Whole body nitrogen kinetics in man: determination from plasma [guanidino-15N]arginine

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. 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. 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.(ABSTRACT TRUNCATED AT 250 WORDS)