Ureide Metabolism in Leaves of Nitrogen-Fixing Soybean Plants

In leaf pieces from nodulated soybean (Glycine max [L] Merr cv Maple Arrow) plants, [(14)C]urea-dependent NH(3) and (14)CO(2) production in the dark showed an approximately 2:1 stoichiometry and was decreased to less than 11% of the control (12-19 micromoles NH(3) per gram fresh weight per hour) in the presence of 50 millimolar acetohydroxamate, a urease inhibitor. NH(3) and CO(2) production from the utilization of [2-(14)C] allantoin also exhibited a 2:1 stoichiometry and was reduced to a similar extent by the presence of acetohydroxamate with a concomitant accumulation of urea which entirely accounted for the loss in NH(3) production. The almost complete sensitivity of NH(3) and CO(2) production from allantoin and urea metabolism to acetohydroxamate, together with the observed stoichiometry, indicated a path of ureide assimilation (2.0 micromoles per gram leaf fresh weight per hour) via allantoate, ureidoglycolate, and glyoxylate with the production of two urea molecules yielding, in turn, four molecules of NH(3) and two molecules of CO(2).