Studies on an Iodinating Enzyme from Calf Thyroid

Abstract

A soluble iodide peroxidasetyrosine iodinase has been derived from calf thyroid tissue by treatment of mitochondria and microsomes with deoxycholate, trypsin and acetone. The enzyme system is stabilized by the presence of KI (10^-6M) through all preparative procedures. The “protective” iodide is not bound to the protein by covalent bonds. Tyrosine and KSCN are also protective. Enzyme activity can be partially purified by chromatography on Sephadex G-200 and diethylamino ethyl cellulose. The purified preparation appears to be a mixture of low weight proteins (Svedberg 2–4) with equal or smaller amounts of cytochrome than the original soluble enzyme. Flavin nucleotide is present in amounts too small to act as a cofactor. No evidence has been found for dissociation of the iodinating enzyme into an apoenzyme and iron-porphyrin prosthetic group. Preparation of the soluble enzyme system from thyroid cell particles is accompanied by a loss of DPNH oxidase, DPNH-cytochrome c reductase, and cytochrome oxidase activity, reflecting a separation from mitochondrial elements. TPNH-cytochrome c reductase content is increased, but this enzyme does not appear to be an integral part of the iodinating system. Iodination by the enzyme preparation is not stimulated by flavin nucleotides. Iodination is inhibited by bishydroxycoumarin and antimycin A. Thyroxine and triiodothyronine also inhibit iodination. Their action is due to dilution of tracer 131I by stable iodide, and direct inhibition of the enzyme system. (Endocrinology76: 632, 1965)