Serum tartrate-resistant acid phosphatase isoforms in rheumatoid arthritis.

Objectives: Our objective was to evaluate the significance and source of serum tartrate-resistant acid phosphatase (TRACP) in patients with rheumatoid arthritis (RA).

Methods: Thirty-five RA, 32 osteoarthritis (OA) and 16 control subjects were studied. Serum TRACP-5b activity and total TRACP protein were determined by immunoassay. TRACP isoforms were analyzed by non-denaturing polyacrylamide gel electrophoresis (PAGE). Serum bone alkaline phosphatase (BAP), cross-linked N-terminal telopeptides (NTx), and C-terminal telopeptides (ICTP) of type I collagen were estimated as markers of bone turnover. C-reactive protein (CRP) was measured as a marker of chronic inflammation. Macrophages and dendritic cells (DC) were developed from peripheral blood monocytes. Cell lysates and culture supernatants were analyzed for TRACP isoforms by immunoassay and PAGE.

Results: In RA, mean TRACP-5b activity was normal, but median total TRACP protein was increased twofold (p<0.001). In OA, TRACP-5b activity and protein were normal. In RA, TRACP-5b activity correlated weakly with ICTP (r=0.56) while TRACP protein levels correlated weakly with NTx (r=0.43). Additionally, TRACP protein, but not TRACP-5b activity correlated significantly with CRP (r=0.42). Macrophage and DC lysates contained TRACP-5b, while tissue culture supernatants contained TRACP-5a.

Conclusions: Increased total TRACP protein in RA sera was probably due to TRACP-5a and not derived from osteoclasts. Rather, it could be a secreted product of inflammatory macrophages and DC.