Oestrogenic activity of CPRG (chlorophenol red-beta-D-galactopyranoside), a beta-galactosidase substrate commonly used in recombinant yeast oestrogenic assays

The finding that a variety of chemicals display oestrogenic activity has re sulted in the development of in vitro and in vivo assays to assess oestroge nic activity. One such assay, the yeast oestrogen assay (YES) makes use of recombinant yeast cells that harbour an oestrogen receptor expression casse tte and a reporter construct, coding for beta -galactosidase. The induction mechanism starts with the binding of oestrogenic compounds to the oestroge n receptor. This complex activates the production of beta -galactosidase. T he beta -galactosidase activity is thus a measure of the oestrogenic activi ty of chemical compounds. In the YES assay, the beta -galactosidase activit y may be quantified with the chromogenic substrate chlorophenol red-beta -D -galactopyranoside (CPRG). In the present study it is reported that CPRG or its beta -galactosidase degradation product chlorophenol red act in the YE S as an oestrogenic compound itself. The implications of this finding are d escribed. It is especially argued that chlorophenol red production after pr olonged incubation of the assay might be misinterpreted as an oestrogenic e ffect of the test compound.