Fibronectin associated with Clq in a Clq isolation procedure

The complement component Clq, prepared by euglobulin precipitation of serum to which EDTA or EGTA had been added, contained fibronectin (FN) as detected by radioimmunoassay and immunodiffusion methods. The FN contents of the Clq preparations varied between 3 and 29% by weight of the Clq contents. Adsorptions of sera with polymerized IgG (an absorbent for Clq) in the presence or absence of EDTA removed all detectable Clq and between 12 and 95% of the FN. In a similar manner, adsorptions of sera and Clq preparations with insolubilized gelatin (to which FN will bind) reduced greatly or removed completely the FN component but also strikingly reduced the Clq contents. High salt concentration or the addition of EDTA did not alter the gelatin absorption results indicating that the association was not sensitive to high ionic condition and that Clq was equally bound as Clq or as the Cl complex. The results suggest that FN and Clq bind individually to both gelatin and IgG or that FN and Clq co-associate, accounting for removal of one component when the other is bound to its expected adsorbent.