Inhibition of the interaction between the complement component Clq and immune complexes.

Several groups of compounds were examined for their ability to inhibit in vitro the binding of Clq to insoluble immune complexes. As expected from previous studies, polyinosinic acid, liquoid, sodium pentosan polysulfate, aliphatic diamines and heparin are good inhibitors. This study shows that compounds with much lower toxicity, such as certain amino acids and substances with vitamin B6 activity (pyridoxal-5-phosphate:P5P) were also capable of decreasing the binding of Clq. Using methods of equilibrium dialysis and difference spectra, it was shown that this compound binds to both, Clq und IgG antibody by forming Schiff bases. Clq binds approximately 10 times more P5P when compared to IgG. Immune complexes prepared with IgG antibody modified by P5P and stabilized with sodium borohydride had the same complement-fixing and Clq-binding capacity as normal immune complexes. This suggests that the inhibition of Clq-binding to immune complexes by P5P is due to a modification of lysyl resides in Clq. Collagen and IgG fragments derived from Fc were also found to inhibit Clq binding to immune complexes, but at higher concentrations than the above small-molecular compounds and with a different mode of action.