Measuring in vivo intracellular protein degradation rates in animal systems1

Continual synthesis and breakdown or remodeling of proteins (also called protein turnover) is a principal characteristic of protein metabolism. During animal production, the net differences between synthesis and breakdown represent the actual marketable muscle foods. Because protein synthesis is a highly end-ergonic and protein breakdown is metabolic energy dependent, efficiency of production can be markedly enhanced by lower muscle protein breakdown rates. Herein, various methodological approaches to studying protein breakdown, with particular emphasis toward food-producing animals, are presented. These include whole-animal tracer AA infusions in vivo, quantifying marker AA release from muscle proteins, and in vitro AA release-based methodologies. From such methods, protein synthesis rates and protein breakdown rates (mass units/time) may be obtained. The applications of such methods and innovations based on traditional methods are discussed. Whole-animal in vivo approaches are resource intensive and often not easily applied to high-throughput metabolic screening. Over the last 25 yr, biochemical mechanisms and molecular regulation of protein biosynthesis and protein breakdown have been extensively documented. Proteolysis is dependent in part on the extent of expression of genes for components of cellular proteolytic machinery during skeletal muscle atrophy. It is proposed that high-throughput methods, based on emerging understanding about protein breakdown, may be useful in enhancing production efficiency.