Hog thyroid tautomerase (EC 5.3.2.1, phenylpyruvate keto-enol isomerase), an enzyme which is believed to play a role in the biosynthesis of thyroxine, has been purified over 1000-fold. The purification procedure involves a heat step at 70°, gel filtration on Sephadex G-100, ion exchange chromatography on carboxymethyl Sephadex C-50, and, finally, gel filtration on Sephadex G-50. The enzyme is homogeneous by the following criteria. It gives a single peak in sedimentation velocity analysis and shows a single band on disc polyacrylamide gel and starch gel electrophoresis, and the sedimentation equilibrium plot is linear, even at the lowest protein concentrations. The pH optimum of the purified tautomerase is 6.2. Its molecular weight is 44,000. The amino acid composition of the enzyme, as well as some of its physico-chemical properties, are presented. The enzyme is able to substitute for borate ions in the coupling reaction of thyroxine synthesis
