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1 The Research Institute, Hospital for Sick Children, Toronto, Ontario M5G 1X8; Departments of 2 Nutritional Sciences and 3 Pediatrics, University of Toronto, Ontario M5S 3E2; 5 Department of Agricultural, Food & Nutritional Sciences, University of Alberta, Edmonton, Alberta, Canada T6G; and 4 PDZ Europa Scientific Ltd, Crewe CW1 6ZA, United Kingdom
The threonine dehydrogenase (TDG) pathway is a
significant route of threonine degradation, yielding glycine in
experimental animals, but has not been accurately quantitated in
humans. Therefore, the effect of a large excess of dietary threonine,
given either as free amino acid (+Thr) or as a constituent of protein
(+P-Thr), on threonine catabolism to CO2 and to glycine was
studied in six healthy adult males using a 4-h constant infusion of
L-[1-13C]threonine and
[15N]glycine. Gas chromatography-combustion
isotope ratio mass spectrometry was used to determine
[13C]glycine produced from labeled threonine.
Threonine intakes were higher on +Thr and +P-Thr diets compared with
control (126, 126, and 50 µmol · kg
1 · h1,
SD 8, P < 0.0001). Threonine oxidation to CO2
increased threefold in subjects on +Thr and +P-Thr vs. control (49, 45, and 15 µmol · kg1 · h1,
SD 6, P < 0.0001). Threonine conversion to glycine tended to be higher on +Thr and +P-Thr vs. control (3.5, 3.4, and 1.6 µmol · kg1 · h1,
SD 1.3, P = 0.06). The TDG pathway accounted for only
7-11% of total threonine catabolism and therefore is a minor
pathway in the human adult.
threonine oxidation; threonine flux; stable isotopes; plasma threonine concentration; glycine
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