Kotaro KONNO, Chikara HIRAYAMA, Hiroshi SHINBO
National Institute of Sericultural and Entomological Science,
1-2 Ohwashi, Tsukuba, Ibaraki 305, Japan
We found that a considerable amount of free glycine exists in the digestive juice of several leaf-feeding Lepidopteran larvae. Evidence suggests that the glycine is secreted by larvae into gut lumen. The concentration of glycine has very clear correlation with the protein denaturing activity of the host plant. We found that the leaf extract of the privet tree, Ligustrum obtusifolium, has extremely strong protein denaturing activity enough to make a protein innutritive. The denaturing activity closely resembles those of polyphenolics, general defense agents widespread throughout plants including leaves of the privet tree. The species which feed on this host plant have an extremely large amount of glycine in their digestive juice. The concentration reaches 50-60 µmoles/ml (0.4%) in Dolbina tancrei (Sphingidae) and Brahmaea wallichii (Brahmaeidae), both of which are specialists to the privet tree, though they belong to different families. We demonstrate that free glycine can completely inhibit the denaturing activity of the privet leaf in vitro, and that free glycine can prevent the protein from becoming innutritive using bioassay. These data suggest that the larvae secrete glycine against chemical defense (denaturing activity) of host plants.
Polyphenolics, which are also found in the privet tree, are known to combine with amino residue in side chain of lysine in protein and are known to aggregate protein. We observed that there is great decrease in the amount of lysine in protein when protein is denatured by the leaf extract of the privet tree This reduction in lysine is also inhibited by free glycine. So, it is very likely that the denaturing activity of the privet leaf derives from polyphenolics and that amino residue in free glycine competes with amino residue in side chain of lysine in protein, therefore inhibits the denaturing activity of the privet leaves. We will also discuss such a chemical mechanism of the phenomena.
This is the first demonstration of the fact that such a simple molecule as a glycine can work as, and is actually used by herbivores as a counteracting agent against denaturing activity of plants. We believe that this is a good example of chemical adaptation and coevolution between plants and herbivores.