Partial characterization of the sialic acid-free forms of alpha-1-acid glycoprotein from human plasma

Abstract

Some of the properties of sialic acid-free alpha(1)-acid glycoprotein prepared by mild acid hydrolysis (pH1.6 at 80 degrees for 1hr.) were compared with those of neuraminidasetreated alpha(1)-acid glycoprotein. Chemically, the former contained less fucose (15%) and amide (2%) residues. Physicochemically, it had undergone certain changes primarily pertaining to the secondary structure, so that the specific optical rotation was more negative than that of the latter. A further expression of this change is probably the difference in the pH range of the resolution into two bands on electrophoresis. The resolution of the glycoprotein prepared by mild acid hydrolysis seems to be extended to more acidic pH values both by starch-gel and free moving-boundary electrophoresis. On ultracentrifugation both preparations appeared homogeneous and sedimented with a rate of 3s. Removal of sialyl residues at different pH values, in the range 1-7, showed that 2moles of sialic acid/mole of protein are very strongly bound. The two variants of alpha(1)-acid glycoprotein were isolated from pooled sialic acid-free alpha(1)-acid glycoprotein by preparative starch-gel electrophoresis and from selected blood of normal adults by fractionation by solubility and chromatography. Ultracentrifugal and starch-gel electrophoretic analyses at pH5, with incubation times of 1 or 24hr., demonstrated that no dissociation-association equilibrium (constant sedimentation coefficient and molecular weight) or isomerization (constant apparent electrophoretic mobilities) exist between the two variants. Therefore these variants are not sub-units of native alpha(1)-acid glycoprotein but represent modifications of naturally occurring proteins. Further, it was shown that the difference in the electrophoretic mobilities between the two variants was not due to any difference in amide content. Immunochemically, the two variants share the same determinants.