A calcium‐protease activator associated with brain microsomal‐insoluble elements

Abstract

A factor which markedly activates Ca²⁺-dependent thiol protease (calpain) is associated with Triton X-100-insoluble materials, presumably structural elements such as cytoskeletons, of bovine brain microsomal fraction. This factor is extracted with 0.6 M KCl, and purified partially by sucrose density gradient centrifugation and hydroxyapatite column chromatography. The factor appears to be a heat-stable protein with an approximate M _r, of 15000. With casein as substrate this factor activates both calpain I and calpain II severalfold up to more than 10- fold without alteration of their affinity to Ca²⁺. Calmodulin is unable to substitute for this factor. A similar factor is associated with human platelet insoluble materials.