An avian serum .alpha.1-glycoprotein, hemopexin, differing significantly in both amino acid and carbohydrate composition from mammalian (.beta.-glycoprotein) counterparts

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Abstract
We report here on physicochemical characteristics of chicken hemopexin, which can be isolated by heme-agarose affinity chromatography [Tsutsui, K., and Mueller, G.C. (1982) J. Biol. Chem. 257, 3925-3931], in comparison with representative hemopexins of rat, rabbit, and human. The avian polypeptide chain appears to be slightly longer (52 kDa) than the human, rat, or rabbit forms (49 kDa), and also the glycoprotein differs from the mammalian hemopexins in being an .alpha.1-glycoprotein instead of a .beta.1-glycoprotein. This distinct electrophoretic mobility probably arises from significant differences in the amino acid composition of the chicken form, which, although lower in serine and particularly in lysine, has a much higher glutamine/glutamate and arginine content, and also a higher proline, glycine, and histidine content, than the mammalian hemopexins. Compositional analyses and 125I concanavalin A and 125I wheat germ agglutinin binding suggest that chicken hemopexin has a mixture of three fucose-free N-linked bi- and triantennary oligosaccharides. In contrast, human hemopexin has five N-linked oligosaccharides and an additional O-linked glycan blocking the N-terminal threonine residue [Takahashi, N., Takahashi, Y., and Putnam, F.W. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 2021-2025], while the rabbit form has four N-linked oligosaccharides [Morgan, W.T., and Smith, A. (1984) J. Biol. Chem. 259, 12001-12006]. In keeping with the finding of a simpler carbohydrate structure, the avian hemopexin exhibits only a single band on polyacrylamide gel electrophoresis under both nondenaturing and denaturing conditions, whereas the hemopexins of the three mammalian species tested show several bands. In contrast, the isoelectric focusing pattern of chicken hemopexin is very complex, revealing at least nine bands between pH 4.0 and pH 5.0, while the other hemopexins show a broad smear of multiple ill-defined bands in the same region. Like its mammalian counterparts, chicken hemopexin resists digestion by trypsin and plasmin when saturated with heme. Each of the four hemopexins forms a distinct peptide map on sodium dodecyl sulfate-polyacrylamide gel electrophoresis following tryptic digestion. These results indicate that the hemopexin of avians differs substantially from the hemopexins of mammals, which, as a group, show a notable similarity with regard to carbohydrate structure and amino acid composition.