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Determination of the subunit composition of haptoglobin 2-1 polymers using quantitative densitometry of
Abstract:
Human haptoglobin (Hp), a hemoglobin-binding glycoprotein containing two types of polypeptide chains, alpha and beta, in equimolar amounts linked by disulfide bonds, exists in three major phenotypes determined by the properties of the alpha chain: Hp 1-1 (alpha1), Hp 2-2 (alpha2), and Hp 2-1 (alpha1 and alpha2). Hp 2-2 and Hp 2-1 form a series of alpha-disulfide-linked polymers. The subunit composition of the Hp 2-1 series was studied by isolation of single Hp 2-1 polymers by polyacrylamide gel electrophoresis. After reductive disulfide cleavage and alkylation the relative content of alpha2 and alpha1 polypeptide chains was determined by quantitative densitometry of acid/urea polyacrylamide gels stained with Coomassie brilliant blue R250. The molar ratios alpha2/alpha1 for the Hp 2-1 polymers. P1 through P5 (in order of decreasing electrophoretic mobility), were found to be: P1, 0.0 (alpha1 only); P2, 0.48; P3, 0.97; P4, 1.6; P5, 2.0. Since one alphabeta-Hb half-molecule is known to bind to each Hp beta chain, the beta polypeptide chain content of each of the Hp 2-1 polymers could be estimated by by counting the number of Coomassie blue bands formed after electrophoresis of isolated Hp 2-1 polymers fractionally saturated with cyanmethemoglobin (Hb). The number of beta chains present in Hp 2-1 polymers P1 through P4 was determined to be: P1, 2; P2, 3; P3, 4 and P4, 5. Molecular weights of the Hp 2-1 polymers were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresus using as standards the almost homologous Hp 2-2 polymer series whose molecular weights are known from ultracentrifugation studies. Molecular weights for the first five Hp 2-1 polymers were estimated to be 107,000; 162,000; 217,000, 274,000; and 331,000, respectively. These data are consistent with the previously proposed model for the subunit composition of the Hp 2-1 polymer series when P1 = (alpha1 beta)2 and the subsequent polymers in order are represented as (alpha1 beta)2(alpha2 beta)n where n = 1,2,3,4...