Abstract

Isoflavone aglycones daidzein (Dein) and genistein (Gein) are present primarily as glucuronides and sulfates in human plasma; however, very little is known about the plasma pharmacokinetics of isoflavone conjugates after soy ingestion. The aim of this study was to investigate metabolism and disposition of the isoflavone conjugated metabolites glucuronide or sulfate or both after ingestion of kinako (baked soybean flour) by 10 volunteers. The quantifications of 16 metabolites in plasma and urine were performed by our previously reported high-performance liquid chromatography-UV-diode-array detector method. Plasma concentrations of total Dein and Gein metabolites reached maximal values of 0.64 ± 0.18 μM at 4.7 ± 2.5 h and 1.58 ± 0.55 μM at 5.4 ± 2.1 h, respectively. The area under the curve from 0 to 48 h demonstrated that daidzein-7-glucuronide-4′-sulfate (D-7G-4′S) (53.3%) was a major metabolite of Dein and that genistein-7-glucuronide-4′-sulfate (G-7G-4′S) (54.0%) and genistein-4′,7-diglucuronide (G-4′,7-diG) (26.6%) were major metabolites of Gein in plasma. The compositions of isoflavone metabolites in urine and plasma were greatly different. Approximately half of the 48-h urinary excretion of total Dein metabolites consisted of daidzein-7-glucuronide. The total amounts of genistein-7-glucuronide and genistein-4′-glucuronide were half the total amount of the urinary Gein metabolites. Excretion into urine of D-7G-4′S and G-7G-4′S accounted for only 16% each of the total Dein and Gein metabolites, respectively. The plasma and urine profiles of 16 metabolites of Dein and Gein demonstrate the involvement of desulfation and deglucuronidation of the conjugated metabolites D-7G-4′S, G-7G-4′S, and G-4′,7-diG in the process of renal excretion.