Role of calcium in the release of EC 3.4.24.15 and EC 3.4.24.16 from endothelial cells

The two closely related soluble zinc metalloendopeptidases EC 3.4.24.15 (EP24.15) and EC 3.4.24.16 (EP24.16) readily hydrolyze the vasoactive peptide bradykinin in vitro, and therefore may play a role in cardiovascular regulation. Although primarily soluble cytosolic enzymes, both secreted and membrane-associated forms of both peptidases have been reported. However, these enzymes have neither a transmembrane domain nor a signal sequence; thus, the mechanisms of membrane anchoring and secretion are unknown. In the present study, secreted/released EP24.15 and EP24.16 activity from aortic endothelial cells in culture was assessed by the cleavage of a specific quenched fluorescent substrate. An increase in enzyme activity released from endothelial cells, which express both peptidases, was seen following incubation with calcium-free media. In the AtT-20 endocrine cell (mouse pituitary corticotrope), which predominantly expresses EP24.15, the release of activity into media was unaffected by calcium removal. The release of enzyme activity from endothelial cells was inversely proportional to calcium concentrations ranging between 0.01 mM (activity equivalent to calcium-free media) and 0.5 mM (activity equivalent to normal media). Cleavage of the EP24.16-specific substrate AcNT_8-13 indicated that the increase in enzyme activity released upon incubation with calcium-free medium was due at least in part to the release of EP24.16. These results suggest that EP24.15 and EP24.16 are secreted from endothelial cells, and that removal of calcium selectively enhances the release of EP24.16 by an as yet unknown mechanism.