Conjugation of the Plant Estrogens Formononetin and Daidzein and Their Metabolite Equol by Gastrointestinal Epithelium from Cattle and Sheep

Conjugation of the plant estrogens formononetin and daidzein and their metabolite equol has been investigated in gastrointestinal epithelium of cattle and sheep to explain species differences in susceptibility to plant estrogens. Conjugative activity occurred in all tissues investigated except abomasum. In sheep, the activity was 3-20 times higher than in cattle in all epithelial tissues studied except small intestine, where bovine activity was twice as high. With 1-naphthol as a reference substrate, only minor differences in conjugation rate were found between sheep and cattle rumen epithelium. Furthermore, in sheep rumen wall, 1-naphthol was more efficiently conjugated than formononetin but was only half as efficient as equol. The results show that significant glucuronidation of plant estrogens, and probably other phenolic compounds, occurs in the gastrointestinal tract, which reduces the role of hepatic glucuronidation of ingested substances. The prevailing conception that sheep are more susceptible than cattle to plant estrogens was not explained by the results.