The microheterogeneity of human plasma alpha1-acid glycoprotein.

alpha1-Acid glycoprotein was isolated in the homogeneous state from the plasma of 33 normal individuals and subjected to analytical isoelectric focusing before and after treatment with neuraminidase. The native glycoprotein preparations, resolved into 6 to 8 bands, were quantitated and grouped into two classes according to the patterns obtained: One class exhibited a relatively anodic and the other a relatively cathodic distribution of the protein bands. The isoelectric points of these bands ranged from pH 2.90 to 3.30. After treatment with neuraminidase the resulting asialo-glycoproteins were also quantitated and afforded two types of fundamentally different patterns from those mentioned above, namely one type with one and the other type with two main bands and both exhibiting several minor components. The isoelectric points of the main bands were found to be of pH 4.55 and 4.70 while those of the minor bands were at both the anodic and cathodic side of the major bands. No apparent relationship between the patterns of the native and those of the asialo-glycoproteins could be established. In addition, a new variant was noted whose major band focused at a pH of 5.0. The microheterogeneity of alpha1-acid glycoprotein is thus interpreted to be due to the amino acid replacements of this protein in combination with the linkages of the sialyl to the galactosyl residues in the native protein.