THE BIOCHEMICAL BASIS FOR L-CANAVANINE TOLERANCE BY THE TOBACCO BUDWORM HELIOTHIS-VIRESCENS (NOCTUIDAE)

The tobacco budworm, Heliothis virescens (Noctuidae), a destructive in sect pest, is remarkably resistant to L-canavanine, L-2-amino-4-(guani dinooxy) butyric acid, an arginine antimetabolite that is a potent ins ecticide for nonadapted species, H. virescens employs a constitutive e nzyme of the larval gut, known trivially as canavanine hydrolase (CH), to catalyze an irreversible hydrolysis of L-canavanine to L-homoserin e 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 b een isolated from the excised gut of H. virescens and purified to homo geneity; it exhibits an apparent K-m value for L-canavanine of 1.1 mM and a turnover number of 21.1 mu mol . min(-1). mol(-1). This enzyme h as 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-(guanidinoo xy) 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.