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

Abstract

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 K m value for L-canavanine of 1.1 mM and a turnover number of 21.1 mol⅐min ؊1 ⅐mol ؊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.

The natural non-protein amino acid N-β-methylamino-l-alanine (BMAA) is incorporated into protein during synthesis

“…Similarly, canavanine, an arginine analogue and plant non-protein amino acid, is incorporated into proteins by pests that feed on the plant tissues (Rosenthal and Dahlman 1986;Huang et al 2011). In each of these cases, the amino acid that was substituted into protein for another, such as canavanine for arginine, is structurally quite similar to the original (Melangeli et al 1997). Recently researchers have proposed that mischarging of the transfer RNA is one potential mechanism for amino acid misincorporation (Yadavalli and Ibba 2013).…”

The Mechanism of l-Canavanine Cytotoxicity: Arginyl tRNA Synthetase as a Novel Target for Anticancer Drug Discovery

“…While L-canavanine still retained the ability to be incorporated into H. virescen proteins, the low frequency of this event presumably protected the insect from the detrimental effects of L-canavanine seen in other organisms. 40 The bruchid beetle, Caryedes brasiliensis (Bruchidae), and the weevil, Stemechus tuberculatus (Curculionoidea), are examples of L-canavanine-utilizing organisms. Both of these organisms feed exclusively on legumes, which contain significant amounts L-canavanine.…”

Plant chemistry and insect sequestration

“…These plant compounds can be exploited by adapted herbivores, which have evolved different ways to overcome the plant defences (Brown and Trigo 1995;Schoonhoven et al 2006). Phytophagous insects either (1) avoid the plant defences by feeding only on those plant parts which contain minimal amounts of the chemical (Hesbacher et al 1995); (2) develop guts that are not permeable to the allelochemicals and allow a fast excretion of toxins (Self et al 1964;Scudder and Meredith 1982b;Weber et al 1986); (3) detoxify plant metabolites by a variety of metabolic means (Dowd et al 1983;Wheeler et al 2001;Wittstock et al 2004;Agerbirk et al 2006) or with the help of endosymbiotic microorganisms (Shen and Dowd 1991); (4) tolerate (Melangeli et al 1997); or (5) accumulate, modify or concentrate these chemicals for their own benefit, either actively or passively and often highly selectively (Duffey 1980;Brown and Trigo 1994;Nishida 2002). In the case of accumulation, plant compounds are taken up by the herbivores and stored in specific parts of the body tissues or integument.…”