Comparison of the enzyme oxidizing thyroid hormone with l-amino acid oxidase

Abstract

An oxidase, which catalyzes mainly the deamination of thyroxine, 3,3′,5-triiodothyronine and 3,5-diiodothyronine, has been partially purified from rat-kidney mitochondrial extracts by successive (NH₄)₂SO₄ fractionation and diethylaminoethyl-cellulose-column treatment. The enzyme preparation was contaminated with catalase (H₂O₂: H₂O₂ oxidoreductase, EC 1.11.1.6) in relatively high concentration, but contained no thyroid-hormone transaminase. The enzyme preparation had no effect on either diiodotyrosine or tyrosine, leucine and lactic acid were well oxidized with the formation of keto acids.

The elution pattern of the partially purified oxidase on diethylaminoethyl-cellulose column indicates that the enzyme oxidizing thyroid hormone is identical to the mammalian l-amino acid oxidase (l-amino acid: O₂ oxidoreductase, EC 1.4.3.2) which catalyzes chiefly the deamination of leucine and the dehydrogenation of lactic acid. Furthermore, the prosthetic group of the purified enzyme oxidizing thyroid hormone was identified as riboflavin 5′-phosphate, which is well known as the prosthetic group of the mammalian l-amino acid oxidase.