[en] Objective: Protein nitration is a prominent feature of inflammatory processes in the joint. We have developed immunoassays specific for a peptide of the alpha-helical region of type II collagen (108)HRGYPGLDG(116) (Coll 2-1) and its nitrated form (108)HRGY(NO2)PGLDG(116) (Coll 2-1 NO2) in biological fluids. Design: Coll 2-1 and Coll 2-1 NO2 peptides were injected into rabbits. Two antisera (D3 and D37) were selected for their specificity and affinity and used to develop specific immunoassays. Coll 2-1 and Coll 2-1 NO2 were measured in sera of 242 healthy subjects (N), 67 patients with primary knee osteoarthritis (OA) and 19 patients with rheumatoid arthritis (RA). Results: In healthy subjects, Coll 2-1 and Coll 2-1 NO2 concentrations were 125.13 +/- 3.71 nM and 0.16 +/- 0.08 nM, respectively. In OA and RA, Coll 2-1 and Coll 2-1 NO2 serum levels were found to be significantly increased compared to controls of the same range of age (Coll 2-1: OA: 200.80 +/- 8.98 nM, RA: 172.30 +/- 19.05 nM, normal: 126.60 +/- 6.70 nM and Coll 2-1 NO2: OA: 0.26 +/- 0.02, RA: 0.38 +/- 0.05, normal: 0.12 +/- 0.01 nM). Coll 2-1 NO2 levels were significantly more elevated in RA than in OA patients (P < 0.05). As a consequence, the ratio Coll 2-1 NO2/Coll 2-1 was 1.6 times higher in RA than in OA subjects. No relationship was found between the radiological OA severity and the levels of Coll 2-1 and Coll 2-1 NO2 in serum. Coll 2-1 NO2, but not Coll 2-1, was correlated with C-reactive protein in the sera of OA and RA patients. Conclusions: The determination of both Coll 2-1 and Coll 2-1 NO2 in serum of arthritic patients seems to be a promising useful tool for the detection of oxidative-related cartilage degradation episode. Further, these markers could be helpful for monitoring the effects of anti-inflammatory or antioxidant drugs on cartilage degradation. (c) 2004 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.