Unusual sialilation of three different rare genetic variants of serum DBP: Gc1A17, Gc1A16, and Gc1A11.

The proteins of three anodal Gc1 variants, Gc1A16, 1A11, and 1A17, are characterized by the most acidic isoelectric points observed so far among the different Gc mutants. Stepwise removal of N-acetylneuraminic acid (NANA) by treatment with neuraminidase was performed to estimate the degree of sialilation of these Gc variants. The results indicate that both proteins, the anodal and the cathodal component of these Gc1 mutants, carry sialic acid residues. This observation is remarkable in so far as usually only the anodal component of the Gc1 protein contains NANA and only a single residue. From the experiments carried out it can be deduced that Gc1A16 has two NANA residues in the anodal and one NANA residue in the cathodal component. Gc1A16 was found in four members of three generations in a Danish family; the variant segregated as a Mendelian trait. More difficult to interpret are the results obtained with the variants Gc1A11 and Gc1A17. Gc1A11 probably has three NANA residues in the anodal and two NANA residues in the cathodal component. Gc1A11 has been observed in two mother-child pairs and is presumably also a simple genetic trait. Gc1A17 has also several NANA residues in both Gc proteins; it is suggested that the anodal component has either three or four NANA residues and the cathodal component either two or three NANA residues. Family information on this variant is not yet available.