Membrane defects in Duchenne dystrophy: protease affecting sarcoplasmic reticulum.

Human muscle sarcoplasmic reticulum (SR) yields three major protein bands. The percent distribution of the mean values of the bands from 15 normal human muscles was 55.4, 14.6, and 30.0 for the 100, 55, and 45-kDa mass proteins, respectively. A mean distribution similar to that in normal muscle SR was found in preparations from 7 patients with polymyositis and from 7 patients with myotonic dystrophy. In 12 preparations from patients with Duchenne dystrophy, the protein distribution differed from that of preparations from normal muscle. The 100-kDa mass protein band was decreased, whereas the 55- and 45-kDa mass bands were increased. Protease inhibitors pepstatin A, antipain, and leupeptin, as well as ethyleneglycol-bis(aminoethyl ether)-N,N,N',N'-tetraacetic acid or ethylenediaminetetraacetic acid, significantly reduced this change. However, some of the changes cannot be prevented by the addition of inhibitors and must be expressed in vivo. Neither protease inhibitors nor chelators affected SR preparations from normal muscle. We found a five- to ten-fold increase in calcium-activated neutral protease activity in Duchenne dystrophic muscles that degraded the calcium-adenosinetriphosphatase of SR. The active protease was identified as the cytoplasmic calpain II. The increased activity in Duchenne muscles may explain many reported abnormalities.