Thyroxine 5'-deiodinase in human anterior pituitary tumors

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 [125I]T4 and 20 mmol/L dithiothreitol (DTT) at 37C 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 labile 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 T4 5'-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 T4 5'-D activity.