Iodohistidine Formation in Ribonuclease A

Abstract Iodination of bovine pancreatic ribonuclease A at 0° and pH 8.5 leads to the formation of 3 iodinated tyrosyl residues and 1 iodinated histidyl residue. The yield of iodinated amino acids is not significantly different at 24°. Carboxymethylation of histidine-119 reduces the total organic iodine from ∼8 to ∼6 g atoms per mole. This decrease is almost entirely accounted for by a lack of iodohistidine formation and suggests that it is this histidyl residue that is iodinated in the native protein. Subtilisin treatment of iodinated RNase A produces iodinated S-protein and uniodinated S-peptide, suggesting that histidine-12 is not iodinated. Phosphate, pyrophosphate, and cytidine monophosphate reduce total iodine incorporation largely by decreasing iodohistidine formation. Enzymatic activity against 2',3'-cyclic CMP is essentially unaffected by the incorporation of 2 g atoms of iodine per mole, but decreases as a function of iodine content with increasing iodination. Preliminary evidence suggests that the initial loss of activity is due primarily to tyrosine iodination.