FIBROINASE FROM SILK GLAND IN THE 4TH MOLT STAGE IN THE SILKWORM, BOMBYX-MORI

1. High activity of fibroinase, an enzyme to hydrolyze the fibroin mol ecule, 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 silk worm, Bombyx mori. Combined with cytological observations (Akai, 1965 Bull. Seric. exp. Sta. 19, 375-484) of the formation of vacuoles in th e gland cells, followed by digestion of peripheral layers of fibroin a nd sericin in the lumen contents of the gland in the corresponding dev elopmental stage, this result suggests that fibroinase is a secretory enzyme. 2. Fibroinase in extracted enzyme preparation was subject to c omplete inhibition by antipain, leupeptin, chymostatin and E-64, all o f which are potent protease inhibitors for cathepsins. This was the sa me result observed in the previous study on fibroinase from degenerati ng silk glands at day 1 (pharate adult).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 fibroinas e in the silkworm, Bombyx mori were discussed.