Over 250 nonprotein amino acids have been identified in plants (Swain, 1977). A number of these compounds are intermediates in the synthesis and catabolism of the protein amino acids (Lea and Norris, 1976). However, many of these non-protein amino acids may play roles as defensive agents.

The best characterized examples of nonprotein amino acids in plants are L-canavanine and L-canaline (Rosenthal, 1982; 1990). Massive accumulation of L-canavanine, the 2-amino-4-(guanidinooxy)butyric acid structural analog of L-arginine, occurs in the seeds of many legumes. This offers protection against predation, but the compound also serves as a nitrogen seed storage reserve which can be rapidly mobilized during seed germination. This requires special adaptations, including a modified L-arginase which can accept L-canavanine as a substrate, and an ability to metabolize L-canaline, a deleterious ornithine analog, to homoserine and ammonia. The enzyme catalyzing the detoxification of canaline, canaline reductase, has a mass of approximately 167 kDa and is composed of 82-kDa dimers. The enzyme catalyzes a NADPH-dependent reductive cleavage of L-canaline to L-homoserine and ammonia, and is the only enzyme known to use reduced NADP to cleave an O-N bond. Canaline reductase performs at least three important functions for canavanine-synthesizing legumes; it detoxifies canaline; it increases by one-half the overall yield of ammoniacal nitrogen released from canavanine; and it permits the carbon skeleton of canavanine, a secondary plant metabolite, to support vital primary metabolic reactions (Rosenthal, 1992). The cleavage of canaline to homoserine and ammonia does not appear to be reversible.

The bruchid beetle (Caryedes brasiliensis) can feed on canavanine-laden tissues. In addition to canavanine and canaline catabolic activities this insect also possesses an arginyl-tRNA synthetase which will not recognize L-canavanine (Rosenthal, 1982; 1990; Rosenthal et al, 1987). The tobacco budworm, Heliothis virescens, a destructive insect pest that is remarkably resistant to L-canavanine, employs a constitutive enzyme of the larval gut, known trivially as canavanine hydrolase (CH), to catalyze an irreversible hydrolysis of L-canavanine to L-homoserine and hydroxyguanidine (Berge and Rosenthal, 1991; Melangeli et al, 1997).

L-Canaline (L-2-amino-4-(aminooxy)butyric acid) is a structural analog of L-ornithine and a powerful antimetabolite which reacts vigorously with the pyridoxal phosphate moiety of vitamin B6-containing enzymes to form a covalently-bound oxime that inactivates, often irreversibly, the enzyme (Rosenthal, 1981; 1997). Canaline is not only capable of inhibiting ornithine-dependent aminotransferase activity (Rosenthal and Dahlman, 1990), but it also can function as a lysine antagonist (Rosenthal, 1997).

Consistent with the metabolic scheme of canavanine synthesis shown above, the ornithine carbamoyltransferase (OCT) [EC 2.1.3.3] of canavanine-containing plants showed high canaline-dependent activities, while the OCT of the canavanine-deficient legume, kidney bean, shows a very low activity towards canaline (Lee et al, 1998).

Berge MA, Rosenthal GA 1991 Metabolism of L-canavanine and L-canaline in the tobacco budworm, Heliothis virescens [Noctuidae]. Chem. Res. Toxicol. 4: 237-240.

Lea PJ, Norris RD 1976 The use of amino acid analogues in studies of plant metabolism. Phytochem. 15: 585-595.

Lee Y, Jun BO, Kim SG, Kwon YM 1998 Purification of ornithine carbamoyltransferase from kidney bean (Phaseolus vulgaris L.) leaves and comparison of the properties of the enzyme from canavanine-containing and -deficient plants. Planta 205: 375-379.

Melangeli C, Rosenthal GA, Dalman DL 1997 The biochemical basis for L-canavanine tolerance by the tobacco budworm Heliothis virescens (Noctuidae). Proc. Natl. Acad. Sci. U.S.A. 94: 2255-2260.

Rosenthal GA 1981 A mechanism of L-canaline toxicity. Eur. J. Biochem. 114: 301-304.

Rosenthal GA 1982 Plant nonprotein amino and imino acids. Biological, Biochemical and Toxicological Properties. Academic Press, New York.

Rosenthal GA 1990 Metabolism of L-canavanine and L-canaline in leguminous plants. Plant Physiol. 94: 1-3.

Rosenthal GA 1992 Purification and characterization of the higher plant enzyme L-canaline reductase. Proc. Natl. Acad. Sci. U.S.A. 89: 1780-1784.

Rosenthal GA 1997 L-canaline: a potent antimetabolite and anti-cancer agent from leguminous plants. Life Sci. 60: 1635-1641.

Rosenthal GA, Berge MA, Bleiler JA, Rudd TP 1987 Aberrant, canavanyl protein formation and the ability to tolerate or utilize L-canavanine. Experientia 43: 558-561.

Rosenthal GA, Dahlman DL 1990 Interaction of L-canaline with ornithine aminotransferase of the tobacco hornworm, Manduca sexta (Sphingidae). J. Biol. Chem. 265: 868-873.

Swain T 1977 Secondary compounds as protective agents. Annu. Rev. Plant Physiol. 28: 479-501.