GASTRIC PEROXIDASE - LOCALIZATION, CATALYTIC PROPERTIES AND POSSIBLE ROLE IN EXTRA-THYROIDAL THYROID-HORMONE FORMATION

A highly active peroxidase (EC 1.11.1.7) was localized in the mitochondria isolated from the fundic region of mouse stomach. The stomach also has the property of concentrating iodide significantly. The peroxidase is oriented outside the mitochondrial membrane. The enzyme is strongly inhibited by antithyroid drugs like methimazole and thiouracil. Azide and cyanide completely inactivate the enzyme. The activity is inhibited by SH-blocking reagents like mersalyl or p-chloromercuribenzene sulfonate, but not by N-ethylmaleimide. The enzyme is also sensitive to the action of some proteolytic enzymes. It can catalyze the formation of mono- or diiodotyrosine from tyrosine or monoiodotyrosine as substrate, respectively. The enzyme is capable of synthesizing thyroxine and triiodothyronine on the backbone of a protein, such as thyroglobulin or albumin.