Studies on the Biosynthesis of Iodotyrosines: A Soluble Thyroidal Iodide-Peroxidase Tyrosine-Iodinase System

Abstract

A soluble enzyme functioning as an iodide-peroxidase and tyrosine-iodinase complex has been prepared from sheep thyroid tissue. The enzyme is detached from cell particles by chymotrypsin digestion in the presence of deoxycholate, followed by a butanol and acetone extraction procedure. It can be purified by diethyl-amino-ethyl cellulose column chromatography. During preparation specific activity of the enzyme is enhanced fiftyfold. The enzyme appears to be a basic protein which does not contain a porphyrin group. It has no cytochrome oxidase activity. Iodinase activity is thermolabile, and dependent on an H₂O₂ source. Under suitable conditions the enzyme forms both diiodotyrosine and monoiodotyrosine from soluble tyrosines, and iodinates proteins. Tyrosine and monoiodotyrosine are its only well-defined amino acid substrates. Methimazole behaves in the enzyme assay system as a strict competitive inhibitor of iodide.