The earliest model of whole-body protein metabolism in humans was applied by San Pietro and Rittenberg in 1953 using [ 15N]glycine (65). Glycine was used as the first tracer because glycine is the only amino acid without an optically active a-carbon center and therefore is easy to synthesize with a 15N label. At that time, measurement of the tracer in plasma glycine was very difficult. Thus, San Pietro and Rittenberg proposed a model based upon something that could be readily measured, urinary urea and ammonia. The assumption was that the urinary N end-products reflected the average enrichment in 15N of all of the free amino acids being oxidized. Although glycine 15N was the tracer, the tracee was assumed to be all free amino acids (assumed to be a single pool). However, it quickly became obvious that the system was more complicated and that a more complicated model and solution were required.
In essence, the method languished until 1969, when Picou and Taylor-Roberts (66) proposed a simpler method that also followed the glycine 15N tracer into urinary N. Their method dealt only with the effect of the dilution of the 15N tracer in the free amino acid pool as a whole, rather than invoking solution of tracer-specific equations of a specific model. Their assumptions were similar to those of the earlier Rittenberg approach in that they assumed that the 15N tracer mixes (scatters) among the free amino acids in some distribution that is not required to be known but that represents amino acid metabolism per se. This distribution of 15N tracer could be measured in the end products of amino acid metabolism, urea and ammonia. These assumptions allow the model to become "fuzzy" as shown in Fig.yie...2,11, in that an explicit definition of the inner workings is not required. The [ 15N]glycine tracer is administered (usually orally), and urine samples are obtained to measure the 15N dilution in the free amino acid pool (67). The 15N in the free amino acid pool is diluted with unlabeled amino acid entering from protein breakdown and from dietary intake. The turnover of the free pool (Q, typically expressed as mg N/kg/day) is calculated from the measured dilution of 15N in the end products via the same approach illustrated in F.i9.u£e... .2.9:
Figure 2.11. Model for measurement of protein turnover using [15N]glycine as the tracer and measurement of the dilution of the 15N tracer in urinary endproducts, urea and ammonia. (From Bier DM, Matthews DE. Fed Proc 1982;41:2679-85, with permission.)
where i is the rate of [15N]glycine infusion (mg 15N/kg/day), and EUN is the 15N enrichment in atom % excess 15N in urinary N (urea and/or ammonia). The free pool is assumed to be in steady state (neither increasing or decreasing over time), and therefore, the turnover of amino acid will be equal to the rate of amino acids entering via whole-body protein breakdown (B) and dietary intake (I) and also equal to the rate of amino acids leaving via uptake for protein synthesis (S) and via amino acid oxidation to the end products urea and ammonia (C):
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