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The discovery of a myopathic mutation in a colony of strain 129 mice (1, 2) provides an opportunity for the study of possible abnormalities in the metabolism of muscle proteins with the use of labeled amino acids. Cell proteins miiay show various kinds of behavior. Some of them are constantly being renewed, either secreted, like digestive enzymes or serum albumin from the liver, or subjected to a molecular turnover, as in the case of muscle aldolase and probably all the water-extractable muscle proteins. The other proteins apparently stay unchanged during the life span of the cell, as in the case of hemoglobin in red blood cells (3), of collagen in the rat (4), or of myosin in the myofibril, a subcellular unit (5. 6). We have previously shown in the rat that myosin has a very slow renewal and that its metabolic behavior permits the measurement of the life span of the myofibril, which is close to 30 days in the rat. If, after the injection of a radioactive amino acid, the radioactivity of the protein is plotted against time, the first kind of protein shows an exponential decline, but hemoglobin and m-vosin show a plateau followed by a rapid decline. These two kinds of curves are combined if the protein is slowly renewed and belongs to a cell or part of a cell with a relatively short life span. In this work we have compared the metabolism of myosin and water-extractable muscle proteins in dystrophic mice, in their normal litter mates, and in another normal strain of mice. Wehave not prepared pure aldolase, which would have required too large a number of animals, but we

ABNORMALITIES OF MUSCLE PROTEIN METABOLISM IN MICE WITH MUSCULAR DYSTROPHY *