Appearance of Serine upon Alkaline Incubation of Iodinated Polytyrosine

Abstract

3,5-DIIODOTYROSINE is known to be a precursor of thyroxine^1,2. The formation of thyroxine from two diiodotyrosine residues leads obviously to the formation of a three-carbon α-amino-acid. Thus simple dismutation would lead to alanine^2,3, while an oxidative mechanism involving the formation of a labile intermediate of quinonic structure should yield serine^1,2,4. In the latter case, serine could be formed either by the direct hydrolysis of the labile intermediate⁴, or by the hydration of dehydroalanine formed upon molecular dissociation of the labile intermediate¹. If the unsubstituted 3,5-diiodotyrosine is used in the reaction, dehydroalanine formed may also be hydrolysed to pyruvic acid and ammonia. Johnson and Tewkesbury identified these two products in their reaction mixture⁴. Pitt-Rivers identified alanine among the products of the aerobic incubation of N-acetyl-L-diiodotyrosine chromatographically, but stated that alanine could have been formed afterwards from pyruvic acid⁵.