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-balance d control diet or a threonine-deficient diet supplemented (Glu) or not (LT) with glutamic acid during constant infusion of L-[1-C-13]-threon ine, [1-C-14]glycine and [1-C-14]alpha-ketobutyrate for 10 h. During t hese infusions, 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 tha t 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 w ith the assumption of both hepatic and extrahepatic oxidation. Plateau values of plasma threonine, glycine and alpha-ketobutyrate enrichment s and specific radio activities allowed estimations of threonine oxida tion through the TDG and threonine dehydratase (TDH) pathways. Threoni ne oxidation into glycine was 12.16 +/- 2.06, 2.89 +/- 0.61 and 2.13 /- 0.44 mu mol/(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 mu mol/(kg . h) for the contr ol, LT and Glu groups, respectively. Total threonine oxidation rates w ere 75 and 81% lower in the LT and Glu groups, respectively, than in t he control group. Liver TDG and TDH activity measured in vitro were no t affected by either the level of dietary threonine supply the additio n of glutamic acid. On the basis of plasma data, it may be concluded t hat 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 rela ted to plasma threonine concentration.