Allantoic acid transport compound

Plants also form the ureides allantoin and allantoic acid, and in some legumes, such as soy beans, these compounds account for 70-80% of the organic nitrogen in the xylem. They appear to function in nitrogen transport.337 As indicated in Fig. 25-18, the hydrolysis to glyoxylate, NH4+, and C02 follows a different pathway than in animals. See also Chapter 24, Section C. 

Asparagine and glutamine are the major transport compounds found in plants, with arginine frequently being present particularly in perennials (e.g., trees). Nonprotein amino acids, homoserine, canavanine, and y-methy-leneglutamine are utilized in specific plants. The ureides allantoin and allantoic acid are frequently found in nodulated tropical legumes. 

The ureides allantoin and allantoic acid are well known as major N containing compounds of plants (Reinbothe and Mothes, 1962 Hegarty and Peterson, 1973). However there has recently been an increase in interest in these compounds as they are major transport compounds in economically important legumes (e.g., soybean and cowpea). 

Ureides are a class of cyclic or acyclic nitrogenous organic compounds derived from or structurally related to urea. Representatives of this class which have been found in plants include allantoin, allantoic acid, citrulline, uric acid, hypoxanthine, xanthine, caffeine, hydroxycitruUine, and albizziine. The structures of allantoin (ALN), allantoic acid (ALA), and citrulline (CIT), ureides which occur throughout the plant kingdom, are presented in Fig. 1. These ureido compounds play an essential role in the assimilation, metabolism, transport, and storage of nitrogen in plants. In this chapter, attention will be focused on the occurrence, function, synthesis, and metabolism of these key metabolites. 

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