Lysosomal protective protein/cathepsin A (PPCA) is a lysosomal serine carboxypeptidase that forms an intralysosomal enzyme-complex with b-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 platelets and lymphocytes, has been shown to function on the inactivation of selected neuropeptides, like substance P, oxytocin, and endothelin I. The autosomal recessive genetic deficiency of PPCA causes galactosialidosis, a neurodegenerative lysosomal storage disorder, resulting in the secondary deficiencies of b-galactosidase and NEU1.
