Estrogenic activity originating in unexpected places was encountered on three occasions during an investigation of whether Saccharomyces cerevisiae synthesized estrogens. In each instance, estradiol found in the conditioned yeast culture medium originated from an exogenous source and was not synthesized by the yeast. In the first instance, yeast grown in the laboratory showed a time-dependent increase in estradiol in the conditioned medium. However, the culture medium supplement Bacto-peptone was found to contain large amounts of estrone. When added to yeast cultures in the form of YPD medium (yeast extract, Bacto-peptone, and dextrose), S. cerevisiae converted the estrone to estradiol leading to the accumulation of estradiol over time. In the second instance, commercially purchased S. cerevisiae grown in a molasses medium exhibited substantial amounts of estradiol. However, corn and beet molasses contained sufficient estrone and estradiol to account for the findings. As in the first instance, the yeast converted the estrone into estradiol. In the third instance, autoclaving culture medium in polycarbonate plastic flasks was found to cause an estrogenic substance to be added to the medium, whether yeast were present or not. It was determined that the autoclaving process leached bisphenol-A (BPA) out of the polycarbonate plastic. BPA was shown to bind to estrogen receptors and to induce estrogenic activity, including stimulation of MCF-7 breast cancer-cell proliferation and induction of the expression of progesterone receptors. The three instances highlight potential problems for investigators who might inadvertently add estrogens to experimental systems confounding their results. The BPA findings raise concerns about the possible addition of this estrogenic molecule to the food supply since polycarbonate plastic is used in myriad applications in the packaging of food and beverages. Although we are unaware of the substantial contamination of food products with BPA, we believe this possibility should be carefully investigated.

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