Identification of zymogen and mature forms of human carboxypeptidase H A processing enzyme for the synthesis of peptide hormones

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Abstract
Carboxypeptidase H (CPH) is one of several processing enzymes required for the conversion of peptide hormone precursors into their smaller active forms. In this study, high levels of CPH activity was found in a liver metastasis of a human ideal carcinoid which expresses β‐preprotachykinin mRNA and the tachykinin neuropeptides, substance P and substance K. This human CPH showed properties of a zinc‐metallopeptidase that is structurally similar to bovine and rat CPH. Immunoblots of the human ileal carcinoma with anti‐bovine CPH showed that CPH activity is represented by two proteins of apparent molecular masses 57 and 55 kDa. Cell‐free translation of poly(A)+ RNA followed by immunoprecipitation with anti‐bovine CPH showed that human CPH mRNA encodes a precursor protein of apparent molecular mass 75 kDa. These data demonstrate that human CPH is synthesized as a zymogen, prepro‐CPH, which must be cleaved to form catalytically active CPH.