Intrahepatic precursor form of rat alpha 1-acid glycoprotein. Isolation and properties.

Abstract

al-Acid glycoprotein (AGP) was purified to homogeneity from rat serum.Using antiserum raised against the purified AGP, immunoelectrophoretic analyses of serum showed one precipitin arc, while buffer extracts from liver homogenate showed an additional, slow moving form of AGP.During a 15-min incubation of liver cells in suspension with ['*C]leucine, only the liver-specific, slow moving form of AGP became highly labeled.During the subsequent chase, radioactivity in the liver form decreased while that in the serum form increased, suggesting that the former is a biosynthetic precursor of the latter.The liver-specific AGP was isolated by DEAE-cellulose chromatography, ammonium sulfate fractionation, and polyacrylamide gel electrophoresis.Its apparent molecular weight is 39,500 in contrast to 43,500 for AGP from serum, as determined by electrophoresis in dodecyl sulfate-polyacrylamide.Each of the two forms of AGP shows four bands in isoelectric focusing, in the pH range of 4.7 to 5.1 for the liver form and 3.3 to 3.6 for AGP from serum.The amino acid composition of the two forms of AGP is very similar, but their carbohydrate composition differs greatly.The liver-specific AGP contains 6% glucosamine, 30% neutral sugars, but no sialic acid, while AGP from serum contains 12% glucosamine, 15% hexose, and 10% sialic acid.This implies that, during AGP secretion from the liver cell, several of the neutral sugars initially attached to liver AGP are removed and, subsequently, other sugars such as glucosamine and sialic acid are added.This finding conforms with current ideas on the glycosylation of secretory proteins containing asparaginyl-linked, complex-type oligosaccharide chains.al-Acid glycoprotein has been described as a constituent of mammalian and avian serum (for review, see Ref. 2).Due to its remarkably high carbohydrate content of about 40% and the fact that its synthesis rate increases severalfold during inflammatory processes, this protein has attracted much attention.The primary structures of the single polypeptide chain (3) and of the five complex-type oligosaccharide side chains (4) of human AGP' are known.Its function, though, still remains to be determined.AGP is synthesized in the liver and is secreted via rough and smooth endoplasmic reticulum and Golgi appratus (5, 6).AGP from serum migrates like an al-globulin during elec-