12] C1q

Abstract

This chapter describes the assay, purification, and properties of Clq. Because Clq plays both a binding role and an activation role in the C1 complex, its activity may be estimated either by the use of an agglutination assay employing IgG-coated latex particles, of a binding assay employing I-labeled Clq and immune aggregates, or of a hemolytic assay. Outdated human plasma may be used in the preparation of Clq without any apparent significant lowering of the final yield of protein or activity. Three main types of procedure can be successfully used in the preparation of human Clq: (a) conventional purification procedures involving euglobulin precipitation, ion-exchange chromatography, and gel filtration; (b) precipitation procedures involving primarily the use of DNA, or low ionic strength buffers containing the chelating agents EGTA and EDTA, to precipitate out specifically the Clq; (c) affinity chromatography on IgG-Sepharose 4B. In general procedures (a) or (c) might be used when relatively large amounts of serum (>500 ml) are available and quantities greater than 10 mg of Clq are routinely required, or if it is desired to purify proteins other than Clq at the same time. If a small amount of serum (<50 ml) is available, or if only a small amount of Clq is required, procedure (b) employing low ionic strength buffers containing chelating agents should be considered.