IODINATION OF LYSOZYME .2. REACTIVITY AND POSITION OF TYROSINE RESIDUES

Tryptic digest of iodinated lysozyme, in which two trosine residues were diiodinated, was chromatographed with Dowex-1 × 2. The amino acid analysis of the peptide containing the unreacted tyrosine residue showed that the non-reactive tyrosine residue was Tyr 53. Tyr 20 and Tyr 23 were completely diiodinated.The peptide analysis of lysoxyme iodinated at 0°C in 0.05 M dibasic sodium phosphate, which contains apparently one diiodinated tyrosine residue, showed that a part of both Tyr 20 and Tyr 23 was diiodinated. This suggested that, when either Tyr 20 or Tyr 23 was first diiodinated, iodination of the other was strongly inhibited, consequently the preparation contains one mole of diiodinated tyrosine residue derived from either Tyr 20 or Tyr 23. Furthermore, it was evidenced that this iodinated lysozyme did not contain monoiodotyrosine at all. This phenomenon is similar to that observed by Crestfield et al for ribo-nuclease on carboxymethylation of histidine residues. © 1968 by the Journal of Biochemistry.