Cocoonase

Cocoonase is a proteolytic enzyme produced by silk moths during adult development and used for softening the end of the cocoon to permit exit of the adult moth. It is deposited on the surface of the galeae (mouth parts) as a partially crystalline deposit. In nature, it is dissolved in a buffer produced by labial glands; for study, it can be picked off the animal in the dry form with forceps. Cocoonase crystals contain an active enzyme (80%) and an inactive protein or peptide or mixture of proteins or peptides (20%). The inactive material is probably derived, at least in part, from the enzyme itself. The active component seems to be a single enzyme as judged by several criteria. It has a molecular weight near 25,000 and an amino acid composition very similar to that of trypsin, except for fewer half-cystine residues. There appears to be only one disulfide bridge. The stability of cocoonase under a variety of conditions is rather different from trypsin, an enzyme which it resembles in other respects. Cocoonase is quite stable to autodigestion at neutral and mildly alkaline pH, but is rapidly deactivated at low pH. It is much less stable than trypsin to heating and to treatment with urea.