The biochemical basis for L-canavanine tolerance by the tobacco budworm Heliothis virescens (Noctuidae)

The tobacco budworm, Heliothis virescens (Noctuidae), a destructive insect pest, is remarkably resistant to L-canavanine, L-2-amino-4-(guanidinooxy)butyric acid, an arginine antimetabolite that is a potent insecticide for nonadapted species. H. virescens 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. As such, it represents a new type of hydrolase, one acting on oxygen-nitrogen bonds (EC 3.13.1.1). This enzyme has been isolated from the excised gut of H. virescens and purified to homogeneity; it exhibits an apparent Km value for L-canavanine of 1.1 mM and a turnover number of 21.1 micromol x min(-1)x micromol(-1). This enzyme has a mass of 285 kDa and is composed of two subunits with a mass of 50 kDa or 47.5 kDa. CH has a high degree of specificity for L-canavanine as it cannot function effectively with either L-2-amino-5-(guanidinooxy)pentanoate or L-2-amino-3-(guanidinooxy)propionate, the higher or lower homolog of L-canavanine, respectively. L-Canavanine derivatives such as methyl-L-canavanine, or L-canaline and O-ureido-L-homoserine, are not metabolized significantly by CH.