Milan A. Berge

Tobacco budworm, Heliothis virescens [Noctuidae] resistance to l-canavanine, a protective allelochemical

Abstract l -Canavanine, the toxic guanidinooxy structural analog of l -arginine found in many leguminous plants was fed to fifth instar larvae of the tobacco budworm, Heliothis virescens [Noctuidae] in agar-based artificial diet. These feeding studies revealed H. virescens to be resistant to the potentially insecticidal effects of this allelochemical in larval stages and established its LC 50 in the terminal instar larvae at 300 m M . The canavanine-treated larvae did not show any of the pupal developmental aberrations exhibited by a canavanine-sensitive insect such as the tobacco horworm, Manduca sexta . Parenterally injected canavanine evaluations complemented the dietary toxicity data and established the LD 50 in the terminal instar larvae at 10.7 g kg −1 . H. virescens stressed by exposure to 150 m M -canavanine-containing diet for 48 hr, excreted only 0.6% of the canavanine consumed during a subsequent 24-hr exposure to this diet. Parenterally injected canavanine cleared from the hemolymph with a t 1 2 of 135 min. It was evident that canavanine was not merely excreted or sequestered but rather was metabolized effectively by this insect. Feeding studies employing other nonprotein amino acids revealed that this destructive generalist feeder could not cope with all potentially toxic nonprotein amino acids.

Aberrant, canavanyl protein formation and the ability to tolerate or utilize L-canavanine

L-Canavanine, 2-amino-4-(guanidinooxy)butyric acid, and L-arginine incorporation into de novo synthesized proteins was compared in six organisms. Utilizing L-[guanidinooxy14C]canavanine and L-[guanidino14C]arginine at substrate saturation, the canavanine to arginine incorporation ratio was determined in de, novo synthesized proteins.Caryedes brasiliensis andSternechus tuberculatus, canavanine utilizing insects;Canavalia ensiformis, a canavanine storing plant; and to a lesser extentHeliothis virescens, a canavanine resistant insect, failed to accumulate significant canavanyl proteins. By contrast,Manduca sexta, a canavanine-sensitive insect, andGlycine max, a canavanine free plant, readily incorporated canavanine into newly synthesized proteins. This study supports the contention that the incorporation of canavanine into proteins in place of arginine contributes significantly to canavanines antimetabolic properties.

A novel mechanism for detoxification of l-canaline

Abstract Canavanine-containing legumes such as the jack bean, Canavalia ensiformis (L.) DC. (Leguminosae) employ arginase to hydrolytically cleave this non-protein amino acid to yield L -canaline, a potentially poisonous natural product. Canavalia ensiformis detoxifies this potent toxicant by the formation of a stable oxime between l -canaline and glyoxylic acid. This reaction enables one to rationalize the absence of a potentially beneficial allelochemical such as canaline in jack bean. To our knowledge, this is the first example of detoxification of a toxicant by conjugation with a 2-oxo derivative.