I purify a small GTPase from E.coli and it copurifies with a molecule of 609 or 610 Daltons. It's not GTP, does anyone have an idea about what it might be?
To Victor: In other words this might be an oxidized form of glutathione (GSSG). It's possible (if it binds GTPase) as GSH is synthesized in E.coli (and then can be oxidized to GSSG). Interesting choice.
I meant. From the MW it should be a hexa-peptide. Could be a fragment of your GTPase.
I would try electrophoresis with Urea to see how "pure" is your preparation. Gradient of poly-acrylamide would be preferable. Then elute the band out of the gel and analyze the "polypeptide" separately, try MALDI-TOF.