ABSTRACT
The tacit assumption frequently made when iodinating a protein is that the attached label resides in one or several of the tyrosyl residues. The quantity of methionine sulfone produced is determined by amino acid analysis after acid hydrolysis, and it is directly proportional to the amount of methionine sulfoxide originally present in the protein. Histidine is considered in this section as the free amino acid in two regards: its labeling and the site(s) of substitution. Formation of iodohistidines may be promoted, or discouraged, by factors that are either intrinsic to the amino acid itself or to its environment in the polypeptide chain. Proteolysis by a broad-spectrum protease provides information on the overall composition of a protein with respect to iodamino acids. Radioactive iodotyrosines can be produced by a variety of methods, the simplest probably being iodination of the free amino acid by radioactive triiodide.
