Mass spectrometric determination of genistein tissue distribution in diet-exposed Sprague-Dawley rats

Genistein, the principal soy isoflavone, was administered in the diet to ma le and female Sprague-Dawley rats as part of a multigeneration study of pot ential endocrine modulation. The rats were exposed to genistein in utero, t hrough maternal milk, and as adults through postnatal d 140 via essentially isoflavone-free feed (similar to 0.5 mu g/g) fortified at 5, 100 and 500 m u g/g with genistein aglycone. Analytical methods based on liquid chromatog raphy, mass spectrometry and the use of deuterated genistein were developed and validated for use in measuring genistein in serum and tissues. Pharmac okinetic analysis of serum genistein showed a significant difference (P < 0 .001) in the elimination half-life and area under the concentration-time cu rve between male [2.97 +/- 0.14 h and 22.3 +/- 1.2 mu mol/(L . h), respecti vely] and female rats [4.26 +/- 0.29 h and 45.6 +/- 3.1 mu mol/(L . h), res pectively, +/- SEM]. Endocrine-responsive tissues including brain, liver, m ammary, ovary, prostate, testis, thyroid and uterus showed significant dose -dependent increases in total genistein concentration. Female liver contain ed the highest amount of genistein (7.3 pmol/mg tissue) and male whole brai n contained the least (0.04 pmol/mg). The physiologically active aglycone f orm was present in tissues at fractions up to 100%, and the concentration w as always greater than that observed in serum in which conjugated forms pre dominated (95-99%). These results for measured amounts of genistein, presen t as aglycone and conjugates, in putative target tissues provide a link wit h other studies in which blood concentrations and physiologic effects of ge nistein are measured.