Identification of a membrane-bound carboxypeptidase as the mammalian homolog of duck gp180, a hepatitis B virus-binding protein.

A unique membrane-bound carboxypeptidase was discovered and characterized in membrane fractions of human skin fibroblasts and the mouse monocyte-macrophage cell line J774A.1 and was partially purified from human placenta. Enzymatic characterization identified it as a new member of the regulatory B-type metallocarboxypeptidases, different from carboxypeptidases B, E, M, N and U. It is, however, similar to the newly described bovine carboxypeptidase D, suggested to be a homolog of duck gp180, a 180 kDa hepatitis B virus-binding protein. To prove this, a partial cDNA encoding a 20 kDa fragment of the human homolog of duck gp180 was expressed in bacteria and the recombinant protein was purified. Antibodies raised to the protein immunoprecipitated 94% or 72% of the low pH carboxypeptidase activity in human skin fibroblasts or J774A.1 cells and gave a 175 kDa protein band in Western blots. Thus, carboxypeptidase D is the mammalian homolog of duck gp180 and is distributed in a variety of different cell types.