Abstract
Alpha-amylases are ubiquitously distributed throughout microbials, plants and animals. It is widely accepted that omnivorous crustaceans have higher α-amylase activity and number of isoforms than carnivorous, but contradictory results have been obtained in some species, and carnivorous crustaceans have been less studied. In addition, the physiological meaning of α-amylase polymorphism in crustaceans is not well understood. In this work we studied α-amylase in a carnivorous lobster at the gene, transcript, and protein levels. It was showed that α-amylase isoenzyme composition (i.e., phenotype) in lobster determines carbohydrate digestion efficiency. Most frequent α-amylase phenotype has the lowest digestion efficiency, suggesting this is a favoured trait. We revealed that gene and intron loss have occurred in lobster α-amylase, thus lobsters express a single 1830 bp cDNA encoding a highly conserved protein with 513 amino acids. This protein gives rise to two isoenzymes in some individuals by glycosylation but not by limited proteolysis. Only the glycosylated isoenzyme could be purified by chromatography, with biochemical features similar to other animal amylases. High carbohydrate content in diet down-regulates α-amylase gene expression in lobster. However, high α-amylase activity occurs in lobster gastric juice irrespective of diet and was proposed to function as an early sensor of the carbohydrate content of diet to regulate further gene expression. We concluded that gene/isoenzyme simplicity, post-translational modifications and low Km, coupled with a tight regulation of gene expression, have arose during evolution of α-amylase in the carnivorous lobster to control excessive carbohydrate digestion in the presence of an active α-amylase.
Highlights
Different to other crustaceans that exhibit transitions in their feeding habits through ontogeny, spiny lobsters are carnivorous since first feeding
Crustaceans offer a wide platform for advancing the understanding of adaptive digestive physiology as they include species with remarkably different feeding habits from mostly herbivorous to strict carnivorous
We previously reported the occurrence of four enzymes with amylolytic activity in the carnivorous spiny lobster P. argus [11]
Summary
Different to other crustaceans that exhibit transitions in their feeding habits through ontogeny, spiny lobsters are carnivorous since first feeding. In other studies the α-amylase activity of some carnivorous crustaceans [19,20] including spiny lobsters [8,11,18] has been observed as high as in herbivorous or omnivorous crustaceans. This high α-amylase activity in spiny lobsters seems to contradict the limited metabolic use of dietary carbohydrates (e.g., high and prolonged hyperglycemia after feeding) [21,22], probably because reduced activity of enzymes involved in both glycolysis and glycogen synthesis [22]
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