Hydrolysis of solubilized fibroin and silk proteins in the midgut of the pharate adult of Bombyx mori

Midgut protease in the pharate adult hydrolysed native silk proteins and solubilized fibroin by ethylenediamine cupric hydroxide or lithium bromide. By agar gel electrophoresis one to three protease bands moving toward the anode were detected, and the number of bands and the electrophoretic mobility were different among the various strains. Optimal activity of the enzyme was at about pH 8·3. The protease activity was found to decrease in higher concentrations of the substrates. One peak of protease activity was seen in Sepharose 6B chromatography, and the elution pattern and peak position of the enzyme were very similar to those of protease activity with casein. In DEAE-cellulose chromatography, the peak of activity for casein overlapped but did not coincide with a broad peak of protease hydrolysing native silk proteins. The results obtained support the assumption that the midgut protease in the pharate adult is one of the sources of the cocoon-digesting enzyme.