Excretion of 15N and incorporation into plasma proteins after high-dosage pulse labelling with various tracer substances in infants.

The suitability of a biosynthetically produced [15N] yeast-protein-thermitasehydrolysate ([15N] YPTH), [15N] yeast protein and [15N] glycine for use as 15N-tracers was tested in three groups of four infants each. The [15N] YPTH was obtained by hydrolysing 15N-labelled yeast protein with thermitase, a proteinase from Thermoactinomyces vulgaris. Following oral single-pulse labelling in a dosage of 10 mg 15N/kg body weight the 15N-excretion in stools and urine as well as the 15N-abundance in plasma proteins and in the TCA-soluble plasma fraction were determined. The [15N] YPTH differs from [15N] glycine in terms of the complete distribution of 15N among all 20 amino acids. This could be demonstrated by a distinctly lower [15N]ammonia (0.5 per cent) and a higher [15N2] urea excretion (5.0 per cent) compared with [15N] glycine (1.2 and 3.4 per cent respectively). The faecal loss of 15N from the [15N] YPTH was 3.7 per cent of the tracer dose, while the corresponding value after administration of [15N] yeast protein was found to be 7.4 per cent on average. There were no differences between the tracer substances in terms of the measured 15N-abundance in the plasma proteins (mean: 0.07 atom per cent excess) and in the TCA-soluble fraction (mean: 0.21 atom per cent excess).