Reactivity of the individual tyrosine side chains of myoglobin toward lodination

Abstract

Horse and sperm whale myoglobin have been iodinated under conditions favoring the iodination of tyrosine, with the amounts of iodine added chosen to obtain only partial iodination. The iodinated proteins were cleaved with cyanogen bromide, and the peptides were separated on Sephadex. The amount of tyrosine and iodinated tyrosine in each fraction was determined from the results of amino acid analysis before and after performic acid oxidation. The results indicate that in horse myoglobin, tyrosine 146 (H22) is much less reactive than 103 (G4), while in sperm whale myoglobin, 146 and 151 (HC3) together are about as reactive as 103 alone. A peptide map of a tryptic digest of iodinated sperm whale myoglobin indicated the absence of the peptide containing 151, and the presence of the peptide containing 146. These results are therefore in agreement with the fact that the side chain of tyrosine 146 has been found to be buried in the interior of the molecule in the crystal.