In vivo threonine oxidation rate is dependent on threonine dietary supply in growing pigs fed low to adequate levels.

Threonine oxidation was examined in 12 growing pigs fed a well-balanced control diet or a threonine-deficient diet supplemented (Glu) or not (LT) with glutamic acid during constant infusion of L-[1-(13)C]-threonine, [1-(14)C]glycine and [1-(14)C]alpha-ketobutyrate for 10 h. During these infusion, liver glycine enrichment was significantly lower than plasma enrichment. Moreover, the pancreas to plasma glycine enrichment ratio was higher than the liver to plasma ratio (70-89%), showing that an important part of glycine de novo synthesis in pancreas occurred through the threonine dehydrogenase (TDG) pathway. These results imply that calculation of threonine oxidation into glycine should be made with the assumption of both hepatic and extrahepatic oxidation. Plateau values of plasma threonine, glycine and alpha-ketobutyrate enrichments and specific radio activities allowed estimations of threonine oxidation through the TDG and threonine dehydratase (TDH) pathways. Threonine oxidation into glycine was 12.16 +/- 2.06, 2.89 +/- 0.61 and 2.13 +/- 0.44 mumol/(kg.h), respectively, in pigs fed the control, LT and Glu diets, and threonine oxidation into alpha-ketobutyrate was 1.80 +/- 0.31, 0.88 +/- 0.02 and 0.55 +/- 0.06 mumol/ (kg.h) for the control, LT and Glu groups, respectively. Total threonine oxidation rates were 75 and 81% lower in the LT and Glu groups, respectively, than in the control group. Liver TDG and TDH activity measured in vitro were not affected by either the level of dietary threonine supply the addition of glutamic acid. On the basis of plasma data, it may be concluded that the addition of glutamic acid to a threonine-deficient diet had no significant effect on threonine oxidation but did reduce the rate of threonine release from protein breakdown. Oxidation appears to be related to plasma threonine concentration.