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NONRUMINANT NUTRITION |
* Center for Nutrition Modeling, University of Guelph, Guelph, Ontario, Canada N1G 2W1;
and
Department of Animal Science, North Carolina State University, Raleigh 27695;
and
Department of Animal Sciences, Purdue University, West Lafayette, IN 47907; and
Universidad Autonoma Baja California, Mexicali, Baja California, Mexico 21100
Domestication of both animals and plants approximately 13,000 yr ago marked the first major milestone of the evolving animal production, which had advanced ancient humans from a hunter-gatherer society challenged by survival to a prolonged farming society (Diamond, 2002
). Modern science and technology has facilitated the second major milestone of developing industrialized intensive animal production systems driven by use of fossil fuels during the last century, especially during the last 2 decades. Although industrialization and globalization are reaching the corners of the earth, current intensive animal production is facing new challenges, including 1) economical and social viability in rural regions faced with increasing labor and living costs; 2) resource and environmental sustainability; and 3) human health management, partly due to an increase in the consumption of animal products.
Animal production is, in essence, a mass and energy conversion process using farm animals as a biological converter. Modern intensive animal production systems have increased the speed and scale but have not increased the efficiency very much of this mass-energy conversion. A large proportion (60 to 80%) of the total animal farming operation cost is associated with animal feeding, primarily due to a poor efficiency of utilization of N and energy needed (Fan et al., 2006). At the whole-body level, CP deposition is the net balance among synthesis, degradation, and the fecal gastrointestinal endogenous loss (Fan et al., 2006). Skeletal muscle is the largest body CP pool. Protein synthetic activity of skeletal muscle decreases dramatically during postnatal growth from suckling to weaning and maintains at a low level for most of the duration of production (Reeds et al., 1993). Protein degradation is the principal negative component in the whole body CP deposition equation. As the primary pathway, the ubiquitin-proteasome protein degradation is an ATP-demanding process, costing approximately 300 to 400 mol of ATP for breaking down 1 mol of a typical protein (Benaroudj et al., 2003). The energy cost for synthesizing protein is approximately 0.90 KJ/g, calculated from the pathway stoichiometry summarized by Yang et al. (2008), whereas the energy need for degrading protein is about 0.40 KJ/g of protein with a molecular weight similar to β-casein (24 kD) calculated according to the work by Benaroudj et al. (2003). The increases in postnatal endogenous CP loss are largely from the high protein synthetic activity of the gut and other viscera that can be further augmented under challenged conditions (Fan et al., 2006).
The following papers were published in follow-up to the corresponding presentations made at the Nonruminant Nutrition Symposium, "Understanding protein synthesis and degradation and their pathway regulations," held on July 10, 2007, at the joint annual meeting of the American Society of Animal Science, the American Dairy Science Association, the Asociación Mexicana de Producción Animal, and the Poultry Science Association in San Antonio, Texas. These papers address some of the key topics in understanding protein synthetic and degradation pathways and regulation of those pathways.
Davis et al. (2008) reviewed hormonal and nutritional factors regulating postnatal muscle protein synthesis through protein translation pathways and signaling molecules in the postnatal pig. High rates of protein deposition and protein synthesis in skeletal muscle in young nursing pigs (i.e., d 7) are associated with elevated protein translational capacity and efficiency compared with pigs during the weaning transition (i.e., d 26). Research conducted on developing pigs showed that the postnatal decline in the stimulation of muscle protein synthesis by insulin and AA was, in part, due to decreases in the activation of the signaling pathways regulating translation initiation, although mechanisms are yet to be determined (Davis et al., 2008).
Bergen (2008) addressed challenges and methodological considerations for measuring intracellular protein degradation in animals. Several in vivo techniques were reviewed, including core issues on infusion of AA tracers, quantification of marker AA release from muscle proteins, and tracer kinetic analyses. The whole-animal in vivo approaches for measuring protein degradation rates are resource-intensive and limited for their applications to high-throughput metabolic screening; thus, both transcriptional profiling and proteomic assessment-based molecular endpoints need to be further developed (Bergen, 2008).
Goll et al. (2008) overviewed major pathways of protein degradation with emphasis on the nonlysosomal Ca2+-dependent protein degradation system in muscle and implications for meat quality. Skeletal muscle contains 4 proteolytic systems involved in protein degradation, including the lysosomal system, the caspase system, the Ca2+-dependent calpain system, and the proteasome. The first 2 systems play very limited roles in muscle protein degradation under normal physiological conditions (Goll et al., 2008). Research during the past 3 decades has shown that the calpain system is essential in disassembling myofibrillar proteins for subsequent access to the proteasome, whereas the proteasome is responsible for further degrading the disassembled proteins into short peptides; however, the mechanisms whereby the calpains and the proteasome are coordinated to carry out the breakdown of myofibrillar proteins remained to be elucidated (Goll et al., 2008).
Yang et al. (2008) reviewed the mammalian target of rapamycin (mTOR) as a central regulator in regulating cellular metabolism, hyperplasic-hypertrophic protein growth, and ribosome biogenesis at transcriptional and translational levels by sensing and integrating signals from mitogens, stressors, and nutrients. Hormonal and stress factors can affect the mTOR-signaling pathway via their receptors and signal transduction pathways, whereas nutritional regulation of the mTOR-signaling pathway is mediated by their corresponding plasma membrane transporters, other unknown mechanisms (Yang et al., 2008), or both. How mTOR senses AA is still poorly understood, and the roles of mTOR signaling in protein degradation as well as cell apoptosis and division also require further exploration (Yang et al., 2008).
Although the biosynthetic pathway and methods for measuring fractional synthesis rates of proteins have been well established, rate-limiting steps and mechanisms regulating efficiency and capacity of hypertrophic protein synthesis remained to be clarified. Tissue- or organ-specific integration of multiple proteolytic systems appears to be essential, and their roles may be dependent on physiological status. The mTOR-signaling pathway is central to the regulation of metabolic processes as well as hyperplasic-hypertrophic cellular growth, as affected by hormonal factors, stressors, and nutrients. Increased understanding of protein metabolic pathway regulations will help in development of novel strategies for altering these metabolic pathways and ultimately improve efficiency and quality of intensive animal production as well as human health management.
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Footnotes |
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2 Related research activities have been supported by grants (to MZF) from the Natural Sciences and Engineering Research Council of Canada Discovery Program and the Ontario Ministry of Agriculture, Food and Rural Affairs-University of Guelph Animal Research Program.
3 Corresponding author: mfan{at}uoguelph.ca
Received for publication November 15, 2007. Accepted for publication November 28, 2007.
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