Physiological significance of α-amylase expressed in livers

Abstract

The physiological significance of sugar chains in human salivary amylase was found previously to be maltotriose hydrolysis and glucose formation. Rat liver amylases are glycosylated, suggesting that the amylase may contribute to glycogen metabolism in liver. In fact, the rat liver amylase showed higher affinity to glycogen and hydrolyzed maltotriose more rapidly than the salivary amylase did. The liver amylase has been expressed from a rat fetus in levels of catalytic activity, protein and mRNA. Moreover, in livers with accelerated amylase activity, e.g., at 2-4 weeks after birth or during liver regeneration after partial hepatectomy, it was found that an amylase protein, which was electrophoreticaly identical to pancreatic type, was expressed in addition to salivary type amylase. However, a restriction endonuclease assay using EcoRI after RT-PCR amplification revealed that the liver expressed only one type of amylase mRNA, but not a pancreatic type. Therefore, the pancreatic type amylase in the liver might be a different protein from the authetic pancreatic amylase.
Liver amylase activity in humans or pigs was found to be lower than in rodents. However, the human liver amylase has been expressed in levels of protein and mRNA. A PCR product (474bp) of human liver amylase cDNA fragment was sequenced and referred to the sequence homology. The sequence was identical to the corresponding cDNA of AMY-2B, which was known to expressed in tumorous tissues. In situ hybridization also revealed the expression of AMY-2B mRNA in non-tumorous human liver.
In evolution, livers naturally might produce high levels of amylase activity, which might contribute to glycogen metabolism. The liver amylase may, as it were, be an ancestral isozyme. As the functional specialization of salivary and pancreatic type amylases, the liver amylase thought to be degenerated in the physiological function, especially in human.