Biosynthesis and Secretion of α1-Antitrypsin in Primary Cultures of Rat Hepatocytes.

The biosynthesis and secretion of α₁-antitrypsin was studied in rat hepatocyte primary cultures. After labeling with [³⁵S]methionine an α₁-antitrypsin with an apparent molecular weight of 49000 estimated by sodium dodecyl sulfate polyacrylamide slab gel electrophoresis was immunoprecipitated from the cell homogenate. This intracellular form of α₁ -antitrypsin could be deglycosylated by endoglycosidase H treatment indicating that its oligosaccharide chains were of the high-mannose type. Pulse-chase experiments showed that about 30 min after its synthesis the transformation of the 49000-M_r α₁ -antitrypsin to a protein with an apparent molecular weight of 54000 began. Only this 54000-M_r protein was secreted by the hepatocytes. The 54000-M_r α₁ -antitrypsin was not sensitive to endoglycosidase H, but sensitive to neuraminidase, and it incorporated [³H]galactose and [³H]fucose indicating that its oligosaccharide chains were of the complex type. In the presence of tunicamycin, which blocks the formation of N-asparagine-linked oligosaccharide chains, an unglycosylated α₁-antitrypsin with an apparent molecular weight of 41000 was found in the cells as well as in the medium. However, tunicamycin decreased the secretion of α₁-antitrypsin by 60–70%, whereas the secretion of albumin remained unaffected. In the presence of colchicine the secretion of both α₁-antitrypsin and albumin was impaired. The results demonstrate the importance of glycosylation for the secretion of α₁-antitrypsin. [ABSTRACT FROM AUTHOR]