The mammalian purple
acid phosphatases (also called
tartrate-resistant
acid phosphatases) are expressed primarily in actively resorbing osteoclasts and activated macrophages. The
enzymes are characterized by the presence of a binuclear
iron center at the active site. Recent studies on transgenic mice lacking
purple acid phosphatase implicate the osteoclast
enzyme in both
bone resorption and bone mineralization. To characterize the mammalian
enzymes in more detail, particularly with respect to their substrate specificity at the low pH of the osteoclastic resorptive space (2.5-3), we have purified the recombinant human and mouse
enzymes from baculovirus-infected insect cells. The properties of the recombinant mouse
enzyme are compared with those of the nonrecombinant
enzyme isolated from mouse spleen. The kinetics of hydrolysis of the substrates
p-nitrophenyl phosphate,
phosphotyrosine, and
pyrophosphate and a phosphotyrosyl
peptide by the recombinant human and mouse
enzymes and the nonrecombinant mouse and pig
enzymes were analyzed. For all the
enzymes the ratio k(cat)/Km was typically approximately 10(6) M(-1) s(-1) and was higher at pH 2.5 than at 4.9. The increase was attributable to a large decrease in Km at the lower pH value. The results indicate that the
enzyme exhibits high catalytic efficiency toward substrates such as
pyrophosphate and acidic
phosphotyrosine-containing
peptides, particularly at low pH values typical of the bone resorptive space. The implications of the results for the physiological function of the
enzyme are discussed.