Modification of the Methionine Residue in the Peptide Component of Ribonuclease-S*T PAUL J. VITHAYATHIL AND FREDERIC

Various derivatives of the peptide component of subtilisinmodified ribonuclease have been described (1). Changes were produced in one or more of the three primary amino groups on Residues 1 and 7 in the parent compound.1 A derivative, in which acetylation of all three amino groups had been effected, was the only one showing a significant change in ability to regenerate enaymic activity when mixed with the protein component. There is a single methionine residue in position 17 of the 20residue peptide (1, 2). This paper reports the preparation and properties of some derivatives involving additions at the sulfur atom of this residue. The sulfide has been oxidized to the sulfone with performic acid, and has been converted to the carboxymethyl and carboxamidomethyl sulfonium salts by treatment with iodoacetic acid and iodoacetamide, respectively. The sulfonium salt derivatives, particularly, have a markedly altered ability to bind to the protein component of RNase-S. However, the enzymic activity of the complexes is not very different from that formed by the protein and the unmodified peptide.