ACP5

ACP5 encodes tartrate-resistant acid phosphatase (TRAP), a binuclear iron-containing acid phosphatase secreted by osteoclasts and activated macrophages. Uniquely among acid phosphatases, its activity is not inhibited by tartrate, and in bone-resorbing osteoclasts it is delivered in ruffled-border vesicles, where it can dephosphorylate matrix proteins such as osteopontin during resorption. Serum TRAP5b is an established clinical marker of osteoclast number and bone resorption used in osteoporosis management, and TRAP also contributes to macrophage functions in inflammation and immunity. Recombinant ACP5 is employed in tartrate-resistance enzyme assays, bone turnover biomarker development, and histochemical detection of osteoclasts.