I remember speaking with a woman, I believe that it was at ABRF '97 in
Baltimore (I can't find my copy of the program from that meeting, so I'm
not entirely certain), who presented a poster including peptides
analyzed by MALDI which presented the same 48 Da loss; she ascribed it
to fragmentation of oxidized methionine. I had also seen this in ESI
with samples which had been prepared by an inexperienced associate;
during the reduction/carboxymethylation step, the reaction tube was not
sparged, and the resulting maps showed numerous additional peaks, all
eluting just before and all presenting masses 48 Da less than the
methionine-containing peptides. Without any supporting evidence, the
woman and I both assumed that our MS conditions produced fragmentation
responsible for the observed lower mass. We hadn't considered the
possibilities raised by Jacek and Richard that methionine
carboxymethylation brought about the mass loss; nevertheless, it is
still important to ask whether the 48 Da loss occurred before or after
introduction into the mass spec, especially to define the modification.
Mike Klein
Amgen, Inc.
> ----------
> From:
> Barbara_J._Root@ccmail.bms.com[SMTP:Barbara_J._Root@ccmail.bms.com]
> Sent: Tuesday, July 28, 1998 6:08 AM
> To: Recipients of ABRF List
> Subject: ProtSeq and MS: Prot modification
>
> I would like to see if anyone has observed a modied methione
> which I
> may be seeing. In a recent peptide map, a new peak was observed.
>
>
> Analysis by protein sequencing shows very low amount of a known
> peptide sequence but with low amount a methionine (8-10 x less
> than
> expected) at its one site of occurrence. At that site there was
> no
> other peak of the height of the rest of the sequence, although
> there
> was a small amt of a peak at PTH-ILE and a slight increase in
> dmptu.
>
> MALDI MS shows the mass anticipated minus 48 daltons. Further
> fragmentation confirms the mass change to be at the site of
> methionine.
>
> Here is my question: has anyone had experience with a methionine
>
> degrading to dehydroamino butyric acid? Any other thoughts?
>
> Thanks for your help!
>
> Barb Root
> Bristol-Myers Squibb
> Syracuse NY
>