1. Whole body nitrogen turnover and protein synthesis were calculated by the method of D. Picou & T. Taylor-Roberts [Clinical Science and Molecular Medicine (1969) 36, 283–296] except that plateau plasma enrichment of [guanidino-15N]arginine was used in place of the [15N]urea enrichment after a constant infusion of [15N]-glycine. With this approach metabolic pool turnover and protein synthesis were 637.2 ± 73.0 mg of N day−1 kg−1 and 2964.0 ± 409.5 mg of protein day−1 kg−1 respectively.
2. Virtually identical isotopic enrichment in [guanidino-15N]arginine and [15N]urea were observed in a healthy young adult who took repeated oral doses of [15N]glycine for a period of 60 h: 0.47 (arginine) and 0.48 (urea) atom% excess.
3. The turnover of glycine nitrogen and of urea, determined from the constant infusion of [15N]glycine and [13C]urea, was 66.2 ± 3.3 mg of N day−1 kg−1 and 156.2 ± 4.3 mg of urea day−1 kg−1 respectively. The ratio of steady-state enrichment in arginine to that in glycine, reflecting the fraction of arginine derived from glycine, was 10.5%. By using the [guanidino-15N]arginine enrichment as representative of the expected enrichment in [15N]urea at plateau, it was calculated that approximately 25% of glycine N flux is directed toward the synthesis of urea, with the remainder directed to protein and quantitatively minor products like haem and creatinine.
4. Unlike steady-state [15N]urea labelling, which is achieved only after infusion of [15N]-glycine for several days, plateau isotopic abundance in [guanidino-15N]arginine was attained after only 1–2 h of [15N]glycine infusions, thereby allowing estimation of whole body nitrogen kinetics and the rate of transfer of glycine nitrogen to urea in a relatively brief experiment.