Carboxypeptidase B-like activity for the processing of enkephalin precursors in the membrane component of bovine adrenomedullary chromaffin granules

Trypsin and carboxypeptidase B-like (CPB-like) peptidases should be involved in processing proenkephalin to form the small biologically active enkephalins. A carboxypeptidase B-like activity from the soluble fraction of bovine adrenomedullary chromaffin granules which converts 125I-(Met)-enkephalin-Arg6 to 125I-(Met)enkephalin has previously been described and characterized (1,2). In this study, CPB-like activity in the membrane bound component of chromaffin granules is characterized and compared with that in the soluble fraction. Membrane and soluble CPB activities cleaved 125I-(Met)enkephalin-Arg6 or 125I-(Met)enkephalin-Lys6 to form 125I-(Met)enkephalin. Like the soluble enzyme, the CPB-like activity in the membrane component had a pH optimum of 6.0, was inhibited by thiol agents (PCMPSA, CuCl2) and metal ion chelators (EDTA, 1,10-phenanthroline), and was stimulated by Co++. The membrane CPB-like activity appeared to be an intrinsic membrane protein, since 80% of the activity remained with the membranes after washing with 1.0 M NaCl. Membrane and soluble CPB-like activities in chromaffin granules appear to be similar enzymes.