1. The protein caeruloplasmin inhibits certain free radical reactions, in part probably due to its ability to oxidize Fe2+ into Fe3+ (ferroxidase activity). Since caeruloplasmin is present in synovial fluid from rheumatoid patients, we investigated its properties in relation to protection of the joint from damage by oxygen radicals produced by activated phagocytes.
2. The ferroxidase and ascorbate oxidase activities of serum from both normal and rheumatoid patients could be accounted for by the caeruloplasmin present, as determined immunologically. More caeruloplasmin is present in the serum of rheumatoid patients than in normal serum.
3. Synovial fluid from rheumatoid patients contains caeruloplasmin protein but its ferroxidase and ascorbate oxidase activities are abnormally low. It is suggested that ferroxidase deficiency contributes to radical damage in the rheumatoid joint.