Changes in Endogenous and Exogenous Iron-Reducing Capability of Soybean Hull During Development

Cereal Chern. 68(\):21-24 Hulls (seed coats) of soybean (Glycine max) seeds contain relatively high levels of bioavailable iron. The presence of some iron in the Fe(II) oxidation state may be a contributing factor to its availability. Apparent Fe(II) and Fe(III) content of hulls of several soybean cultivars was followed throughout development. Iron was extracted from hulls with 2N HCI. Fe(II) and Fe(III) content of extracts was measured with bathophenanthrolinedisulfonic acid or by ion chromatography. Extracts of immature Soybean seed coats (hulls) have been identified as a highly concentrated source of dietary iron (Levine et al 1982, Weaver et al 1984). Unlike the extremely poor bioavailability of iron in soy meal (Welch and Van Campen 1975, Lynch et al 1984, Beard et al 1988), hull iron is readily assimilated by humans (Jacob et al 1980, Johnson et al 1985, Lykken et al 1987). The reason for this difference is unknown. The composition of these two tissues with respect to iron-chelating components is quite different. Soybean cotyledons contain phytic acid (Lolas et al 1976) and a range of iron-binding proteins (Funk et al 1986, Sczekan and Joshi 1987, Yoshida 1989), whereas the hull is composed largely of cell wall polymers (Aspinall and Whyte 1964; Aspinall et a1 1966, 1967; Rasper 1979). Phytic acid (Cheryan 1980, Morris and Ellis 1982, Zemel 1984), soy protein (Picciano et al 1984, Morr and Seo 1986), and plant cell walls (Reilly 1979; Reinhold et al 1981; Fernandez and Phillips 1982a,b; Reinhold et al 1986) have all been identified as potential inhibitors of iron absorption. The oxidation state of dietary iron also can have a profound influence. Fe(II) is believed to be far more bioavailable than Fe(III) (Lee and Clydesdale 1979). Thus, the difference in iron availability between cotyledon and seed coat tissue could be attributed to many causes. Previous work (Laszlo 1988) indicated that soybean hulls contain a substantial quantity of Fe(II). Hull Fe(II) is solubilized to a greater extent than hull Fe(III) under simulated gastrointestinal conditions (Laszlo 1989). These observations of hull Fe(II) and good bioavailability provide an appealing correlation. The Mossbauer study by Ambe and co-workers (1987) seemingly 'Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 1815 N. University St., Peoria, IL 61604. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1991. Century and Williams 82 hulls had higher levels of Fe(II) than fully mature hulls. Iron was all Fe(II) in extracts of Sooty, Wilson, and Peking hulls from late growth stages (late linear seed fill to harvest maturity). Conclusive assessment of the in situ hull iron oxidation state was not possible because extracts of all cultivars and stages contained reductants that were endogenous to the tissue or were generated during extraction. These reductants may influence bioavailability to a greater extent than hull iron valence. provides corroborative evidence for the presence of Fe(II) in soybean seeds. Their work, which did not distinguish between signals arising from seed coat or cotyledons, suggested that immature seeds contain a higher percentage of Fe(II) than mature seeds. Given the high concentration of iron in the hull relative to the rest of the seed, much of the Fe(II) signal may have originated from the seed coat. One objective of the present work was to examine hull Fe(II) and Fe(III) content during development. Mature soybean cotyledons contain reductants or antioxidants (Hayes et al 1977, Pratt 1985) that may influence the oxidation state of iron. Although the hull is not known to contain significant quantities of these chemicals, the possibility that endogenous reductants contribute to the formation of Fe(II) in hulls was explored. MATERIALS AND METHODS The soybean (Glycine max (L.) Merrill) cultivar Century was field grown locally under standard cultivation practices. Cultivars Williams 82, Wilson, Sooty, and Peking were greenhouse grown with supplemental lighting. Greenhouse conditions are detailed elsewhere (Laszlo 1990). Developing soybean seeds were collected and segregated following the classification scheme of Fehr and co-workers (1971): R4, initial seed formation; R5, beginning linear seed fill; R5.5, intermediate linear seed fill; R6, late linear seed fill; R6.5, maximum seed fresh weight; R7, physiological maturity; and R8, harvest maturity. Hulls were removed from the seeds and placed immediately in a container on ice, then freeze-dried overnight. The dry hulls were ground to a coarse powder and stored at room temperature under vacuum. Colorimetry was the principal method used to measure sample Fe(II) and total iron content. Hull samples (50 mg) were extracted with 2N HCl (5 ml) in 6-ml screw-capped vials. Vials were masked with black tape to avoid inadvertent photoreduction of Fe(III) during extraction. Sample extracts were passed through a 5-~m nitrocellulose filter (Alltech, Deerfield, IL). After filtration, Vol. 68, No.1, 1991 21