Tissue enrichments and protein turnover measured with ¹⁵N-glycine

Abstract

VALUES for total body protein turnover (TBPT) measured in man using ¹⁵N-glycine^1–3 are comparable with those obtained using ¹⁴C-amino acids⁴. Because ¹⁴C is radioactive it is of limited clinical use. The stable isotope ¹⁵N has a potentially wider application, although its use has been restricted for technical and conceptual reasons⁴. When a tracer labelled with ¹⁵N is used to measure TBPT the results are based on the enrichment of urea. This assumes that both urea and newly synthesised protein are derived from the same precursor N pool. Because urea is synthesised in the liver the enrichment of the precursor N pool for protein synthesis in the liver may determine the enrichment of urea. Thus the measurement of TBPT using ¹⁵N-glycine may be weighted heavily by the rate of hepatic protein turnover^4,5. Similar problems may arise when urinary ammonia is the end product (M.H.N.G. and J. C. Waterlow, to be published). The extent of labelling of the precursor N pool is affected by at least two factors. First, after infusion of a ¹⁴C-amino acid, its specific radioactivity at plateau is lower in tissues than in plasma^6–9, due to dilution with unlabelled amino acid derived from intracellular protein breakdown. This is termed internal recycling and also applies when ¹⁵N-glycine is infused. Second, when ¹⁵N-glycine is infused, extensive rapid intracellular transamination occurs especially in liver and kidney, so that the ¹⁵N is distributed to other amino acids, which will have a higher enrichment within the cell than in the plasma. Thus, depending on the balance between internal recycling and transamination, the intracellular enrichment of amino acids may be greater or less than the plasma enrichment. To investigate this problem we have infused ¹⁵N-glycine, and measured the enrichment of α-amino-N urea-N and ammonia-N in various tissues. The TBPT calculated from the liver urea-N enrichment gives a value which we consider representative and which agrees with published values based on ¹⁴C.