FUNCTIONAL AFFINITY CONSTANTS OF REACTION BETWEEN I-125-LABELED C1Q AND C1Q BINDERS AND THEIR USE IN MEASUREMENT OF PLASMA C1Q CONCENTRATIONS

Abstract

Purified 125I-labeled C1q [q fragment of the 1st complement component] reacted with glutaraldehyde-treated red cells (glut-RBC), with a value for the functional affinity constant, K, of 1-3 .times. 108 M-1, based on measurement of concentrations of bound and free reactants at equilibrium. Values of K obtained for other C1q-binders were as follows: diaminopropane, 2 .times. 102 M-1; monomer IgG [immunoglobulin G] 5 .times. 104 M-1; heat-aggregated IgG, 0.5-2.5 .times. 108 M-1; IgG-anti-IgG complexes, 0.31 .times. 108 M-1. The functional rate constant for association (ka) between 125I-labeled C1q and glut-RBC was 5 .times. 105 M-1 s-1 at 37.degree. C in 0.17 M NaCl. The rate of dissociation of the C1q-glut-RBC complex was biphasic with rate constants (kd) of 2 .times. 102 s-1 and 2 .times. 105 s-1. Calculated values of K from the ratio ka/kd gave values of 2.5 .times. 107 M-1 and 2.5 .times. 1010 M-1. The range of values of K apparently reflects the involvement of 1, 2 or more binding sites on the C1q molecule. Reduction of the ionic strength of the medium from 0.17 M to 0.14 M increases the rate of association of C1q and glut-RBC 11-fold, indicating involvement of ionized groups at the binding site. A method is described for measuring plasma C1q concentrations by saturation assay, using 125I-labeled C1q and glut-RBC. Plasma C1q concentrations fell in the range 170-250 .mu.g/ml.