15N-labelled yeast protein--a valid tracer for calculating whole-body protein parameters in infants: a comparison between [15N]-yeast protein and [15N]-glycine.

The validity of [15N]-glycine and 15N-labelled yeast protein as tracers for investigating the parameters of nitrogen metabolism in man was studied by comparisons of each tracer in three infants on different diets. Both tracers were administered with the food as a single oral dose of 0.2 mmol 15N-excess nitrogen per kg body weight. Cumulative 15N-excess excretion in the urine was measured by emission spectrometry and a three-compartment model was used to calculate the pool sizes by computer. In all three comparisons the values calculated for protein synthesis, protein breakdown, protein turnover and reutilization after administration of 15N-labelled yeast protein were slightly lower than those calculated after administration of [15N]-glycine. The particular advantage of applying a highly enriched, completely labelled [15N]-protein instead of [15N]-glycine as a tracer is that the protein, containing some 20 amino acids, doubtless gives a more accurate picture of protein metabolism than the use of a single amino acid labelled with heavy nitrogen. However, the small differences between the whole-body parameters calculated from 15N-labelled yeast protein and [15N]-glycine do not justify the general replacement of [15N]-glycine by 15N-labelled protein.