The effects of limited proteolysis by trypsin on human haptoglobin

Trypsin digestion of haptoglobin resulted in four glycopeptides. The glycopeptides were characterized by amino acid composition and molecular weight, as determined by thin-layer chromatography, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis in the presence or absence of 2-mercaptoethanol. Hemoglobin-binding capacity and immunological properties were investigated. Glycopeptides I and II did not form an active complex with hemoglobin and they inhibited the reaction of haptoglobin with specific antiserum by over 70%. Glycopeptides III and IV showed 11 and 4% of the hemoglobin-binding capacity and 82 and 67% of antigenic reactivity of native haptoglobin, respectively. Glycopeptide IV contained three antigenic determinants, whereas glycopeptides III contained four, one of them being exposed by trypsin digestion. In crossed two-dimensional immunoelectrophoresis, glycopeptide III showed at least four components reacting with antihaptoglobin serum, and glycopeptide IV, two components.