engleski

Characterization of intra-peptide disulfide bridged peptides resulting from acid cleavage of ammodytoxin by MALDI-TOF PSD and MALDI-TOF high-energy CID (20 keV) experiments

Ammodytoxins (Atxs) are presynaptically neurotoxic phospholipases present in Vipera ammodytes ammodytes snake venom. Atxs have a high sequence homology and contain 14 cysteines which are forming 7 biologically relevant disulfide bridges connecting non-neighboring cysteines. Atx sample was reduced and subjected to formic acid cleavage in order to confirm protein sequences. This resulted in 95.6% sequence coverage exhibiting only few by-products caused by formylation of peptides and water or ammonia loss. Interestingly, cysteine containing peptides showed adjacent signals 2 and/or 4 Da lower (according to the number of cysteines present in the peptide) than the theoretical molecular weight indicating forming of intra-peptide disulfide bridges. When Atx sample was reduced and alkylated prior to acid cleavage these additional peaks in front of theoretical monoisotopic peaks were not present confirming our hypothesis of forming intra-peptide bridges. This kind of disulfide rearrangement has not been reported so far in the case of acid cleavage of reduced proteins. Interestingly Atx derived peptides contain adjacent cysteines what could facilitate their oxidation (resulting in formation of eight membered ring) but it has to be pointed out that a peptide with non-neighboring cysteines also exhibited intra-disulfide bridging. Peptides with intra-disulfide bridges were subjected to MS/MS analysis to get further sequence confirmation and to study fragmentation behavior under low and high-energy conditions. Post source decay (PSD) and high-energy collision induced dissociation (CID) at 20 keV experiments showed fragmentation pattern unique for the reduced, thiol group containing and the oxidized, disulfide bridge harboring peptide. Beside typical low-energy fragment ions observed during the PSD experiments (a, b, y), additional high-energy fragment ions (c, x, w, d and internal fragments) of mentionable intensity were generated during fragmentation at 20 keV. In the case of charge directing N- and C-termini x and w ions were also observed during PSD.

Ammodytoxins; Mass spectrometry; Disulfide bridge

nije evidentirano