Calcium‐Activated neutral protease in the peripheral nerve, which requires μM order Ca2+, and its effect on the neurofilament triplet

There are two types of calcium-activated neutral protease (CANP), m-CANP and mu-CANP, following the nomenclature of Suzuki et al to show that each requires mM and microM Ca2+, respectively, for its activation. We found mu-CANP activity in a crude CANP fraction extracted from the peripheral nerve, which degraded the neurofilament (Nf) triplet (200 K, 160 K, 68 K), especially the 160 K component, at Ca2+ concentrations of 50 microM and 0.1 mM. The triplet was degraded in the order of the 160 K, 68 K, and 200 K components, respectively. In addition, the effects of partially purified mu-CANP of rabbit skeletal muscle, purified natural mu-CANP of bovine liver, derived mu-CANP prepared by autodigestion of chicken muscle m-CANP, m-CANP of chicken skeletal muscle, and cathepsin B of rat liver on the Nf were examined. Among the triplet components, the 160 K component was most rapidly degraded by all proteases so far tested. The difference in the effect of mu-CANP and m-CANP or cathepsin B on susceptibility of the 200 K component to degradation might be due to the difference of the relative amounts of enzymes to Nf.