Calcium-activated neutral protease (CANP), a putative processing enzyme of the neuropeptide, kyotorphin, in the brain.

Kyotorphin (Tyr-Arg) accumulation in the dialysed synaptosol from the rat brain in the presence of an inhibitor of kyotorphin-degrading enzyme, was maximal at neutral pH. This accumulation was activated by calcium ions, but was inhibited by leupeptin and SH-blocking agents, a finding which suggests the involvement of calcium-activated neutral protease (CANP or calpain). In addition, the kyotorphin-precursor protein, being processed by purified mu- or m-CANP, was detected at about 160 kDa on Sephacryl S-300 chromatography of the synaptosol. The present findings seem to be the first evidence for the role of CANP as a processing enzyme of neuropeptide-precursor in nerve terminals.