Threonine dehydrogenase is a minor degradative pathway of threonine catabolism in adult humans.
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 CO(2) and to glycine was studied in six healthy adult males using a 4-h constant infusion of L-[1-(13)C]threonine and [(15)N]glycine. Gas chromatography-combustion isotope ratio mass spectrometry was used to determine [(13)C]glycine produced from labeled threonine. Threonine intakes were higher on +Thr and +P-Thr diets compared with control (126, 126, and 50 micromol x kg(-1) x h(-1), SD 8, P < 0.0001). Threonine oxidation to CO(2) increased threefold in subjects on +Thr and +P-Thr vs. control (49, 45, and 15 micromol x kg(-1) x h(-1), 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 micromol x kg(-1) x h(-1), 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.
Darling
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The Research Institute, Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada.
Grunow
J
J
Rafii
M
M
Brookes
S
S
Ball
R O
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Pencharz
P B
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D000328:Adult; D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D000613:Aminobutyrates / Q000097:blood; D002247:Carbon Isotopes; D004044:Dietary Proteins / Q000008:administration & dosage; D004734:Energy Metabolism; D008401:Gas Chromatography-Mass Spectrometry; D005998:Glycine / Q000008:administration & dosage / Q000097:blood; D006801:Humans; D008297:Male; D009587:Nitrogen Isotopes; D010084:Oxidation-Reduction; D013912:Threonine / Q000008:administration & dosage / Q000097:blood*
D000613:Aminobutyrates; D002247:Carbon Isotopes; D004044:Dietary Proteins; D009587:Nitrogen Isotopes; D013912:Threonine; C012223:alpha-aminobutyric acid; D000429:Alcohol Oxidoreductases; C023159:L-threonine 3-dehydrogenase; D005998:Glycine