PHYTOESTROGENS - NEW LIGANDS FOR RAT AND HUMAN ALPHA-FETOPROTEIN

The binding of the lignans, enterolactone, enterodiol, nordihydroguaiaretic acid (NDGA), and the isoflavonic phytoestrogen equol, to human and rat-alpha-fetoprotein (AFP) was studied. They had differential inhibitory effects (NDGA > equol > enterolactone > enterodiol) on the binding of estrone and estradiol to rat AFP and the binding of unsaturated fatty acid to both rat and human AFP. Inhibition was dose-dependent. The apparent dissociation constants (K(d)) for phytoestrogens binding to AFP were: K(d) NDGA = 5 +/- 1.2.10(-7) M, K(d) equol = 6.7 +/- 0.8.10(-6) M, K(d) enterolactone = 1.7 +/- 0.4.10(-5) M and K(d) enterodiol = 2.2 +/- 0.6.10(-5) M. The K(d) for estrone binding to rat AFP was increased by increasing concentrations of equol, but the number of estrone binding sites remained unchanged. This, plus the results of double-reciprocal plots, suggests that they compete for the same site(s). NDGA also competitively inhibited estrone binding at low NDGA concentrations (increased K(d)), but high concentrations induced conformational changes in rat AFP, as both K(d) and the number of binding sites (n) were altered. Both rat and human AFPs underwent changes in electrophoretic behaviour and loss of immunoreactivity with increasing NDGA, suggesting that NDGA binding induces conformational changes in the AFPs. However, equol did not alter the electrophoretic or immunological properties of either rat or human AFP, providing further evidence for qualitative differences in the effects of these diphenols. These findings indicate that phytoestrogens could play a role in AFP-dependent normal and pathological growth and development.