Human articular cartilage proteoglycan monomers (PG) were purified and used for developing a radioimmunoassay. Its analytical sensitivity is 0.6 ng/tube and its clinical sensitivity is 20 #x3BC;l/tube for serum and 0.12 #x3BC;l/tube for synovial fluid. The intra- and between-assay variation coefficient are less than 10 and 20, respectively, in the linear part of the curve. There is a complete cross reaction with costal, vertebral disk and tracheal cartilage PGs and the PGs extracted from vein and artery. Concerning the latter, inhibition curves are not parallel. No cross reaction exists with PGs from various fetal tissues and small PGs from bone. However, large PGs from bone produce a weak cross reaction. Furthermore, the assay is species specific since cartilage PGs from dog, rat, chicken and calf embryos either do not or weakly cross react in the assay. Other constituents of cartilage: type II collagen, fibronectin, chondroitin sulfate and hyaluronic acid do not interfere with the assay. The antigenic determinants are localized on the protein core of the PG, as shown by the lack of cross reaction with glycosaminoglycans and PG treatment with various enzymes.
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