Pepsinogen--an immunoglobulin binding artefact in 'collagen' preparations.

It has previously been shown that extracts of human articular cartilage, many many of which contain type II collagen, react with heat-aggregated immunoglobulin and artificially prepared immune complexes. Sera from patients with rheumatoid arthritis and psoriatic arthritis, but not from patients with inflammatory bowel disease, react with these extracts. There are two distinct patterns of binding, either as low molecular weight immune complexes or as free antibody directed against collagen. Aggregate-binding activity identified in extracts of human articular cartilage following pepsin digestion was found to be distinct from collagen in its salt solubility. Further purification of this aggregate-binding factor by SDS gel electrophoresis has shown it to be an artefact resulting from the binding of small immune complexes to pepsinogen present in the pepsin preparation used to digest the cartilage.