A Mn(2+)-dependent enzymic breakdown of allantoate has been detected in crude and partially purified extracts of developing soybeans. The products detected were CO(2), NH(3), glyoxylate, labile glyoxylate derivatives, and low levels of urea. Urea is initially produced at less than 10% the rate of urease-independent CO(2) release indicating that the activity is not allantoate amidinohydrolase (i.e. urea is not directly cleaved off allantoate). The urease-independent CO(2) releasing activity has an apparent K(m) of 1.0 millimolar for allantoate. Ethylenediaminetetraacetate, borate, and acetohydroxamate (all at 10 millimolar) inhibit the enzymic production of NH(3), CO(2), and labile glyoxylate derivatives from allantoate. However, the potent urease inhibitor, phenyl phosphordiamidate does not inhibit CO(2) and NH(3) release indicating that the action of acetohydroxamate is not due to its inhibition of urease. That the allantoatedegrading activity was more than 5-fold greater in seed coats than in embryos is consistent with the data of Rainbird et al. (Plant Physiol 1984 74: 329-334) which indicate that available ureides are metabolized before reaching the embryo. 2-Ethanolthio, 2'ureido, acetic acid (NH(2)COHNCHCO(2)HSCH(2)CH(2)OH), the first allantoate-derived product detected by HPLC analysis, is an addition produced of mercaptoethanol with an unidentified enzymically produced ureido intermediate that is not derived from ureidoglycolate or oxalurate.