAKI, KATSUMI YOSHIDA, HIDETOSHI IREDA, KAZURO KAISE, NOBUKO KAISE, MARIRO YAMAMOTO, TOSHIRO SARURADA, KAORU YOSHINAGA, Thyroxine 5′-Deiodinase in Human Anterior Pituitary Tumors, The Journal of Clinical Endocrinology & Metabolism, Volume 71, Issue 2, 1 August 1990, Pages 340–344, https://doi.org/10.1210/jcem-71-2-340
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Abstract
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 × 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 Km 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 Km 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.
