The human plasma metallo-protease carboxypeptidase N of M , 280000 consists of two small, enzymatically active subunits of M , 50000 and two large subunits. Only the large subunits are glycosylated. They may have a function in stabilizing the complex in plasma. The N-terminal sequence of the small subunit was determined from the isolated protein and used to specify a unique 59-mer oligonucleotide probe. A cDNA clone of 1.7 kbp containing the entire coding sequence of the small subunit of carboxypeptidase N was isolated from a humanliver cDNA library. The cDNA clone encodes a signal sequence of 20 amino acids and the 438 amino acids of the mature subunit. There is a remarkable primary structure similarity of 49% to bovine carboxypeptidase E (enkephalin convertase). A more distant relationship to the bovine pancreatic, digestive carboxypeptidases A and B or even to the metallo-endopeptidases is based mainly on the occurrence of conserved, mechanistically important residues.Carboxypeptidase N (arginine carboxypeptidase) cleaves basic amino acid residues from the C-terminal of peptides and proteins. In the circulation, the enzyme plays a central role in regulating the biological activity of peptides such as kinins and anaphylatoxins, and therefore it is also known as kininase 1 and anaphylatoxin inactivator [l, 23.The liver was proposed as the site of origin of the plasma enzyme which is a tetrameric complex of M , 280 000 -320 000. It can be dissociated into two large, identical subunits of M , 83000-98000 and two small, identical subunits of M , 42000 -55000. Enzymatic activity resides only in the small, unglycosylated subunits. The large, glycosylated subunits are supposed to prevent proteolytic degradation of the enzymatically active, small subunits and/or their rapid removal from blood by glomerular filtration [3 -51.The reported M , values of the small subunit of carboxypeptidase N and other mammalian metallo-carboxypeptidases vary clearly in size. This is true for the pancreatic carboxypeptidases A and B ( M , 34400 and 34600, respectively, as calculated from primary structure), human urinary carboxypeptidase (M, 75 700 [6], a membrane-bound carboxypeptidase in human placenta ( M , 67000) [7] and carboxypeptidase E ( M , 50000) from bovine adrenal-medulla chromaffin granules, brain and pituitary [8,9], secretory granules of rat pituitary [lo] and rat insulinoma (M, 54000) [ll]. Because of their common properties as metallo-carboxypeptiCorrespondence to W. Gebhard,
