discolored recombinant protein

[john gately]

Folks--I've been purifying recombinant protein from E. coli grown in
standard LB.  The recombinant proteins are enzymes that use phenolic
compounds as substrates.  I have four enzymes from this class purified
and three of them crystallized.  Three out of these four proteins have a
brownish yellow appearance that becomes very visible when I concentrate
(~10-20mg/ml; no substrate added).  So I am a bit worried that there is
something in there with my protein that I don't know about.  The
purifications are very standard, using combinations of IEX, HIC, and SEC
columns and buffers of tris or phosphate.  The proteins are very clean
on SDS-gels stained with Coommassie: single bands.  Any explanations
would be much appreciated.  Thanks.--John
-- 
jguz at mail.med.cornell.edu