Radioiodinated β1H was demonstrated to bind to sheep EAC4b, EAC4,2,3b and EC3b, but not to E, EA, EAC4,2 or to C3bINA and β1H treated EC3b. This binding was dependent upon the C3b concentration on the surface of the cells and could be inhibited by unlabeled β1H, free C3b or high concentrations of Factor B. The presence of activated properdin on EC3b cells did not affect β1H binding. Binding of radioiodinated C3bINA was demonstrated by stabilizing the enzyme substrate complex EC3b, βIH, C3bINA at 0°C. Binding of 125I-C3bINA to EC3b in the presence of β1H was 30-fold greater than in the absence of this cofactor. C3bINA binding was approximately stoichiometric with respect to bound β1H and this interaction had an apparent binding constant at 0°C of 108 M-1.
Fearon and Austen (J. Exp. Med. 146: 22, 1977) have reported that C3b on rabbit erythrocytes is protected from C3bINA and β1H action.