We had a similar experience a few years ago.  In our case, the adventitious ligand was L-lactate, which happens to be a competitive inhibitor of our enzyme.  We used using L-lactate dehydrogenase assays to show that several commercially available PEGs contain L-lactate.   We also think that acetate ions are present in PEGs, based on crystallographic data.  See these papers for the full story:

M. Zhang and J.J. Tanner
Detection of L-lactate in polyethylene glycol solutions confirms the identity of the active-site ligand in a proline dehydrogenase structure.
Acta Crystallogr.  (2004) D60, 985-986
PMID: 15103160

M. Zhang, T.A. White, J.P. Schuermann, B.A. Baban, D.F. Becker, and J. J. Tanner
Structures of the Escherichia coli PutA proline dehydrogenase domain in complex with competitive inhibitors.
Biochemistry (2004) 43(39) 12539-48.
PMID: 15449943



--
John J. Tanner
Professor of Chemistry and Biochemistry
University of Missouri-Columbia
125 Chemistry Building
Columbia, MO 65211
Phone: 573-884-1280
Fax: 573-882-2754
Email: [log in to unmask]
http://www.chem.missouri.edu/TannerGroup/tanner.html

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From: Michael Jackson <[log in to unmask]>
Reply-To: <[log in to unmask]>
Date: Wed, 29 Oct 2008 10:29:22 -0700
To: <[log in to unmask]>
Subject: [ccp4bb] PEG degradation/properties lit source

Hello,
  I would greatly appreciate anyone who would know of a basic literature source on the properties of PEG and particularly on PEG decomposition or degradation products.  We see densities in our maps  for ethanol molecules, but our condition for the crystals do not have added ethanol.