Labeling of soybeans with the stable isotope 70Zn for use in human metabolic studies

Radiozinc (65Zn) was used to assess the efficiency of incorporation of the label into soybean seeds by means of a noncycling hydroponic system. It was found that 21.3 and 27.6% of the applied dose was incorporated into the mature seeds when the tracer was applied during vegetative or flowering stages of growth cycle, respectively. The latter method was adapted to labeling with the stable isotope 70Zn, and quantitative incorporation of this isotope was investigated within and among hydroponic culture pots each containing four plants. It was found that on the average 17.7% +/- 3.0% of the applied dose was recovered in the mature seeds harvested from each pot. The mass isotope enrichment ratio for 70Zn: 68Zn in these seeds was 2.93 +/- 0.38 (mean +/- SD) compared with the natural ratio of 0.0343, indicating a high degree of stable isotope enrichment. Results of soybean fractionation studies showed that the enrichment was somewhat higher in the defatted meal fraction as compared with the full-fat, the hull, and the whole seed, indicating somewhat preferential translocation of the applied stable isotope in the protein. Despite this small but observable difference, all fractions were highly enriched with 70Zn. The data from this study indicated that noncycling hydroponic culture may be effectively employed to intrinsically label soybeans with the stable isotopes of zinc for use in human feeding trials dealing with the issues of zinc availability. A 100-g portion of such labeled seeds provided 38-43 g protein and 490-620 micrograms 70Zn in various processed products for use in human feeding studies.