Studies on the Active Site of Papain. VIII. Photooxidation of Tryptophan Residues

Abstract

1) Only a tryptophan residue was exclusively modified by photooxidation at pH 4.0 with the loss of enzyme activity. 2) This residue is considered to be identical with the first NBS-oxidizable residue. 3) This residue is different from the residues to be photooxidized at pH 8.0. 4) A tryptophan residue to be photooxidized at pH 4.0 and to be first NBS-oxidized is considered to exist in or nearby the active site of papain. These findings agree with X-ray model of papain. 5) Two tryptophan residues other than the essential residue were modified by photooxidation at pH 8.0 and by NBS oxidation at pH 3.0. These two residues may be exposed in native papain. 6) Other two tryptophan residues were not affected by photooxidation at pH 4.0 and pH 8.0 and by NBS oxidation. It is considered that these two residues are buried in native papain. 7) Based on the interpretation described above, the state of tryptophan residues of papain is illustrated schematically.