The Histidyl Residues of Insulin

Extensive iodination of bovine insulin at 0° and pH 8.5 leads to the formation of substituted tyrosyl and histidyl residues. The iodohistidines consistently appear later than the iodotyrosines. The maximum iodine incorporation approaches 12 g atoms of iodine per mole of insulin: 8 g atoms in the 4 tyrosyl residues and 4 g atoms in the 2 histidyl residues. The presence of zinc in insulin leads to (a) a reduction in the initial rate of iodine incorporation which can be ascribed to an effect on the reactivity of the tyrosyl residues, and (b) a diminution in the total iodine incorporation because 1 of the histidyl residues is unavailable in zinc insulin. The S-carboxymethylated B chain of insulin is much less sensitive to zinc, and both histidyl residues can be iodinated. Carboxymethylhistidine insulin can be iodinated only in its 4 tyrosyl residues. No iodohistidine is formed. It is postulated that both direct chemical and conformational factors are involved in the effect of zinc on the iodination of insulin.