Recombinant Human Aminopeptidase B/RNPEP Protein, CF

Aminopeptidase B (APB; also known as aminopeptidase basic, Arginine aminopeptidase and RNPEP) is a monomeric, secreted member of the metallopeptidase M1 family of enzymes (1-2).  Members of the M1 family are often associated with mitosis, and are characterized by the presence of a catalytic domain that consists of a GxMxN exopeptidase motif, coupled to an extended Zn-binding HExxH[18x]E sequence (1, 3).  Although Zn-dependent, it is activated by divalent cations such as Ni, Mn and Co.  APB is widely expressed, perhaps even ubiquitously, and has been reported in a number of distinct cell types.  These include hepatocytes (4), skeletal muscle cells (5), macrophages and neutrophils (6), pancreatic islet  alpha -cells (7), anterior lobe pituitary basophils (8), and adrenal medullary chromaffin cells (8).  APB has been identified in multiple subcellular locations, including an extracellular association with the plasma membrane, within secretory granules of neuroendocrine cells, and through an NLS, within the cell nucleus (1, 7-11). APB appears to act in concert with at least one other peptidase during the processing of propeptides into active or mature peptides.  The activity attributed to APB involves the preferential cleavage of the basic amino acids (aa) Arg and Lys from the N-terminus of partially processed proforms (9).  This activity is reported to occur in alpha -cell granules where an NRD convertase:APB cooperation converts glucagon into miniglucagon (aa 19-29), and in adrenal medullary chromaffin cell granules where a cathepsin L:APB cooperation converts proenkephalin into Met-enkephalin (7, 8).  The human APB precursor is 650 aa in length.  It contains an atypical 28 aa signal sequence plus a 622 aa mature region that contains a peptidase region over aa 24-634 (10, 12).  A leukotriene A4 hydrolase has also been described between aa 500-644.  Mature APB is reported to run as a 72-76 kDa protein on SDS-PAGE (5, 8, 10, 11).  There are potential isoform variants.  One shows a 12 aa substitution for aa 11-14, while another may utilize an alternate start site at Met291.  Mouse and rat APB share 89% aa sequence identity with human APB.