Two neutral Ca2+-dependent proteinases, differing in molecular size, have been isolated from rabbit liver. Both are recovered as inactive proenzymes that can be converted to the active forms by high (0.1-1.0 mM) concentrations of Ca2+ in the absence of substrate or, in the presence of a protein substrate, by low (1-5 microM) concentrations of Ca2+. The activated proteinases required only 1-5 microM Ca2+ for maximal activity. Substrates hydrolyzed were denatured globin, globin, casein, and to a lesser extent, several extracellular proteins; no digestion was observed with several intracellular cytosolic enzymes tested. Only those proteins that served as substrates were capable of promoting conversion of the proenzymes to the active low-Ca2+-requiring proteinases.

Pontremoli, S., Melloni, E., Salamino, F., Sparatore, B., Michetti, M., & Horecker, B. L. (1984). Cytosolic Ca2+-dependent neutral proteinases from rabbit liver: activation of the proenzymes by Ca2+ and substrate. Proceedings of the National Academy of Sciences, 81(1), 53–56.