Improvement of BF typing and of BF F subtyping after neuraminidase treatment in the unconverted and converted factor B

When neuraminidase‐treated sera are analyzed by agarose gel isoelectric focusing, the factor B (BF) banding pattern is reduced to predominantly one major band without cathodically positioned bands. This not only makes unequivocal typing of BF allotypes possible but also the reliable distinction of all BF F subtype phenotypes with delimitation of “BF F subtype variants”. With this new method, serum aging affects the BF determination to a lesser extent than when applying methods that separate native sera. We show that sialylation is not responsible for the BF F subtype polymorphism. All of the investigated BF allotype bands, including those characteristic of the subtypes, show functional hemolytic activity. The banding pattern after removal of neuraminic acid residues ranges from pH 6.8 to 7.3 for factor B, from pH 5.3 to 5.9 for the Ba fragment, and from pH 8.2 to 8.7 for the Bb fragment. The protein structure of factor B is also discussed. Eliminating the superimposition of bands in different BF allotypes, as demonstrated by these methods, proved to be necessary for the detection of hypomorphic BF gene products (BF QL), which are expressed by assumed BF*Q0 alleles in heterozygous genotypes. This allows investigation of BF*Q0 alleles on a protein level, which complements molecular genetic approaches.