Glycine in digestive juice: A strategy of herbivorous insects against chemical defense of host plants

A large amount of free glycine was found in the digestive juice of several Lepidoptera larvae and appears to be secreted. The amount of glycine differs greatly among species and seems to correlate with the host plant. Several species of Lepidoptera from different families which feed on the privet tree, Ligustrum obtusifolium, show extremely large amounts of free glycine. The concentration of glycine reached 57 mu moles/g (0.4%). The leaves of the privet tree have an extremely strong protein denaturing activity, enough to make protein completely nonnutritive. This denaturing property closely resembles the activity of polyphenolics, digestion inhibitory compounds common among plants. In vitro studies showed that free glycine completely inhibits the denaturing activity of the privet leaves. These results suggest that insects secrete glycine to counter the chemical defenses of host plants. (C) 1997 Published by Elsevier Science Ltd.