Selective secretion of free glycine, a neutralizer against a plant defense chemical, in the digestive juice of the privet moth larvae

The larva of the privet moth, Brahmaea wallichii (Brahmaeidae) is a specialist feeder of the privet tree, Ligustrum obtusifolium (Oleaceae). A very high concentration (50 mM or 0.4%) of free glycine, found in the digestive juice of the larvae, works as a neutralizer against the very strong protein-denaturing activity of privet leaves that is caused by oleuropein, an iridoid that functions in chemical defense. Concentration of free glycine was high in the anterior region of the midgut lumen and low in the posterior region. To examine if some glycine-specific secretion mechanism exists, injection experiments were performed using 15N-labeled amino acids. When 13 μmol (1 mg) of 15N-glycine was injected into hemolymph of fifth instar larvae of B. wallichii, a high concentration of 15N (5 mM or 75 μg/g midgut content) was detected in the anterior parts of the midgut lumen 1 h after injection. 15N-NMR data indicated at least 60% of the 15N found in midgut lumen existed as 15N-glycine. Approximately, 25% of the injected 15N-glycine was estimated to have moved from the hemolymph to the midgut lumen. In contrast, no 15N was detected in the midgut lumen when 13 μmol of 15N-labeled alanine, lysine and glutamate were injected into hemolymph. Glycine was the only amino acid whose concentration was higher in the midgut lumen (50 mM) than in the hemolymph (22 mM). These data suggest the existence of some active and glycine-specific secretory mechanism in the midgut of B. wallichii.