Non protein amino acids (NPAAs) are structural analogues to the 20 protein amino acids (Wink and Schimmer 1999). The structure of some simple NPAAs is given in Figure 5. In protein amino acids the amino group is linked to the acarbon, while in NPAAs the amino group can be bound to the P-carbon as for instance in 3-alanine or to the y-carbon like in 4-amino butyric acid (Figure 5). NPAAs are usually present in the free state (Luckner 1990). The formation of NPAAs is regulated either by specific pathways, the modification of primary amino acids or the modification of biosynthetic pathways leading to primary amino acids (Luckner 1990). About 900 amino acids, which are not linked to the primary metabolism, were identified and thus can be regarded as secondary metabolites (Lambein et al. 2001).
homoserine 3-alanine 4-amino butyric acid
Figure 5. Structure of some simple non-protein amino acids (adapted from Selmar 1999).
Canavanine, a structural analogue of arginine is present in many legume crops and affects in particular the immune system, but is also discussed as a chemotherapeutic agent for human pancreas cancer (Lambein et al. 2001). Besides canavanine, mimosine and canaline are tested as anticancer metabolites (Lambein et al. 2001). Other NPAAs, for example glutamine derivates are tested in pharmacology (Brauner-Osborne et al. 1997).
Toxic NPAAs in plants may offer protection against herbivores (Rosenthal 2001). Possible effects of NPAAs on pests and diseases are, however, obscure at the moment. NPAAs function as storage pools for reduced N (Shelp et al. 1999).
Seeds of Griffonia simplicifolia and Mucuna mutisiana contained for example 14% 5-hydroxy-L-tryptophan and 8% L-DOPA (L-3,4-dihydroxypheny-lalanine), respectively (Luckner 1990). Other functions of NPAAs are the provision of energy by, for example, creatine phosphate, and osmotic regulation, for example by betaines (Luckner 1990).
N fertilisation increased the content of S-acetylornithine in rhizomes of Bistorta bistortoides (Lipson et al. 1996). Leguminosae are a major source for NPAAs such as albizine, canavanine and lathyrine (Wink and Waterman 1999), but the influence of the N nutritional status on their synthesis remains speculative.
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