Tartrate-resistant purple acid phosphatase is synthesized as a latent proenzyme and activated by cysteine proteinases

Abstract

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.

Osteoclastic Tartrate-resistant Acid Phosphatase (Acp 5): Its Localization to Dendritic Cells and Diverse Murine Tissues

“…Clearly, TRAP may retain its putative role in intralysosomal digestion in the processing of complex macromolecular antigens before presentation in the context of surface MHC Class II molecules. Because this process is dependent on the presence of cathepsin S specifically to cleave the invariant MHC Class II chains for surface display of antigenic peptides, and because activation of TRAP occurs by limited endopeptidase cleavage (Ljusberg et al 1999), TRAP may also be activated in situ by the action of local proteinases of this class.…”

Structure, function, and regulation of tartrate-resistant acid phosphatase

“…29,51,59,80,89 Several proteolytic enzymes are able to cleave the exposed loop, but only the cysteine proteinases papain and cathepsin B have been able to cause activation among several tested. Ljusberg et al 75 put forth the view that TRAP, like several other hydrolases, is synthesized as a relatively inactive proenzyme, and cleavage is the physiological mechanism of proenzyme activation in osteoclasts.…”

Widespread expression of tartrate‐resistant acid phosphatase (Acp 5) in the mouse embryo

“…This processing results in the cleavage of the single chain 34 kDa form into a highly active 2 subunit form (Ljusberg et al 1999 a). Most tissues in the mouse embryos shown here to express TRAP are also known to express cysteine proteinases (Qian et al 1991 ;Szomor et al 1995).…”

“…TRAP purified from human osteoclastomas is composed of 2 subunits, initially translated as a single polypeptide (Hayman et al 1989(Hayman et al , 1991. It is thus of relevance that mice deficient in cathepsin K have recently been shown to express a significantly higher proportion of the uncleaved monomeric form of TRAP (Ljusberg et al 1999 b).…”