We analysed the respective influences of age and lysine deficiency on skeletal muscle and liver protein turnover. Growing male broilers were fed ad fibirum on isoenergetic diets containing 2OOg crude protein/kg which varied in their lysine content (77 or 101 g/kg). Fractional rates of protein synthesis (FSR) were measured in vivo in the liver and the pectoralis mapr muscle of 2, 3-and 4-week-old chickens (flooding dose of ~-[4-~H]phenylalanine). Fractional rates of proteolysis (FBR) were estimated for the same tissues as the difference between synthesis and growth. Over the %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 de5ciency 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 lysini?-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 90 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.
Lysine: Protein turnover: ChickenThe poultry industry aims to increase the efficiency of the transformation from feed to animal proteins to provide consumers with a product containing more lean and less fat, and to reduce N excretion which is a source of pollution. At present, synthetic amino acids are added to low-protein diets to obtain a well-balanced feed. A deficiency in a single essential amino acid can indeed disrupt growth mechanisms: it decreases chick growth, feed intake and N balance (Akinwande & Bragg, 1985;Okumura et al. 1985; Kino & Okumura, 1986a, b). It caused a reduced protein deposition in the whole body and in the pectoralis major muscle of 3-week-old chickens mainly because lower amounts of protein were synthesized each day (Kino & Okumura, 1987; Tesseraud er al. 1992). To our knowledge, all of the studies concerning the effect of an amino acid deficiency have compared chicks at the same chronological age, when chicks fed on a control or an amino acid-deficient diet did not only have different body weights but had, also, different tissue protein masses and different rates of tissue development. To understand better the reduction of growth performance associated with the deficiency of a particular amino acid, we analysed the effect of dietary lysine on in vivo protein turnover using 2-, 3...
