Lysosomal protective protein/cathepsin A (PPCA) is a lysosomal serine carboxypeptidase that forms an intralysosomal enzyme complex with ?galactosidase and neuraminidase (NEU1). PPCA is synthesized as a 54 kDa precursor/zymogen, and proteolytically cleaved in the lysosome into a catalytically active 32 and 20 kDa two chain enzyme. The enzyme has cathepsin A activity at acidic pH but maintains also a deamidase/esterase activity at neutral pH. Furthermore, the human enzyme, purified from plate