Demonstration of the interaction of native C1 with monomeric immunoglobulins and C1 inhibitor.

The association of native C1 with physiologically relevant proteins was studied by ultracentrifugation. 125I-C1 was centrifuged through numerous sucrose density gradients, each of which contained a different concentration of monomeric (19S) IgM throughout the gradient. The s-rate of C1 (16S) increased with increasing IgM input to a maximum of 32S. In the absence of C1q, the C1r2s2 subunit did not bind to the Ig. In gradients containing physiologic concentrations of IgM (1.3 mg/ml) at 0.14 M ionic strength, the observed s-rate of C1 was 21S. In the presence of 13 mg/ml IgG, C1 sedimented with an s-rate of 19S. Thus, under physiologic conditions, a significant fraction of native C1 is reversibly bound to monomeric Ig. SDS-PAGE analyses show that this interaction does not lead to C1 activation. The interaction of native C1 with C1 inhibitor (C1-In) was studied by ultracentrifugation at physiologic ionic strength. Purified 125I-C1-In alone sedimented with an s-rate of 4S. However in the presence of excess native C1, one-third of the C1-In co-sedimented with C1 at a 16S position. For these studies, 100 microM nitrophenylguanidinobenzoate (NPGB) was present throughout the sucrose density gradient to prevent C1 activation during centrifugation. As the concentration of NPGB was increased, the percent of 125I-C1-In at 16S decreased, indicating that C1-In was binding (reversibly) to the C1 active site region(s), which is at least partially accessible in uncleaved C1. In controls, when NPGB was omitted or activated C1 was used, the s-rate of 125I-C1-In was only 12S due to the release of C1rC1s(C1-In)2 from activated C1. Thus, under physiologic conditions native C1 is reversibly bound to C1-In.