Behavior of chymotrypsinogen during low pH gel electrophoresis is altered by persulfate

Chymotrypsinogen was observed to have two bands in a low-pH gel electrophoresis system, though the protein was pure by other criteria. Other proteins have also been reported to give artifacts under these conditions. Removal of persulfate from the gel by pre-electrophoresis or by substituting riboflavin eliminated the artifacts. The affected amino acid residue was identified as tryptophan by titration of persulfate-treated proteins with 2-hydroxy-5-nitrobenzyl bromide and by the spectral method of Edelhoch. Persulfate-treated chymotrypsinogen had the same mobility as the artifact, while oxidation of Met-192 with hydrogen peroxide produced a protein with a different mobility.