Tartrate-resistant acid phosphatase forms complexes with alpha(2)-macroglobulin in serum

Tartrate-resistant acid phosphatase (TRAP) is a standard histochemical mark er of differentiated osteoclasts and has been proposed as a serum/plasma ma rker for osteoclast activity. Enzyme assays have been described that show e levated TRAP enzyme activity in the serum or plasma of patient groups known to have increased bone metabolism, However, the poor stability of the enzy me and potential contribution from nonosteoclastic sources make it problema tic to measure in patient samples. Immunoassays developed to measure TRAP i n serum and plasma have yielded widely varying TRAP levels in both normal a nd disease states. It is not clear if this variability is caused by differe nces in assay calibration, antibody specificity, and/or TRAP instability. I n this paper, we report that purified TRAP spiked into serum forms high mol ecular weight complexes, Complex formation results in greatly decreased TRA P enzyme activity and immunoreactivity. The complexing protein in serum has been identified as alpha(2)-macroglobulin (alpha(2)M). Similar complexes a re observed in stored patient samples. In vitro studies with purified compo nents show that TRAP binds to alpha(2)M primarily through noncovalent ionic interactions. Our results demonstrate that one mechanism of TRAP instabili ty in serum is complex formation with alpha(2)M and suggest further that cu rrent TRAP enzyme and immunoassays may not accurately measure the circulati ng level of TRAP.