AENDO May 41/5

Metges, Cornelia C., Yong-Ming Yu, Wei Cai, XiaoMing Lu, Sue Wong, Meredith M. Regan, Alfred Ajami, and Vernon R. Young. Oxoproline kinetics and oxoproline urinary excretion during glycineor sulfur amino acid-free diets in humans. Am J Physiol Endocrinol Metab 278: E868–E876, 2000.—L-5-oxoproline (L-5-OP) is an intermediate in glutathione synthesis, possibly limited by cysteine availability. Urinary 5-OP excretion has been proposed as a measure of glycine availability. We investigated whether 5 days of dietary sulfur amino acid (SAA-free) or glycine (Gly-free) restriction affects plasma kinetics of 5-OP and urinary excretion of Land D-5-OP in 6 healthy men. On day 6, L-5-[1-13C]oxoproline and [3,3-H2]cysteine were infused intravenously for 8 h (3 h fast/5 h fed). In a control study (adequate amino acid mixture), plasma oxoproline fluxes were 37.8 6 13.8 (SD) and 38.4 6 14.8 μmol ·kg21 ·h21; oxidation accounted for 85% of flux. Cysteine flux was 47.9 6 8.5 and 43.2 6 8.5 μmol ·kg21 ·h21 for fast and fed phases, respectively. Urinary excretion of Land D-5-OP was 70 6 34 and 31.1 6 13.3 μmol/mmol creatinine, respectively, during days 3–5, and 46.4 6 13.9 and 22.4 6 8.3 μmol/mmol over the 8-h tracer study. The 5-OP flux for the Gly-free diet was higher (P 5 0.018) and tended to be higher for the SAA-free diet (P 5 0.057) when compared with the control diet. Oxidation rates were higher on the Gly-free (P 5 0.005) and SAA-free (P 5 0.03) diets. Cysteine fluxes were lower on the the Gly-free (P 5 0.01) and the SAA-free diets (P 5 0.001) compared with the control diet. Rates of L-5-OP excretion were unchanged by withdrawal of SAA or Gly for 5 days but increased on day 6 (P 5 0.005 and P 5 0.019, respectively). Thus acute changes in the dietary availability of SAA and Gly alter oxoproline kinetics and urinary 5-OP excretion.