Two major serum components antigenically related to complement factor H are different glycosylation forms of a single protein with no factor H-like complement regulatory functions.

Factor H is a 150-kDa serum glycoprotein with key regulatory functions in the alternative pathway of complement activation. Two glycoproteins with a molecular mass of approximately 42 and 37 kDa that react with an antiserum against factor H were purified from human plasma. The two glycoproteins have identical N-terminal amino acid sequences but differ in glycosylation. Sequence comparisons indicated that they both correspond to a 1.4-kb mRNA recently cloned from human liver cDNA. The serum concentration of the two glycoproteins together was estimated to be approximately 40 mg/liter. They were found not to exert factor H-like regulatory functions in the alternative pathway. Thus, the 42-kDa glycoprotein described here appears to be distinct from the previously characterized factor H-related protein of similar size, suggesting that human serum contains two factor-H related molecules which both have a molecular mass of 41 to 43 kDa but which differ largely in structure.