Ureide Metabolism in Leaves of Nitrogen-Fixing Soybean Plants

Abstract

In leaf pieces from nodulated soybean (Glycine max ILI Meff cv Maple Arrow) plants, '4Cqurea-dependent NH3 and "CO2 production in the dark showed an approximately 2:1 stoichiometry and was decreased to less than 11% of the control (12-19 micromoles NH3 per gram fresh weight per hour) in the presence of 50 millimolar acetohydroxamate, a urease inhibitor. NH3 and CO2 production from the utilization of 12-'4Cj 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 NH3 production. The almost complete sensitivity of NH3 and CO2 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 NH3 and two molecules of CO2.The ureides, allantoin and allantoate, constitute 60 to 80% of xylem-borne nitrogen in nitrogen-fixing tropical legumes (1,3,8,9,14,16) and apparently provide most of the nitrogen used in amino acid and protein synthesis during plant growth (1,2,16). Although the pathway of ureide assimilation in plant tissues has not been critically examined, five metabolic sequences, each giving rise to glyoxylate, NH3, and CO2 are possible based on bacterial metabolism ( Fig. 1; adapted from Refs.

Ureide Catabolism of Soybeans

“…Although reactions (6), (7), and (8) (Table I). These results confirm the results reported by Shelp and Ireland (14). AHA, a urease inhibitor and metal chelator (26), also inhibits allantoate-dependent glyoxylate production in partially purified extracts of soybean seed coats (27).…”

“…The report of Shelp and Ireland (14) suggests that the AHA Figure 1 that is consistent with our data is one in which all C-N bonds are broken independently of urease. We propose that allantoin is catabolized by a series ofreactions outlined in Figure 7, including allantoin amidohydrolase, allantoate amidohydrolase, ureidoglycine aminohydrolase, and ureidoglycolate amidohydrolase.…”

“…Coker et aL (2) found that cultured soybean cotyledons metabolized ['3C-'5N]ALN so that all C-N bonds were broken before reincorporation into protein. Further Shelp and Ireland (14) observed that the urease inhibitor and metal chelator (26) Results from our laboratory, however, favor the action of allantoate amidohydrolase, i.e. the degradation of allantoate directly to NH4' and CO2 without the production of a urea intermediate.…”

“…The metabolism of glyoxylate, a product of ureidoglycolate, is outlined in Figure 2. The amidinohydrolase is generally termed allantoicase and has been proposed to be present in soybeans (14). On the other hand we have previously demonstrated evidence for ALAH in extracts of developing soybean (27).…”

Essential Role of Urease in Germination of Nitrogen-Limited Arabidopsis thaliana Seeds

“…Although it is an -MATERIALS A N D METHODS ~ abundant seed protein in many members of the Gguminosae, Cucurbitaceae, Asteraceae, and Pinaceae (Bailey and Boulter, 1971), urease is also found at lower levels in the vegetative tissues of most or other plants (Hogan et al, with arginase in the utilization of seed protein reserves during germination (Thompson, 1980). A second proposed role is the assimilation of urea derived from ureide metabolism (Shelp and Ireland, 1985). Also, urea is an effective foliar fertilizer.…”