The use of a continuous infusion of 15N for the study of human nitrogen metabolism
Noninvasive techniques are necessary for studying human protein metabolism. Because it is nonradioactive, 15N has frequently been used as a tracer in such experiments. Infusing a 15N-labelled amino acid for a suitable period of time can lead to an approximate isotopic steady state in certain nitrogen pools. In the absence of isotope re-entry, a true steady state should result. Usually there is some re-entry of isotope, so only an approximate isotopic steady state is observed. Methods based on the measurement of an isotopic steady state give the most reliable values for the overall rate of human protein synthesis1,2. A disadvantage of this approach as compared to the single injection procedure (‘pulse label’) is that it is not amenable to a compartmental analysis. The advantages of such an approach where only limited sampling is possible have been summarised by Dietchy and Wilson3 in their review of cholesterol metabolism. Given a valid model and a reliable method of determining the compartmental parameters, it is possible to assess pharmacological, physiological and nutritional effects on cholesterol (nitrogen) metabolism. For these reasons it seemed worthwhile to develop a reliable compartmental method for the study of human N metabolism.
KeywordsHippuric Acid Urea Synthesis Urinary Urea True Steady State Blood Amino Acid
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