Alternative immunoassay for tartrate-resistant acid phosphatase isoform 5b using the fluorogenic substrate naphthol ASBI-phosphate and heparin.

Our purpose was to develop a specific immunoassay for tartrate-resistant acid phosphatase (TRACP) 5b using naphthol ASBI phosphate (N-ASBI-P) as a selective substrate for isoform 5b and heparin as a selective inhibitor of isoform 5a. Serum TRACP 5a and 5b and recombinant TRACP 5a were used to optimize and standardize the immunoassay for specificity, linearity, analytical recovery, sensitivity and reproducibility. Serum N-telopeptide cross-links (NTX) and bone alkaline phosphatase (bone ALP) were also measured. The clinical sensitivity and specificity were assessed in healthy control subjects and patients with osteoarthritis (OA), rheumatoid arthritis (RA) and endstage renal disease (ESRD). TRACP 5b specificity was achieved at pH 6.3 with 2.5 mmol/l substrate and 25 U/ml heparin. Isoform 5b specificity was increased over our original immunoassay using 4-nitrophenyl phosphate (4-NPP) without heparin. The alternative immunoassay was linear with 110% analytical recovery and no serum matrix effects. The average intra-assay error was 10.65%; the average inter-assay error was 10.11% for values of 1-3 U/l and 6.5% for values of 7-11 U/l. Mean serum TRACP 5b in OA and RA were not significantly different from control using either immunoassay. Mean serum TRACP 5b was significantly increased in ESRD with both immunoassays. Serum TRACP 5b levels correlated significantly with NTX and bone ALP in the disease groups, but not always in the control group. This alternative immunoassay for TRACP 5b activity is highly specific. It should have applications in evaluating patients with bone disease and will improve our understanding of the biological significance of TRACP 5b expression in health and disease.