Modification of amino acid residues in anti-p-azobenzenearsonic acid antibody during extensive iodination.

The active site of antibody directed against the negatively charged p-azobenzenearsonic acid group has been shown to contain an iodine-reactive residue. The first evidence was provided by Pressman and Sternberger (1)) who demonstrated that iodination destroys the precipitating capacity of this antibody and that the loss can be prevented by the prior addition of the homologous hapten. Subsequently, Koshland, Englberger, and Gaddone (2) showed that the iodination reduces the binding capacity by a direct attack at the active site, and the reaction thus can be used to label with radioactive iodine the specific amino acid involved (3). However, the identity of this iodine-reactive group has not been established. From studies with other proteins such as pepsin, casein, and lysozyme (4), a variety of side chains is known to be modified during iodination. Iodine may be substituted in the phenolic group of tyrosyl residues (5) and on both the nitrogen and carbon atoms of the imidazole ring of histidyl residues (6). Iodine may also oxidize cysteinyl and cystinyl residues to cysteic acid (7), and tryptophanyl residues to an unidentified compound in which the ring is ruptured (8). Furthermore, on the basis of the reaction of the free amino acid with iodine, the methionyl residues (9) in proteins might also be expected to be oxidized to the corresponding sulfoxide or sulfone, and perhaps the seryl and threonyl residues to carbonyl derivatives (10). In view of these complexities, the first step in the identification of the iodinereactive group of arsenic antibody’ was to determine by extensive iodination both the number and kinds of residues that can react. The results of such investigations are presented in this paper.