Fluorescence and the structure of proteins VIII. Iodination of ribonuclease modified by subtilisin

• 1.

  1. Loss of fluorescence during initial stages of iodination of ribonuclease S was identical with that observed for ribonuclease A. These results indicate that ribonuclease A and ribonuclease S are very similar in general conformation and that each has a minimum of 3 and possible 4 tyrosyl residues initially susceptible to iodination. During more extensive iodination of ribonuclease S, the molecule dissociated into S-peptide and iodinated S-protein. These results suggest that extensive iodination of a protein may not be a reliable guide to the number and identity of exposed tyrosyl residues in the native molecule.

• 2.

  2. Iodination of S-protein gave a loss of fluorescence similar to that observed for urea-denatured ribonuclease A; the results indicate that all six tyrosyl residues of S-protein are susceptible to iodination as though the structure were more open than for ribonuclease A or ribonuclease S.

• 3.

  3. A large increase in fluorescence of partially iodinated ribonuclease A occurred during denaturation by dodecyl sulfate. This change is consistent with the proposal that non-fluorescent tyrosyl residues in the interior of the native molecule are exposed and made fluorescent during denaturation.