What could be the cause for Pyro-carbamidomethyl (N-term C) modification of proteins during 2-DE?

I received some protein spots from 2de to analyse in MS. I can hardly identify some of them. I tryed a search in Mascot by the error tollerant function and I get peptides with Pyro-carbamidomethyl...

07 July 2012 6,417 5 View