Metabolism of glucosinolates and their hydrolysis products in insect herbivores

Abstract

The glucosinolates produced by plants of the order Brassicales are part of a potent activated chemical defense system. These nitrogen- and sulfur-containing glucosides are hydrolyzed by myrosinases upon tissue damage, forming a toxic mixture of compounds consisting mostly of the corresponding isothiocyanates and nitriles. While humans find these compounds pleasantly spicy and beneficial to health, many of them are noxious and deterrent towards microorganisms and insect herbivores. Nonetheless, ingenious and efficient biochemical mechanisms employed by several insect herbivores enable these to feed on glucosinolate-producing plants, circumventing the effects of these plant defenses. Here, we summarize some of the counteradaptations utilized by insects to overcome the defense imposed by these compounds and their hydrolysis products. Insects can divert hydrolysis to less toxic products or desulfate the parent glucosinolates to preclude them from being hydrolyzed by myrosinases. Once hydrolysis occurs, toxic electrophilic hydrolysis products can be conjugated to glutathione and various amino acids. Another insect strategy is the rapid sequestration of ingested glucosinolates to prevent hydrolysis and allow them to be used in their own defense.