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

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.