Fibroinase from silk gland in the fourth molt stage in the silkworm, Bombyx mori

Abstract

1. 1. High activity of fibroinase, an enzyme to hydrolyze the fibroin molecule, with complete hydrolysis of the heavy chain of fibroin within 1 hr in standard assay conditions, was demonstrated in extracted enzyme preparations from the silk gland in the fourth molt stage in the silkworm, Bombyx mori. Combined with cytological observations (Akai, 1965 Bull. Seric. exp. Sta. 19, 375–484) of the formation of vacuoles in the gland cells, followed by digestion of peripheral layers of fibroin and sericin in the lumen contents of the gland in the corresponding developmental stage, this result suggests that fibroinase is a secretory enzyme. 2. 2. Fibroinase in extracted enzyme preparation was subject to complete inhibition by antipain, leupeptin, chymostatin and E-64, all of which are potent protease inhibitors for cathepsins. This was the same result observed in the previous study on fibroinase from degenerating silk glands at day 1 (pharate adult). 3. 3. Low, but detectable, levels of fibroinase activity were observed in extracted enzyme preparations from silk glands at day 2 (fourth larval instar) and at day 0 (fifth larval instar). The results of the physiological function of fibroinase in the silkworm, Bombyx mori were discussed.