Table 1.

Susceptibility of methionine residues in glutamine synthetase to oxidation and the expected decrease in residue yields during simultaneous peptide sequencing

Methionine no.	Oxidized by H2O2	Cycle 1	Cycle 2	Cycle 3	Cycle 4	Cycle 5
8	Yes	Leu	Asn	Glu	His	Glu
48	Yes	Phe	Asp	Gly	Ser	Ser
65	Yes	Val	Leu	Met
68	Yes	Pro	Asp	Ala	Ser	Thr
195	Yes	Cys	Leu	Val	Met
199	No	Glu	Gln	Gln
202	No	Gly	Leu	Val	Val	Glu
228	No	Thr	Lys	Lys	Ala	Asp
256	?	Pro	Lys	Pro	Met
260	No	Phe	Gly	Asp	Asn	Gly
268	No	His	Cys	His	Met
272	No	Ser	Leu	Ser	Lys	Asn
331	Yes	Leu	Ala	Tyr	Ser	Ala
376	No	Ala	Gly	Leu	Asp	Gly
392	Yes	Asp	Lys	Asn	Leu	Tyr
455	Yes	Thr	Pro	His	Pro	Val

Oxidation of a methionine residue prevents cleavage by CNBr, thus causing a decrease in yield of residues in the following peptide. Short, hydrophilic peptides may exhibit poor recovery, and that was the case for P²⁵⁷KPM so that the status of Met-256 could not be determined. Oxidation of the carboxyl-terminal methionine of such peptides causes an increase in recovery of the preceeding residues because the resultant peptide is longer. This phenomenon was useful in monitoring the status of Met-68, whose oxidation caused an increase in valine in cycle 1 because of improved retention of Val-66. Also, methionine sulfoxide was reduced back to methionine under conditions of Edman sequencing so that yields of methionine during sequencing increased as residues were oxidized. For example, no methionine was detected in cycle 3 of the control glutamine synthetase, but yields increased as Met-68 was oxidized. The second column refers to oxidation by exposure to hydrogen peroxide.