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Article

Plasma L-5-Oxoproline Carbon and Nitrogen Kinetics in Healthy Young Adults

Cornelia C. Metges^*,1, Yong-Ming Yu, Wei Cai, Xiao-Ming Lu, Sue Wong, Alfred M. Ajami and Vernon R. Young^*,,23

^* Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge, MA 02139; Boston Burns Hospital, Boston MA 02114; MassTrace Inc., Woburn, MA 01801

³Correspondence to: Prof. Vernon R. Young, Laboratory for Human Nutrition, Bldg. E17-434, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, Phone: 617-253-5801; Fax: 617-253-9658; email: vryoung{at}mit.edu

L-5-oxoproline (OP), an intermediate of the -glutamyl cycle of glutathione synthesis and degradation, may serve as a probe for the state of glutathione kinetics. We explored the whole-body carbon and nitrogen kinetics of OP in five male healthy subjects (75.2 kg; 181 cm; 26 y) after a 5-d adaptation to an adequate L-amino acid-based diet (160 mg N · kg^-1 · d^-1; 188 kJ · kg^-1 · d^-1), using a crossover design. On day 6 of the diet period, we carried out an 8-h tracer protocol (3 h fast; 5 h fed; 2/3 of daily nitrogen intake) with intravenous infusion of L-[1-¹³C]oxoproline and L-[3,3-²H]cysteine or, in randomized order, on the second occasion, L-[¹⁵N]oxoproline and L-[3,3-²H]cysteine. Plasma OP was isolated by cation exchange and after addition of internal standards (DL-[²H₃]-5-oxoproline; L-[¹⁵N, U-¹³C₅]-5-oxoproline; DL-[²H₃]-glutamic acid) derivatized to form TBDMS esters and measured by gas chromatography/mass spectrometry. Plasma OP concentration did not differ between fed and fasted state (fast: 59.4 ± 8.3; fed 59.2 ± 8.9 nmol/mL). ¹³C- and ¹⁵N OP flux during the fasted and fed state were 19 ± 3.6, 21.2 ± 3.2, and 22.6 ± 3.9, 25.8 ± 4.3 µmol · kg^-1 · 30 min^-1, respectively. OP oxidation was 15.6 ± 3.6 and 17.9 ± 3.5 µmol · kg^-1 · 30 min^-1, in fasting and feeding, respectively, (P < 0.05). More than 80% of the plasma flux was oxidized. These findings are compared with the published literature on GSH turnover in plasma of human subjects and underscore the need to define more completely the dynamic aspects of glutathione metabolism and of the intermediates of the -glutamyl cycle.

KEY WORDS: • oxoproline kinetics • glutathione • stable isotopes • oxidation • flux • humans

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				Y.-M. Yu, C. M. Ryan, Z.-W. Fei, X.-M. Lu, L. Castillo, J. T. Schultz, R. G. Tompkins, and V. R. Young Plasma L-5-oxoproline kinetics and whole blood glutathione synthesis rates in severely burned adult humans Am J Physiol Endocrinol Metab, February 1, 2002; 282(2): E247 - E258. [Abstract] [Full Text] [PDF]

				C. C. Metges, Y.-M. Yu, W. Cai, X.-M. Lu, S. Wong, M. M. Regan, A. Ajami, and V. R. Young Oxoproline kinetics and oxoproline urinary excretion during glycine- or sulfur amino acid-free diets in humans Am J Physiol Endocrinol Metab, May 1, 2000; 278(5): E868 - E876. [Abstract] [Full Text] [PDF]