Canaline

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Canaline, or l-Canaline, is a rare amino acid that isn’t used to build proteins. Its full name is 2-amino-4-(aminooxy)butanoic acid, and it is found in legume plants that contain canavanine, especially in jack bean (Canavalia ensiformis). It’s unique because its side chain has an O-alkyl hydroxylamine group, making it the only naturally occurring amino acid with this feature and it is related to ornithine (it is the 5-oxa derivative of ornithine).

Insects are strongly affected by canaline. It acts as an insecticide and can cause developmental problems or death in caterpillars like the tobacco hornworm, and it can also be neurotoxic to some moths. Its toxicity is partly due to its ability to form oximes with aldehydes and with the vitamin B6 cofactor pyridoxal phosphate, which interferes with enzymes. It inhibits ornithine aminotransferase at very low concentrations (as low as 10 nM).

In plants, canaline is a substrate for ornithine aminotransferase, leading to a chain of reactions that produce l-ureidohomoserine, then l-canavaninosuccinic acid, and finally l-canavanine via argininosuccinic acid synthetase. This sequence constitutes a canaline-urea cycle, similar to the ornithine-urea cycle, releasing urea as a by-product to supply ammonia for nitrogen metabolism. Canaline can also be broken down to l-homoserine, another important non-protein amino acid, which helps feed nitrogen into the plant’s metabolism and contributes to building other amino acids.