GLYCINE IN DIGESTIVE JUICE OF HERBIVOROUS INSECTS:PROTECTION AGAINST STRONG PROTEIN-DENATURING ACTIVITYOF OLEUROPEIN, A PHENOLIC COMPOUND IN THE PRIVET LEAF

Kotaro Konno, Hiroe Yasui, Chikara Hirayama and Hiroshi Shinbo
National Institute of Sericulture and Entomological Science, 1-2 Ohwashi,Tsukuba, Ibaraki 305, Japan

Previously, we reported the existence of a high concentration of freeglycine in the digestive juice of several Lepidopteran larvae, particularlyin species which feed on the privet tree, Ligustrum obtusifolium(Oleaceae). The concentration reached 0.4% in Dolbina tancrei (Sphingidae)and Brahmaea wallichii (Brahmaeidae). The water extract ofthe privet leaf showed a very strong protein-denaturing activity and alysine decreasing activity, which closely resembled those of oxidized phenolics.The fact that the addition of 1% glycine to the extract could completelyinhibit these activities suggested that free glycine is secreted into thedigestive juice as an adaptation mechanism to chemical defense of hostplants. We examine here the chemical basis of this phenomenon. We showthat the privet tree is endowed with a defense mechanism in which a stabledenaturing compound in the cytosol or in the vacuoles is activated intoa chemically active denaturant by an enzyme present in the organelles (includingchloroplasts) only after the leaf is eaten by insects and the organellesare broken down by osmotic shock or by digestive mechanisms. Purificationby HPLC and identification by NMR show that the denaturing compound isoleuropein, an o-dihydroxyphenolic compound. This compound is present inthe privet leaf at the rate of 3.0% per wet weight. Our results suggestthat the protein-denaturing activity and the lysine-decreasing activityof the privet leaf are caused by oxidized polyphenolics, and that insectssecrete free glycine to counter the denaturing activity of oxidized phenolics.