Mass balance, plasma pharmacokinetics, tissue distribution, and metabolism of [(14)C]genistein were investigated in male and female rats (n = 5) following an oral dose of [(14)C]genistein (4 mg kg(-1)) to determine potential sites and mechanisms of biological action. Mean total excretion of radioactivity in urine and feces for both sexes was 66 and 33% of the dose respectively at 166 h after administration. Mean and maximal concentrations of radioactivity in plasma were significantly (p < 0.02) higher in male than female rats, with half-lives of 12.4 and 8.5 h, respectively. The concentration of radioactivity was significantly (p < 0.002) higher in liver from females than males and in reproductive (vagina, uterus, ovary, and prostate) compared with other peripheral organs. Analysis of plasma extracts by radio-HPLC-MS indicated that the predominant metabolites were genistein glucuronides, 4-hydroxyphenyl-2-propionic acid, and trace amounts of parent compound (<5%). Radioactive residues in uterus and prostate were predominantly parent compound and 4-hydroxyphenyl-2-propionic acid, respectively. Significantly (p < 0. 05) increased retention of [(14)C]genistein or metabolites was associated with reproductive organs, such as vagina, uterus, ovary, and prostate, likely to contain relatively high concentrations of estrogen receptors or binding proteins compared with other peripheral tissues. Factors liable to influence bioavailability of biologically active genistein or metabolites, such as dietary intake, warrant further investigation to determine the risks or benefits for different consumer groups of phytoestrogen-containing foods.
Copyright 2000 Academic Press.