A study of the role of the asparagine-linked sugar chains of human complement subcomponent C1q in its biological activities.

The sialic acid residues were removed from asparagine-linked sugar chains on the C-terminal non-collagenous globular regions of human C1q by sialidase digestion. Both the haemolytic activity and the binding ability to immunoglobulin G (IgG) (Fc-binding ability) of C1q were unimpaired, even after the complete removal of sialic acid from these sugar chains. On the other hand, the rate of disappearance of C1q from the circulation was greatly accelerated by its desialylation, that is, the radioactivity of the infused intact and desialylated C1q was reduced to half for 200 min and for 140 min in the circulation of rats, respectively. A mixture of entire asparagine-linked sugar chains consisting of neutral, monosialyl and disialyl oligosaccharides was isolated from the intact C1q molecule by hydrazinolysis. The oligosaccharide-mixture isolated, after NaBH4 reduction, was added to assay system of C1q, but neither the haemolytic activity nor the Fc-binding ability was influenced.