Comprehensive enzymatic analysis of the amylolytic system in the digestive fluid of the sea hare, Aplysia kurodai : Unique properties of two α‐amylases and two α‐glucosidases

Sea lettuce ( Ulva pertusa ) is a nuisance species of green algae that is found all over the world. East‐Asian species of the marine gastropod, the sea hare Aplysia kurodai , shows a clear feeding preference for sea lettuce. Compared with cellulose, sea lettuce contains a higher amount of starch as a storage polysaccharide. However, the entire amylolytic system in the digestive fluid of A. kurodai has not been studied in detail. We purified α‐amylases and α‐glucosidases from the digestive fluid of A. kurodai and investigated the synergistic action of these enzymes on sea lettuce. A. kurodai contain two α‐amylases (59 and 80 kDa) and two α‐glucosidases (74 and 86 kDa). The 59‐kDa α‐amylase, but not the 80‐kDa α‐amylase, was markedly activated by Ca 2+ or Cl − . Both α‐amylases degraded starch and maltoheptaose, producing maltotriose, maltose, and glucose. Glucose production from starch was higher with 80‐kDa α‐amylase than with 59‐kDa α‐amylase. Kinetic analysis indicated that 74‐kDa α‐glucosidase prefers short α‐1,4‐linked oligosaccharide, whereas 86‐kDa α‐glucosidase prefers large α‐1,6 and α‐1,4‐linked polysaccharides such as glycogen. When sea lettuce was used as a substrate, a 2‐fold greater amount of glucose was released by treatment with 59‐kDa α‐amylase and 74‐kDa α‐glucosidase than by treatment with 45‐kDa cellulase and 210‐kDa β‐glucosidase of A. kurodai . Unlike mammals, sea hares efficiently digest sea lettuce to glucose by a combination of two α‐amylases and two α‐glucosidases in the digestive fluids without membrane‐bound maltase–glucoamylase and sucrase–isomaltase complexes.