Thank you for your reply.
>From our peptide mapping experiments we did not observe any oxidized
histidine. The map coverage however was about 75%. The signal for the
intact protein was well resolved (>1000, FWHM). If an oxidized product was
present at < 5-10% it would not easily be observed, but 50% would be. We
also measured the mass of enolase against other known proteins to confirm
the mass was as expected without oxidation. Perhaps Sigma have improved
their protein purification methods. Do you happen to recall which His was
oxidized?
Best regards, Helen.