Chromaffin granule aspartic proteinase processes recombinant proopiomelanocortin (POMC)

Our search for proteases responsible for proenkephalin (PE) processing in adrenal medulla led to the isolation of a 70 kDa aspartic proteinase that cleaves PE between the basic residues of the Lys-Arg processing site (1). Studies in pituitary have also identified a similar aspartic proteinase that processes POMC (2,3). To compare the chromaffin granule (CG) 70 kDa aspartic proteinase with that in pituitary, processing of recombinant POMC by the CG enzyme was examined. POMC was expressed in the T7 expression system in E. coli, and purified to homogeneity. The CG 70 kDa aspartic proteinase converted POMC to 27 and 22 kDa bands that were detected by anti-N-POMC immunoblots, and to 26, 22, and 14 kDa bands that were immunoreactive with anti-beta-lipotropin. POMC products represented by these bands indicate appropriate POMC processing by the CG 70 kDa aspartic proteinase. These results, combined with the similar biochemical properties of these two enzymes, suggest that the CG 70 kDa aspartic proteinase resembles the POMC-converting enzyme (PCE), an aspartic proteinase in pituitary (2,3).