Straight Edman sequencing won't work for locating the sites of methionine
sulfoxide (which is the form I'm assuming your client is interested in since
the sulfone rarely occurs). That's because the sulfoxide is reduced back to
methionine during the Edman reactions. A similar reduction is well known to
occur during acid hydrolysis for amino acid analysis, which is why you can't
quantitate methionine sulfoxide in proteins by simple amino acid analysis.
However, if you or your client are willing to add the simple step of cyanogen
bromide treatment, then you can probably get the information you want. CNBr
will oxidze methionine to homoserine, with cleavage of the following peptide
bond. However, methionine sulfoxide is not affected by CNBr. You may have to
repurify the peptide(s) of interest after the CNBr treatment, and then proceed
onto the sequencer. The methionine sulfoxide will be reduced back to
methionine, which will elute at the normal PHT-Met position.
For an alternative approach to locating methionine sulfoxde in proteins with a
sequencer, consider "simultaneous sequencing" of a peptide mixture. See PNAS
93:15036-15040, 1996.
Rod Levine
NIH
Bldg 3, Room 106 MSC 0320
Bethesda, MD 20892-0320
email: rlevine@nih.gov
voice: 1 (301) 496-2310
fax: 1 (301) 496-0599