STRUCTURAL, IMMUNOLOGICAL AND FUNCTIONAL COMPARISONS OF FACTOR-H, RHEUMATOID-ARTHRITIS PROTEIN (RHP), AND ITS APPARENT NORMAL COUNTERPART (N-RHP)

The isolation and characterization of two human serum proteins, RHP an d N-RHP, are described. N-RHP appears to be the normal counterpart of RHP which is found at elevated levels in sera of patients with rheumat oid arthritis [Rosano et al. (1988b) Inflammation 12, 351-360]. Althou gh both proteins crossreact with anti-Factor H and have identical N-te rminal amino acid sequences, they differ from Factor H in pI, solubili ty at low ionic strength, and in glycosylation. RHP differs from Facto r H and N-RHP in antigenicity in the rabbit, in effect on the Clq-anti -Clq precipitin reaction, and in ability to disaggregate Cl, the first component of the complement system. Removal of RHP, N-RHP and Factor H from binding to Clq is a prerequesite for separation of RHP and N-RH P from Factor H by anion exchange chromatography and isoelectric focus ing. The finding of uniquely demonstrable RHP activity (enhancement of Clq-anti-Clq precipitin activity) in unfractionated sera from patient s with rheumatoid arthritis, but not in normal sera, suggests that RHP is not an artefact of Factor H produced during isolation.