IgG is bound by antigen-antibody bonds and some IgG and albumin are bound by intermolecular disulfide bonds to cartilage in rheumatoid arthritis and osteoarthritis.

To elucidate the mechanisms for the presence of immunoglobulins and human serum albumin (HSA) in articular cartilage from patients with rheumatoid arthritis (RA) or osteoarthritis (OA), the recovery of these molecules was determined in several elution steps. These steps included serial elutions with a neutral buffer to extract entrapped molecules, elution with 6 M guanidine hydrochloride to extract molecules bound by noncovalent interactions, and digestion of cartilage with bacterial collagenase to release molecules covalently bound to cartilage matrix proteins. Significantly more IgG than HSA was recovered with 6 M guanidine after serial elutions with neutral buffer from the cartilages of patients with both RA and OA, consistent with the binding of IgG by antigen-antibody bonds. Degradation of cartilage with collagenase released additional IgG and HSA. Analysis of the IgG and HSA, recovered with guanidine or with collagenase, using SDS-PAGE and transfer blotting, indicated for the first time the presence of disulfide bonds between these molecules and cartilage matrix molecules.