A carboxypeptidase processing enzyme for enkephalin precursors

Abstract

Recent studies^1,2 have shown that adrenal medullary chromaffin granules contain relatively large amounts of Met- and Leuenkephalin and several large enkephalin-containing peptides ranging in molecular weight (M_r) from 3,200 to 50,000. The large, biologically inactive enkephalin-containing molecules can be broken down in vitro to give active enkephalins by two enzymes, trypsin and carboxypeptidase B¹: trypsin liberates enkephalins (for example, Met-enkephalin-Arg⁶, Met-enkephalin-Arg⁶-Arg⁷, Met-enkephalin-Lys⁶, Leu-enkephalin-Arg⁶) that are extended at their C-terminus by Lys or Arg. These C-terminal residues can be trimmed off by carboxypeptidase B, an exopeptidase that specifically removes C-terminal basic amino acid residues. Presumably, the enkephalin precursor is processed in vivo by trypsin-like and carboxypeptidase B-like enzymes^1,3; it seems reasonable that these enzymes co-exist with the precursor in chromaffin granules. We describe here a carboxypeptidase obtained from bovine chromaffin granules that converts ¹²⁵I-labelled Met-enkephalin-Arg⁶ to ¹²⁵I-Met-enkephalin. Fulfilment of certain criteria expected of a processing enzyme suggests that this peptidase may be involved in enkephalin precursor processing.