Lysine residues, but not carbohydrates, are required for the regulatory function of H on the amplification C3 convertase of complement

Lysine epsilon-amino groups of human factor H were selectively converted to guanidino groups by treatment with 0.1 M O-methylisourea at pH 10.4. Guanidination resulted in a dose-dependent decrease in the capacity of the regulatory protein to accelerate decay dissociation of P-stabilized amplification C3 convertase sites, to serve as a co-factor for cleavage of cell-bound C3b by I, and to compete for binding of 125I-untreated H to C3b. Modification of approximately 75% lysine epsilon-amino groups suppressed 97% of H functional activity. Biochemical analysis of native H demonstrated a total carbohydrate content of 18.5% (w/w) and the presence in the molecule of 11 biantennary oligosaccharidic chains of the N-acetyl-lactosaminic type. Total desialation of H by using Clostridium perfringens neuraminidase, and total deglycosylation of desialated H by using beta-endo-N-acetylglucosaminidase resulted in a 1.5- to 2-fold increase in H activity on a weight basis. Deglycosylation did not alter the capacity of H to discriminate between activating and nonactivating surfaces of the alternative pathway. Thus, lysine residues are important determinants of the binding capacity of H for cell-bound C3b, whereas the carbohydrate portion of the molecule is not required for the regulatory function of the protein on the amplification C3 convertase.