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A factor which markedly activates Caz+-dependent thiol protease (calpain) is associated with Triton X-lOOinsoluble 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, of 15 000. With casein as substrate this factor activates both calpain I and calpain II severalfold up to more than lo-fold without alteration of their affinity to Ca 2+. Calmodulin is unable to substitute for this factor. A similar factor is associated with human platelet insoluble materials. Calpain Calpain activator Protease

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