The synthesis and degradation of pectoralis myosin have been investigated in chickens 15-23 days after hatching. An essentially nonreutilizable tracer amino acid was used to demonstrate differences in the rates of synthesis and degradation of myosin isolated from normal and hypertrophied, dystrophic breast tissue. The analyses have shown that the dystrophic system synthesizes myosin faster than the normal system and that only myosin in dystrophic pectoralis is degraded during the experimental period. Double label experiments have indicated that the heavy chains of dystrophic myosin are destroyed at a rate greater than that characteristic of the light chains.
Recent studies of turnover rates of mixed skeletal muscle proteins have indicated that these proteins display more metabolic activity in adult tissue than had been previously indicated in the literature. Several laboratories (Goldberg, '69a,b; Millward, '70; Waterlow and Stephens, '67, '68; Young, '70; Young et al., '71) have independently reported half-lives for crude preparations of myofibrillar proteins far less than those previously reported for myosin, actin and tropomyosin (Dreyfus et al., '60; McManus and Mueller, '66; Velick, '56). The present studies characterize the turnover of highly purified myosin and myosin subunits isolated from two developing systems: normal chicken pectoralis (Line 200) and hypertrophying, dystrophic pectoralis (Line 304). The analyses examine the pectoralis major of 15-23-day old female chickens. During this developmental period, Line 304 pectoralis has a low fat content, consistent early hypertrophy, and possesses significant increases in cathepsin B activity and autolysis. In addition, this time interval includes a transition period relative to the ability of dystrophic ribosomes to synthesize proteins (Bettelle and Florini, '73). The present studies concentrate on myosin degradation during this period and use tracer methodology which minimizes label reutilization.
Methods
One-day female chicks of Line 200 and Line 304 were purchased from University