Metabolism of chitin precursors by crayfish tissues during chitin synthesis

AbstractIn order to follow changes in chitin precursors during chitin synthesis in the crayfish, Orconectes sanborni, tissue extracts were analyzed with an anion exchange column before and after injection of 14C‐N‐acetylglucosamine. Glucose concentration was found to be much lower in the blood than in the other tissues. Therefore, any movement into the tissues must occur against a steep concentration gradient. In the epidermis, most of the metabolites of the chitin synthetic pathway were absent in early premolt stage D0, and they were at their highest concentration in late premolt stage D3, when chitin synthesis was underway. Apparently the synthesis of these chitin precursors is initiated after premolt state D0. In epidermis, midgut gland, and muscle, the chitin precursors found in highest concentration, N‐acetylglucosamine‐6‐phosphate and UDP‐N‐acetyl‐glucosamine, decreased in concentration between stage D3 and postmolt stage C1‐2, the stage when rate of chitin synthesis is highest. This suggests the early enzymes in the chitin synthetic pathway, but not the late enzymes, increase in activity in premolt, causing accumulation of these two compounds. Then the late enzymes increase in activity in postmolt, reducing the levels of accumulated precursors and raising the overall rate of chitin synthesis. Chitin precursors were found in midgut gland and muscle, but little incorporation of label into macromolecules was seen. This suggests the possibility that these tissues may synthesize chitin precursors for use in chitin synthesis by the epidermis. Some N‐acetyl‐glucosamine‐6‐phosphate was found in the blood, possibly en route from the tissues that synthesize it, such as midgut gland and muscle, to the epidermis, which uses it for chitin synthesis. Even late in premolt, the chitin of the old cuticle, soon to be shed, became labeled. Therefore, the cuticle may serve as a reserve food supply by its continual breakdown and resynthesis.