Isolation and purification of a Clq-anti Clq precipitin ring enhancing protein from sera of patients with rheumatoid arthritis.

We have isolated and purified to apparent homogeneity a serum protein which appears to be a biological marker for active rheumatoid arthritis. The protein has been found in the sera in all 44 active rheumatoid arthritis patients thus far studied and is absent from, or present in undetected amounts, in sera from normal subjects or from patients with other arthritides. The protein has a molecular weight of 135,000 daltons, an isoelectric pH of 5.1-5.3, and it enhances the size of the C1q-anti C1q ring.