Dietary lysine deficiency greatly affects muscle and liver protein turnover in growing chickens.

We analysed the respective influences of age and lysine deficiency on skeletal muscle and liver protein turnover. Growing male broilers were fed ad libitum on isoenergetic diets containing 200 g crude protein/kg which varied in their lysine content (7.7 or 10.1 g/kg). Fractional rates of protein synthesis (FSR) were measured in vivo in the liver and the pectoralis major muscle of 2-, 3- and 4-week-old chickens (flooding dose of L-[4-3H]phenylalanine). Fractional rates of proteolysis (FBR) were estimated for the same tissues as the difference between synthesis and growth. Over the 2-week period liver FSR and FBR were unchanged, whereas muscle FSR decreased with age. This developmental decline was related to the lower capacity for protein synthesis (Cs) without any modifications of the translational efficiency. Whatever the age, lysine deficiency resulted in significant decreases in body weight, tissue protein content and tissue protein deposition, apparently because of reduced amounts of proteins synthesized. We recorded a difference in the response of the two tissues to lysine deficiency, the pectoralis major being more sensitive than the liver. When comparing birds of the same age, liver FSR and FBR were not modified by the diet, whereas muscle FSR, Cs and FBR were higher in chicks fed on a lysine-deficient diet than in the controls. Conversely, when chicks of similar weights were compared, the main effect of the dietary deficiency was an increase in muscle FBR. The results suggest that lysine deficiency not only delayed chick development so that protein turnover was affected, but also induced greater changes in metabolism. Thus, the principal mechanism whereby muscle mass decreased appeared to be a change in FBR.