Purification and properties of halogenated tyrosine and thyroid hormone transaminase from rat kidney mitochondria.

Halogenated tyrosine transaminase has been purified 30-fold from a sonic extract of rat kidney mitochondria. Purification involves ammonium sulfate fractionation and chromatography on diethylaminoethyl cellulose and hydroxylapatite. The purified enzyme is highly specific for tyrosine analogues with halogen substituents in positions 3 or 3 and 5, for thyroid hormones, and, to a lesser extent, for naturally occurring aromatic amino acids such as tyrosine, tryptophan, and phenylalanine. The identity of the enzyme and thyroid hormone transaminase has been confirmed. Halogenated tyrosine transaminase requires pyridoxal 5’-phosphate as a coenzyme and a-ketoglutarate as amino acceptor. Inhibition studies indicate that a sulfhydryl group is essential for enzyme activity. 3,5-Diiodo-4-hydroxyphenylacetate has been found to be a powerful inhibitor of the enzyme, functioning as a mixed type of inhibitor. The reaction catalyzed by this enzyme is: halogenated tyrosine + oc-ketoglutarate + halogenated p-hydroxyphenylketo acid + glutamate.