The interaction between C1q and immune complexes is inhibited by 1-anilino-8-naphthalenesulfonate (ANS) in the concentration range of 2–4 mM. ANS binds to C1q with a 20-fold higher affinity than to IgG [(1986) Mol. Immunol. 23, 39–44] and therefore it is possible to label only C1q with ANS in the presence of IgG. Under such conditions no inhibition is observed. Addition of monomer IgG to a solution of C1q-bound ANS did not significantly after the fluorescence of the ANS. However when oligomeric IgG was added there was a 2-fold increase in fluorescence over the same IgG concentration range. When C1q was pretreated with diethylpyrocarbonate there was little change in the fluorescence when IgG oligomers were added to C1q:ANS solutions. These results suggest that C1q undergoes conformational changes upon binding to IgG oligomers.
