Electrophoretic Polymorphism of Human C4 is Due to Charge Differences in the α‐Chain, Presumably in the C4d Fragment

Various common C4 gene products were isolated from serum by immunoprecipitation. After reduction the C4 α‐, β‐, and γ‐polypeptide chains were studied by two‐dimensional electrophoresis. Isoelectrofocusing was performed in the first dimension and sodium dodecyl sulphate polyacrylamide gradient gel electrophoresis in the second. The charge differences behind the electrophoretic C4 polymorphism were shown to reside in the 95,000‐u(atmic mass units) α‐chain. Charge variation closely mirroring the α‐chain differences were also found in a 49,000‐u fragment of the α‐chain, most probably C4d. The basic β‐chain could not be studied in detail, but no differences were observed with regard to molecular weight or charge of the γ‐chains of the different C4 gene products.