The C1q inhibitor in serum is a chondroitin 4-sulfate proteoglycan.

An inhibitor of human C1q has been purified from serum and identified as a chondroitin 4-sulfate proteoglycan. A typical preparation contained 22% uronic acid, 20% hexosamine, 12% sulfate, and 9% protein. When chromatographed on Sepharose CL-2B, the proteoglycan was eluted as a broad peak with a mean Kav of 0.6, which indicates that it is polydisperse and has an average Mr = approximately 175,000 (range, 45,000-750,000). Unlike the major species of cartilage proteoglycans, the serum proteoglycan did not form a complex with hyaluronic acid. Additional evidence for the noncartilaginous origin of C1q inhibitor is that its glycosaminoglycan chains totally lack chondroitin 6-sulfate isomers. Furthermore, the glycosaminoglycan component of C1q inhibitor was eluted from Sepharose CL-6B with a Kav of 0.52, indicating that these polysaccharide chains are considerably larger than those of human articular cartilage proteoglycan. The interaction between the proteoglycan and C1q was clearly evident in 0.15 M NaCl, as demonstrated by a radial immunodiffusion technique. The interaction decreased with increasing ionic strength but was not entirely abolished even at 0.3 M NaCl. These findings suggest that the interaction between C1q and the C1q inhibitor may occur under physiological conditions and may be of importance in modulating C1q activity in vivo.