The synthesis of myosin, actin and the major protein fractions in rabbit skeletal muscle

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Abstract
New Zealand White rabbits were infused with [3H]tyrosine for 5-6 h and then different methods of extraction were applied for the purification of the main muscle proteins and protein fractions. Myosin (I), prepared from salt extraction of muscle mince, consistently had a higher specific radioactivity than did myosin (II), isolated by dissociation of actomyosin. Actins (IA) and (IB), extracted from acetone-dried powders prepared by different treatments of myosin-extracted muscle mince, gave specific radioactivities approximately 0.6 that of myosin (I) and 0.7 that of myosin (II). Actin (II), isolated by dissociation of actomyosin, had a specific radioactivity similar to that of myosin (II) from the same source, but higher than those of actins (IA) and (IB). The differences between the specific radioactivities of the proteins, in particular actin, purified by the various methods, are attributed to the loss of newly synthesized material of high specific radioactivity during the initial extraction procedures. Actin (II) and myosin (II) may be representative preparations for the total population of each protein. On this basis, myosin and actin have similar rates of synthesis. Total muscle protein, myofibrils, actomyosin and sarcoplasm all have very similar specific radioactivities at the end of a 6 h infusion.