Characterization and expression of tartrate-resistant acid phosphatase (TRAP) in hematopoietic cells

Acid phosphatase (AcP, EC 3.1.3.2) is represented by a number of enzymes that can be differentiated according to structural and immunological properties, tissue distribution, subcellular location and other features; these AcP isoenzymes share similar catalytic activity toward phosphoesters in an acidic medium. Classically, AcPs have been divided into four types according to their sensitivity to tartrate and to their origin: erythrocytic, lysosomal, prostatic AcP, and an AcP enzyme that was first identified in hairy cell leukemia (HCL). This latter AcP was termed isoenzyme 5 (based on its electrophoretic mobility) or human type 5, tartrate-resistant acid phosphatase (TRAP). Differences in various physicochemical properties, lack of amino acid sequence similarity and different chromosomal locations of the respective genes showed that the four AcP isoenzymes are not related. The biochemical properties of TRAP are unique: resistance to inhibition by tartrate, but inhibition by molybdate; glycoprotein of 30-40 kDa occurring as two similar isoforms with different carbohydrate content, each composed of dissimilar subunits of 16 and 23 kDa in disulfide linkage; active at acid pH (optimum at 5-6) with basic pI (8.5-9.0); presence of an iron active site giving the purified protein a purple color. The TRAPs of different human sources (HCL spleen, osteoclastoma, Gaucher's spleen, placenta) have an 85-94% homology in their amino acid sequences. Full-length TRAP cDNAs (1.4 kb) have been cloned from human placenta and Gaucher's spleen. Variations in TRAP structure appear to result from post-translational modifications and not from the existence of a multigene family as only a single TRAP gene and a single mRNA species have been reported. This notion of a single TRAP gene is supported by the substantial sequence homology found among the various TRAPs from human tissues and from animal sources (e.g. bovine spleen and bone; rat spleen, bone and epidermis; pig uterus). The latter enzyme preparations of animal origin have been described for many years as the purple acid phosphatase (PAP). However, the high degree of sequence homology indicated that TRAP and PAP enzymes represent a single entity belonging to the class of metalloproteins. The human TRAP gene was assigned to chromosome 15 and to chromosome 19 by two groups. TRAP protein is localized in lysosomes or similar organelles and is not secreted. The serum level of TRAP was found to be increased during physiological bone growth, in Gaucher's disease, and in malignancies metastasized to bone (resulting from increased osteoclastic activity).(ABSTRACT TRUNCATED AT 400 WORDS)