In vivo threonine oxidation in growing pigs fed on diets with graded levels of threonine.

Threonine oxidation to glycine was investigated in vivo in twelve growing pigs (27.4 kg live weight) fed on one of the following three diets with graded levels of threonine supply: a low-threonine diet (LT), a control well-balanced diet (C) or a high-threonine diet (HT), during 10 h constant infusion of L-[1-13C]threonine and [2-3H]glycine in the cranial vena cava and [1-14C]glycine in the portal vein. 13C-threonine and glycine enrichments and [3H]glycine and [14C]glycine specific radioactivities (SR) were determined at plateau in peripheral venous plasma, liver and pancreas. Glycine production rates calculated from plasma [2-3H]glycine or [1-14C]glycine SR gave similar values suggesting that [1-14C]glycine SR could be used in order to estimate whole-body glycine flux. The high pancreas [1-13C]glycine enrichment provided evidence that the pancreas may be, with the liver, a major site of threonine oxidation to glycine. Moreover, the present findings suggest that threonine transport into the liver could be the limiting step of threonine oxidation in this tissue when dietary threonine supply is low. Total threonine oxidation to glycine, calculated from plasma values of enrichment and specific radioactivity, was low and constant when the estimated absorbed threonine was lower than 4 g/d and increased for higher amounts of absorbed threonine.