The Excretion of Isotope in Urea and Ammonia for Estimating Protein Turnover in Man with [15N]Glycine

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Abstract
Four normal adults were given [15N]-glycine in a single dose orally or i.v. Rates of whole-body protein turnover were estimated from the excretion of 15N in ammonia and in urea during the following 9 h. The rate derived from urea took account of the [15N]urea retained in body water. In postabsorptive subjects the rates of protein synthesis given by ammonia were equal to those from urea, when the isotope was given orally, but lower when an i.v. dose was given. In subjects receiving equal portions of food every 2 h, rates of synthesis calculated from ammonia were much lower than those from urea, whether an oral or i.v. isotope was given. Comparison of rates obtained during the postabsorptive and absorptive periods indicated regulation by food intake primarily of synthesis when measurements were made on urea, but regulation primarily of breakdown when measurements were made on ammonia. These inconsistencies suggest that changes in protein metabolism might be assessed better by correlating results given by different end products; it is suggested that the mean value given by urea and ammonia is useful for this purpose.

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