Unusually high concentration of free glycine in the midgut content of the silkworm, Bombyx mori, and other lepidopteran larvae

In the course of investigating the digestion and the absorption of dietary protein by the silkworm, Bombyx mori, reared on an artificial diet, we found the existence of an unusually high concentration of free glycine in the midgut content compared with other amino acids. To test whether this free glycine is from diet or from the silkworm itself, silkworm larvae were fed a diet without a protein source for 1 day and the free amino acids in the midgut content were then analyzed. Unexpectedly, the concentration of free glycine in the midgut content was still high (8.98 mu moles/g midgut content). In contrast, the concentrations of other amino acids were very low, and none of them exceeded 1/20 that of glycine. The concentration of free glycine was high in the anterior half of the midgut, and it decreased toward the posterior region. Free glycine was virtually undetected in the feces. To examine if this phenomenon has ally generality, the same experiments were repeated with the sweet potato hornworm, Agrius convolvuli, and the rice armyworm, Pseudaletia separata. A phenomenon similar to that in the silkworm was observed in Agrius convolvuli, but in Pseudaletia separata, no amino acid, including glycine, had a high concentration. We also found that an unusually high concentration of free glycine exists in the digestive juice of Bombyx mori. These results suggest that free glycine is secreted into the intestinal lumen and that the free glycine might have some unknown biological role.