Estimation of arginine oxidation and de novo synthesis in healthy adult men, using L-[5-¹³C]arginine

We describe a new tracer infusion approach, using L-[5-¹³C]arginine, to measure whole-body rate of arginine (arg) oxidation (Ox) and derive its rate of de novo synthesis. A series of 26h tracer infusion studies were conducted in 7 healthy young adults. One pilot subject received an arg-supplemented (2934 nmol/kg/d) diet for two 10-day periods; period 1, L-[5-¹³C]arg, day 7; L-(5-¹³C]glu, day 10; period 2; [5-¹³C]glu day 7; L-[1-¹³C]glu, day 10. We found using plasma [5-¹³C]arg, with correction for unrecovered ¹³C-label from glutamate (glu), gave a close to expected rate of arginine Ox. Then, six subjects received, for 10 days, a control (511.8 nmol/kg/d) or arg-free diet. Infusion of L-[5-¹³C]arg was performed on day 7 and L-[5-¹³C]glu on day 10. Plasma arg fluxes decreased during the arg-free period from 92.3±12.4 to 40.1±4.9 μmol/kg/h and from 87.8±4.4 to 52.0±9.7 umol/kg/h for fast and fed states, respectively. Arg Ox was reduced from 765.6 and 253.4 nmol/kg/day Estimated mean daily arg de novo synthesis was 165.5 nmol/kg/day for the control and ≤ 157.3 μmol/kg/day for the arg-free diet. Therefore, arg Ox adaptively declines, with possible maintenance of arg synthesis during a short term arg-free diet. De novo arg synthesis remains a small fraction of the plasma arg flux at 7.6% and ≤14.2% for the control and arg-free diets, respectively.