Urinary excretion of Bowman-Birk inhibitor in humans after soy consumption as determined by a monoclonal antibody-based immunoassay

The Bowman-Birk inhibitor (BBI) found in soybeans is a serine protease inhibitor with anticarcinogenic activity. In the present study, an ELISA for BBI was developed with the use of a monoclonal antibody against a reduced form of BBI. This newly developed ELISA method was used to measure the urinary levels of BBI metabolites in nine human subjects after consumption of 36-oz or 60-oz soymilk (containing 105 or 175 mg of BBI) at two time points 36 h apart. The results demonstrate that urinary BBI excretion rates peaked within 6 h and decreased to baseline levels within 12-24 h after soymilk ingestion. The changes in BBI:creatinine ratios in urine closely paralleled the changes in urinary BBI excretion rates after soymilk consumption. These data suggest that BBI ingested p.o. is absorbed and could be bioavailable for cancer chemoprevention in other organs in addition to those in the gastrointestinal tract.