As indicated above, proteins in the body are not static. Just as every protein is synthesized, it is also degraded. Schoenheimer and Rittenberg first applied isotopically labeled tracers to the study of amino acid metabolism and protein turnover in the 1930s and first suggested that proteins are continually made and degraded in the body at different rates. We now know that the rate of turnover of proteins varies widely and that the rate of turnover of individual proteins tends to follow their function in the body, i.e., proteins whose concentrations must be regulated (e.g., enzymes) or that act as signals (e.g., peptide hormones) have relatively high rates of synthesis and degradation as a means of regulating concentrations. On the other hand, structural proteins such as collagen and myofibrillar proteins or secreted plasma proteins have relatively long lifetimes. However, there must be an overall balance between synthesis and breakdown of proteins. Balance in healthy adults who are neither gaining nor losing weight means that the amount of N consumed as protein in the diet will match the amount of N lost in urine, feces, and other routes. However, considerably more protein is mobilized in the body every day than is consumed ( Fig 2.6).
Figure 2.6. Relative rates of protein turnover and intake in a healthy 70-kg human. Under normal circumstances, dietary intake (IN = 90 g) matches N losses (OUT = 90 g). Protein breakdown then matches synthesis. Protein intake is only 90/(90 + 250) » 25% of total turnover of N in the body per day. (Redrawn from Hellerstein MK, Munro HN. Interaction of liver and muscle in the regulation of metabolism in response to nutritional and other factors. In: Arias IM, Jakoby WB, Popper H, et al., eds. The liver: biology and pathobiology. 2nd ed. New York: Raven Press, 1988;965-83.)
Although there is no definable entity such as "whole-body protein," the term is useful for understanding the amount of energy and resources spent in producing and breaking down protein in the body. Several methods using isotopically labeled tracers have been developed to quantitate the whole-body turnover of proteins. The concept and definition of whole-body protein turnover and these methods have been the subject of entire books (e.g., [ 42]). An important point of Hgure 2..6. is that the overall turnover of protein in the body is several fold greater than the input of new dietary amino acids ( 43). A normal adult may consume 90 g of protein that is hydrolyzed and absorbed as free amino acids. Those amino acids mix with amino acids entering from protein breakdown from a variety of proteins. Approximately a third of the amino acids appear from the large, but slowly turning over, pool of muscle protein. In contrast, considerably more amino acids appear and disappear from proteins in the visceral and internal organs. These proteins make up a much smaller proportion of the total mass of protein in the body but have rapid synthesis and degradation rates. The overall result is that approximately 340 g of amino acids enter the free pool daily, of which only 90 g come from dietary amino acids. The question is how to assess the turnover of protein in the human body? As noted from Figure..2.6, the issue quickly becomes complex. Much effort has been spent in devising methods to quantify various aspects of protein metabolism in humans in meaningful terms. The methods that have been developed and applied with success to date are listed in Table..2.8. These methods, which range from simple and noninvasive to expensive and complicated, are described below.
Table 2.8 Methods of Measuring Protein Metabolism in Humans
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