tailieunhanh - Quantitative aspects of ruminant digestion and metabolism - Phần 11

All cellular proteins are in a continuous state of turnover in which they are synthesized and degraded (Waterlow et al., 1978). Thus, the intracellular concentration of any protein, and the tissue, organ or whole-body protein mass, are determined by the relative synthetic and | 14 Protein Metabolism and Turnover D. Attaix 1 D. Rémond1 and . Savary-Auzeloux2 Institut National de la Recherche Agronomique Unité de Nutrition et Métabolisme Proteique Theix 63122 Ceyrat France 2Institut National de la Recherche Agronomique Unite de Recherches sur les Herbivores Theix 63122 Ceyrat France Introduction All cellular proteins are in a continuous state of turnover in which they are synthesized and degraded Waterlow et al. 1978 . Thus the intracellular concentration of any protein and the tissue organ or whole-body protein mass are determined by the relative synthetic and degradation rates. It should be pointed out that a change in the size of a given protein pool only depends on the imbalance between both processes of protein turnover. In other words an increase or a decrease in such a protein pool does not necessarily correlate with only an enhanced rate of either protein synthesis or protein breakdown respectively. For example the anabolic agent trenbolone acetate decreased rates of both protein synthesis and breakdown and resulted in net muscle protein gain Vernon and Buttery 1976 . The cyclical nature of protein turnover also implies that rates of protein synthesis and degradation are considerably greater than the net flux protein deposition or loss through the protein turnover cycle. For example a large proportion of free amino acids arising from protein breakdown is reutilized for protein synthesis so that the rate of whole-body protein synthesis is much greater than the rate of dietary influx of amino acids. Both protein synthesis and breakdown require energy see below . However the process of protein turnover provides the organism with several adaptive mechanisms that clearly outweigh the metabolic costs 1. Growth and mobilization of tissue organ and whole-body protein mass is easily achieved depending on the physiological status. 2. Large amounts of free amino acids can be mobilized from skeletal muscle and used to provide energy and .