Measuring in vivo intracellular protein degradation rates in animal systems 1 W. G. Bergen 2 Program in Cellular and Molecular Biosciences, Department of Animal Sciences, Auburn University, Auburn 36849-5415 ABSTRACT: 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 synthe- sis and breakdown represent the actual marketable muscle foods. Because protein synthesis is a highly end- ergonic and protein breakdown is metabolic energy de- pendent, efficiency of production can be markedly en- hanced 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, Key words: marker amino acid, proteolysis, protein synthesis, tracer methodology, transcriptional and translational control, ubiquitin-proteasome ©2008 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2008. 86(E. Suppl.):E3–E12 doi:10.2527/jas.2007-0430 INTRODUCTION The protein component of lean mass (operationally defined as water plus protein) in the whole body, irre- spective of tissue or anatomical location, undergoes a process of remodeling characterized by continual pro- tein synthesis, breakdown, and resynthesis (Schoen- heimer, 1942). This overall process, often referred to as protein turnover, is driven by 2 separate but concerted bioprocesses, de novo protein synthesis (i.e., transla- tion; Hershey, 1991) and protein breakdown (Schimke, 1970). Growth or protein accretion of an animal de- pends on the rate of protein synthesis exceeding that of protein breakdown (Zak et al., 1979; Waterlow, 2006). 1 Presented at the Nonruminant Nutrition symposium “Under- standing protein synthesis and protein degradation and their path- way regulations for improving monogastric production efficiency and product quality”, at the annual meeting of the American Society of Animal Science, San Antonio, TX, July 8 to 12, 2007. 2 Corresponding author: bergewg@auburn.edu Received July 17, 2007. Accepted August 29, 2007. E3 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 ap- proaches 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 break- down 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 understand- ing about protein breakdown, may be useful in enhanc- ing production efficiency. In mature humans and animals, body protein mass is more or less constant but aging is accompanied by a slow loss of protein mass in particular skeletal muscle protein mass (Solomon and Bouloux, 2006). Farm ani- mals in production situations, however, are rarely in a negative N equilibrium except for females in early lactation or all animals during periods of insufficient feed intake usually related to strategies of limit feeding/ compensatory growth or lack of feedstuffs related to weather and economics. In general, however, periods of negative N equilibrium result in lowered perfor- mance by animals and, as such, are not deemed as contributing to food production and sustainability of animal agriculture. During disease states or in periods when energy and AA are overtly lacking, both protein biosynthesis and protein breakdown are regulated to maintain as much of the cellular and tissue infrastruc- ture as possible and to contribute to critical metabolic needs of the organism (Waterlow, 2006). ROLE OF PROTEIN TURNOVER IN ANIMAL PRODUCTION Although early workers recognized that body protein mass in growing animals was a net product of protein Published December 5, 2014