Digestive sites of dietary proteins and absorptive sites of amino acids along the midgut of the silkworm, Bombyx mori

The present study was conducted to identify where dietary proteins are digested and where the digested amino acids are absorbed in the midgut of the silkworm, Bombyx mori. Analysis by electrophoresis on SDS-polyacrylamide gels shows that few of the protein bands existing in the artificial diet were found among the contents of the foremost part of the midgut; instead, this region contained a cluster of low molecular protein bands, probably proteolytic products from the dietary proteins. This suggests that digestion of dietary proteins has already taken place in the anterior part of the midgut. This result is confirmed by the fact that an extract from the contents of the anterior part of the midgut is capable of digesting soybean proteins, a protein source in the artificial diet, as are extracts from the middle and posterior regions. Estimates of the absorption ratio of amino acids in the midguts were observed using chromic oxide as a non-absorbed reference substance. Cumulative absorption ratios of all the amino acids, except glycine, increased gradually as the diet proceeded through the midgut. The results suggest that absorption of overall amino acids, free amino acids and those in the form of digested peptides, takes place throughout the whole length of the midgut.