Two distinct forms of the chitin-degrading enzyme N-acetyl-β-d-glucosaminidase in the Antarctic krill: specialists in digestion and moult

Abstract

In the Antarctic krill Euphausia superba two forms of the chitinolytic enzyme N-acetyl-β-d-glucosaminidase (NAGase, EC 3.2.1.52) have been described, previously identified as NAGase B and NAGase C. Here, we demonstrate the organ-specific distribution and physiological relevance of both forms using a polyclonal antibody preparation which allows them to be distinguished immunologically. While NAGase B was localized in the integument and displayed a pattern of activity related to the moult cycle, the activity of NAGase C was independent of the moult cycle and was predominantly found in the gastrointestinal tract. Accordingly, NAGase B played a significant role in chitin degradation during the krill's moult, whereas NAGase C participated in the digestion of chitin-containing dietary components. Chromatographic elution profiles of isolated organs confirmed the immunological results by displaying characteristic organ-specific patterns in NAGase activity. The molecular characteristics of the moulting form, NAGase B, may further indicate a vesicular transport of moulting enzymes from the epidermis into the ecdysial space. Based on our results we develop a hypothesis explaining the concurrent processes of simultaneous chitin degradation and chitin synthesis occurring during moult.