NADH-cytochrome b5 reductase and cytochrome b5 associated with slow-muscle sarcoplasmic reticulum and liver microsomal fraction were identified with discrete protein bands of molecular weights 33000 and 16700 by polyacrylamide-gel electrophoresis. Purified detergent-extracted cytochrome b5 from muscle sarcoplasmic reticulum is indistinguishable from liver microsomal cytochrome b5 with respect to spectral properties, pI values and immunological reactivity with antibody to the liver cytochrome b5. Reaction of the antibody with membrane-bound cytochrome b5 inhibits the sarcoplasmic-reticulum NADH-cytochrome c reductase activity.
