Some specialized herbivores tolerate L-canavanine either because they metabolize it efficiently (cf. L-canaline) or avoid its incorporation into their own nascent proteins.
By metabolic detoxification Herbivores may be able to metabolize canavanine efficiently. The beetle
Caryedes brasiliensis is able to break canavanine down to canaline, then further detoxifies canaline by reductive deamination to form homoserine and ammonia. As a result, the beetle not only tolerates the chemical, but uses it as a source of nitrogen to synthesize its other amino acids to allow it to develop.
By selectivity An example of this ability can be found in the larvae of the tobacco budworm
Heliothis virescens, which can tolerate large (
lethal concentration 50 or LC50 300 mM) amounts of dietary canavanine. These larvae fastidiously avoid incorporation of L-canavanine into their nascent proteins due to gastrointestinal expression of canavanine hydrolase, an enzyme that cleaves L-canavanine into L-homoserine and hydroxyguanidine, and L-arginine kinase, which phosphorylates L-canavanine. In contrast, larvae of the tobacco hornworm
Manduca sexta can only tolerate tiny amounts (1.0 microgram per kilogram of fresh body weight) of dietary canavanine because their arginine-
tRNA ligase has little, if any, discriminatory capacity. No one has examined experimentally the arginine-tRNA synthetase of these organisms. But comparative studies of the incorporation of radiolabeled L-arginine and L-canavanine have shown that in
Manduca sexta, the ratio of incorporation is about 3 to 1.
Dioclea megacarpa seeds contain high levels of canavanine. The beetle
Caryedes brasiliensis is able to tolerate this however as it has the most highly discriminatory arginine-tRNA ligase known (as of 1982). In this insect, the level of radiolabeled L-canavanine incorporated into newly synthesized proteins is barely measurable. Moreover, this beetle uses canavanine as a nitrogen source (see above). ==See also==