Imidazole buffer has esterase activity?

[Steven Goldberg]

Our group has been trying to purify a recombinant mammalian liver esterase
from Pichia pastoris.  The enzyme appears to be expressed but at very low
levels so we have added a His-tag at the 3' end to help us purify it.  We
test activity using the colorimetric substrate p-nitrophenyl butyrate, but
are finding very significant activity in our control that contains binding
buffer only (which includes imidazole).  Has anyone else seen this or give
an explanation why this is happening. Are there any good substitutes for
imidazole?

Thanks for any information.

Steve