Characterization of Enzymes That Degrade Crab Exoskeleton: I. Two Alkaline Cysteine Proteinase Activities

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Extracts from integumentary tissues of the Bermuda land crab, Gecarcinus lateralis, degraded proteins extracted from the membranous layer (ML), the innermost layer of the exoskeleton that serves as the interface between epidermal cells and calcified exoskeleton. Separation of the extracts by HPLC hydrophobic chromatography and characterization using 125I‐casein as substrate identified two peaks of degradative activity. The activities showed pH optima of 7 to 8, were sensitive to inhibitors of cysteine proteinases, and required the reducing agent, dithiothreitol, for activity, indicating that they are alkaline cysteine proteinases (ACPs). The pH optima of ACPs I and II indicated that they could function in the slightly alkaline extracellular environment of the proecdysial animal when the old exoskeleton is broken down. Although ACPs I and II are distinctly different from other crustacean cysteine proteinases (CP) they are similar to a CP activity present in extracts from the ML. A 23‐kDa protein from the ML was particularly susceptible to degradation by these enzymes. Activity of these enzymes in the extracellular environment might be controlled by the concentration of reducing agents as they are inactive unless in a reduced state.