Cellular carboxypeptidases

This article focuses on four human carboxypeptidases (CPs): two metallo-CPs and two serine CPs. The metallo-CPs are members of the so-called B-type regulatory CP family as they cleave only the C-terminal basic amino acids Arg or Lys. The plasma membrane-bound CPM and the mainly, but nor exclusively, intracellular CPD are surveyed from this group of enzymes. These enzymes can regulate peptide hormone activity at the cell surface and possibly intracellularly after receptor-mediated endocytosis and may also participate in peptide hormone processing. The serine CPs, as their name indicates, contain a serine residue in the active center essential for catalytic activity that reacts with organophosphorus inhibitors. Prolylcarboxypeptidase (PRCP) (angiotensinase C) and deamidase (cathepsin A, lysosomal protective protein) are discussed here. These two enzymes are highly concentrated in lysosomes; however, they may also be active extracellularly after their release from lysosomes in soluble form or in a plasma membrane-bound complex. Whereas deamidase cleaves a variety of peptides with C-terminal or penultimate hydrophobic residues (e.g. substance P angiotensin I, bradykinin, endothelin, fMet-Leu-Phe), PRCP cleaves only peptides with a penultimate Pro residue (e.g. des-Arg(9)-bradykinin, angiotensin II). These enzymes may also be involved in terminating signal transduction by inactivating peptide ligands after receptor endocytosis.