Protein degradation: an important consideration

The growth rates of chicks can be varied by dietary manipulations. The rates of synthesis of protein in selected muscles can be measured by using radioactive amino acids. Measurements are made of expired air, plasma, and muscle tissues. Synthesis rates are most rapid in the young chick and turkey. As the bird ages, synthesis rates and degradation rates decrease; but these two parameters gradually come together so that the net gain in protein is zero. Synthesis rates remain fairly similar in chicks fed a complete diet or a diet lacking in lysine. This is true even if the growth rate is extremely low. However, the degradation rates are extremely high in the deficient birds so that the gain in protein deposition is very low. When the energy is restricted, growth is reduced dramatically, but synthesis rates are reduced only a small amount. Again, degradation rates are extremely high. Chickens with muscular dystrophy exhibit very high synthesis rates in breast muscle. The leg muscle changes compare with a normal bird. There are also high degradation rates in the breast muscle throughout so that the growth rate of the muscle was only greater during the early stages of growth. Additional work needs to be conducted to develop a reliable accurate chemical method for determining the degradation rates. It is apparent that the emphasis in research to increase the rapidity of growth should center on the degradation rates rather than on increasing synthesis rates.