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Tartrate-resistant acid phosphatase (TRAP) is a metallophosphoesterase participating in osteoclast-mediated bone turnover. Activation of TRAP is associated with the redox state of the di-iron metal center as well as with limited proteolytic cleavage in an exposed loop domain. The cysteine proteinases cathepsin B, L, K, and S as well as the matrix metalloproteinase-2, -9, -13, and -14 are expressed by osteoclasts and/or other bone cells and have been implicated in the turnover of bone and cartilage. To identify proteases that could act as activators of TRAP in bone, we report here that cathepsins K and L, in contrast to the matrix metalloproteinases, efficiently cleaved and activated recombinant TRAP in vitro. Activation of TRAP by cathepsin K/L was because of increases in catalytic activity, substrate affinity, and sensitivity to reductants. Processing by cathepsin K occurred sequentially by an initial excision of the loop peptide Gly Tartrate-resistant acid phosphatase (TRAP), 1 also known as type 5 acid phosphatase (EC 3.1.3.2) or uteroferrin, belongs to the purple acid phosphatase (PAP) subfamily of the non-heme dinuclear metallophosphatases (1-3). The metals of the catalytic center of all PAPs consist of a common ferric ion and a divalent metal cation in an active enzyme, where mammalian PAPs characteristically contain a redox-active iron in the M(II) site (4 -6).The TRAP enzyme is abundantly expressed by bone-resorbing cells, osteoclasts, and certain subpopulations of monocytes/ macrophages and dendritic cells (7-10). The precise role of osteoclastic TRAP is not fully understood, but studies on TRAP knock-out mice showed disturbed endochondral ossification with decreased resorptive activity of osteoclasts (11, 12), whereas overexpression of TRAP was associated with increased bone turnover (10). Different functions have been suggested for TRAP, e.g. as an osteopontin phosphatase (13-15), generation of reactive oxygen species (16 -19), iron transport (20 -24), and as a growth/differentiation factor for hematopoietic (25) and osteoblastic (26) cells.Mammalian PAPs are synthesized as 35-37-kDa monomers but are commonly isolated from tissues as proteolytically cleaved two-subunit forms consisting of a 23-kDa N-terminal domain disulfide-linked to a 16-kDa C-terminal domain. The monomeric form exhibits properties of a proenzyme with low phosphatase activity that is converted to a high activity, twosubunit form upon proteolytic cleavage in the intervening loop domain with either serine proteases, e.g. trypsin or chymotrypsin (27), or members of the cathepsin family (28,29). Mutagenesis studies suggested that proteolysis removes or alters repressive interactions between loop amino acids and active site residues because replacement of Asp 146 of the exposed loop domain with Ala resulted in activation of unproteolyzed TRAP (30).Several lines of evidence indicate a role for cathepsin K in bone resorption. Cathepsin K is highly expressed in osteoclasts near the ruffled border membrane and has been shown to participate i...

The tartrate-resistant acid phosphatase (TRAP) of rat osteoclasts has been shown to exhibit high (85-94%) identity at the amino acid sequence level with the purple acid phosphatase (PAP) from bovine spleen and with pig uteroferrin. These iron-containing purple enzymes contain a binuclear iron centre, with a tyrosinate-to-Fe(III) charge-transfer transition responsible for the purple colour. In the present study, production of rat osteoclast TRAP could be achieved at a level of 4.3 mg/litre of medium using a baculovirus expression system. The enzyme was purified to apparent homogeneity using a combination of cation-exchange, hydrophobic-interaction, lectin-affinity and gel-permeation chromatography steps. The protein as isolated had a purple colour, a specific activity of 428 units/mg of protein and consisted of the single-chain form of molecular mass 34 kDa, with only trace amounts of proteolytically derived subunits. The recombinant enzyme had the ability to dephosphorylate bone matrix phosphoproteins, as previously shown for bone TRAP. Light absorption spectroscopy of the isolated purple enzyme showed a lambda max at 544 nm, which upon reduction with ascorbic acid changed to 515 nm, concomitant with the transition to a pink colour. EPR spectroscopic analysis of the reduced enzyme at 3.6 K revealed a typical mu-hydr(oxo)-bridged mixed-valent Fe(II)Fe(III) signal with g-values at 1.96, 1.74 and 1.60, proving that recombinant rat TRAP belongs to the family of PAPs. To validate the use of recombinant PAP in substituting for the rat bone counterpart in functional studies, various comparative studies were carried out. The enzyme isolated from bone exhibited a lower K(m) for p-nitrophenyl phosphate and was slightly more sensitive to PAP inhibitors such as molybdate, tungstate, arsenate and phosphate. In contrast with the recombinant enzyme, TRAP from bone was isolated predominantly as the proteolytically cleaved, two-subunit, form. Both the recombinant enzyme and rat bone TRAP were shown to be substituted with N-linked oligosaccharides. A slightly higher apparent molecular mass of the monomeric form and N-terminal chain of bone TRAP compared with the recombinant enzyme could not be accounted for by differential N-glycosylation. Despite differences in specific post-translational modifications, the recombinant PAP should be useful in future studies on the properties and regulation of the mammalian PAP enzyme.

Purple acid phosphatases (PAPs) are binuclear acid metallohydrolases also referred to as tartrate-resistant acid phosphatases (TRAPs) or type 5 acid phosphatases. The cDNA sequences of TRAP/PAP enzymes from different species and organs indicate that these enzymes are translated as monomeric polypeptides of approx. 35 kDa, contrasting with the predominantly two-subunit structure observed in purified enzyme preparations. In the present study we have compared certain structural and enzyme-kinetic properties of recombinant rat PAP (monomeric) with those of the native rat bone TRAP/PAP enzyme (two-subunit), and examined effects on these parameters by cleaving the monomeric recombinant PAP with the serine proteinase trypsin or the cysteine proteinases papain or cathepsin B. Cleavage with trypsin resulted in a moderate activation of the recombinant enzyme and shifted the pH optimum to a slightly more basic value (5.0-5.5). Cleavage with papain resulted in complete activation and conferred similar properties to those of the bone PAP variant with regard to pH optimum (5.5-6.0) and sensitivity to reducing agents, as well as in the sizes of the subunits. Substrate specificity studies showed that the two-subunit bone PAP was considerably more active than the monomeric recombinant rat PAP towards a variety of serine-, threonine- and tyrosine-phosphorylated substrates. Of these substrates, bovine milk osteopontin seemed to be the most readily dephosphorylated substrate. In conclusion, the results suggest that the monomeric form of PAP represent a latent proenzyme with low enzymic activity towards both tyrosine- and serine/threonine-containing phosphorylated substrates. Besides being implicated in the catabolism of the extracellular matrix, members of the cysteine proteinase family might also exert a regulatory role in degradative processes involving the PAP enzymes by converting the newly synthesized PAPs to enzymically active and microenvironmentally regulated species.

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