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Metabolism of Glycosylated Human Salivary Amylase: In Vivo Plasma Clearance by Rat Hepatic Endothelial Cells and In Vitro Receptor Mediated Pinocytosis by Rat Macrophages
Author(s) -
Niesen Thomas E.,
Alpers David H.,
Stahl Philip D.,
Rosenblum Jerry L.
Publication year - 1984
Publication title -
journal of leukocyte biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.819
H-Index - 191
eISSN - 1938-3673
pISSN - 0741-5400
DOI - 10.1002/jlb.36.3.307
Subject(s) - amylase , biology , mannose receptor , medicine , human serum albumin , endocrinology , albumin , saliva , receptor , mannose , in vitro , in vivo , biochemistry , hamster , pinocytosis , endocytosis , enzyme , macrophage , microbiology and biotechnology
Salivary‐type amylase normally comprises about 60% of the amylase activity in human serum, but only a small fraction is a glycosylated isoenzyme (amylase A). In contrast, 1/3 of amylase in human saliva is glycosylated. Since glycosylate can affect circulatory clearance, we studied the clearance of amylase A in rats and its uptake by rat alveolar macrophages. Following intravenous injection, 125 l‐labeled amylase A disappeared rapidly from plasma (t 1/2 = 9 min) and accumulated in the liver. Simultaneous injection of mannose‐albumin slowed its clearance to a rate comparable to that of 125 l‐labeled nonglycosylated salivary amylase (t 1/2=45 min). In contrast, galactose‐albumin had no effect. Electron microscope autoradiography of the liver following injection of 125 l‐labeled amylase A revealed a localization of grains over the hepatic endothelial cells. In vitro studies indicated that amylase A is taken up by alveolar macrophages via receptor‐mediated pinocytosis. Uptake was linear over time, saturable, and inhibited by mannan and mannose‐albumin, but not by galactose‐ albumin. We conclude that amylase A, which is a naturally occurring human glycoprotein with at most three terminal L‐fucose residues per molecule, is recognized in rats by a mannose receptor located on hepatic endothelial cells. We speculate that this receptor, by rapidly clearing circulating amylase A, may be responsible for the low level of amylase A in human serum.