Distribution and unspecific protein binding of the xenoestrogens bisphenol A and daidzein

Physiological toxicokinetic (PT) models are used to simulate tissue burdens by chemicals in animals and humans. A prerequisite for a PT model is the knowledge of the chemical's distribution among tissues. This depends on the blood flow and also on the free fraction of the substance and its tissue:blood partition coefficients. In the present study we determined partition coefficients in human tissues at 37°C for the two selected xenoestrogens bisphenol A (BA) and daidzein (DA), and their unspecific binding to human serum proteins. Partition coefficients were obtained by incubating blood containing BA or DA with each of the following tissues: brain, liver, kidney, muscle, fat, placenta, mammary gland, and adrenal gland. Blood samples were analysed by HPLC. For BA and DA, all partition coefficients in non-adipose tissues were similar (average values: BA 1.4, DA 1.2). However, the lipophilic properties of both compounds diverge distinctly. Fat:blood partition coefficients were 3.3 (BA) and 0.3 (DA). These values indicate that with the exception of fat both compounds are distributed almost equally among tissues. In dialysis experiments, the unspecific binding of BA and DA with human serum proteins was measured by HPLC. For BA, the total concentration of binding sites and the apparent dissociation constant were calculated as 2000 and 100 nmol/ml, respectively. Because of the limited solubility of DA, only the ratio of the bound to the free DA concentration could be determined and was found to be 7.2. These values indicate that at low concentrations only small percentages of about 5% (BA) and 12% (DA) are as unbound free fractions in plasma. Since only the unbound fraction can bind to the estrogen receptor, binding to serum proteins represents a mechanism that limits the biological response in target tissues.