14C-labelled lysine, glutamic acid and methionine were each administered as single infusions through catheters in the jugular veins of pigs between 28 and 42 d old, receiving either a low-protein (LP) (140 g protein/kg), or a high-protein (HP) (200 g protein/kg) diet. The pigs received 40 g diet/kg body-weight per d. 2. L-[U-14C]lysine and L-[U-14C]glutamic acid were both rapidly removed from the plasma; 3 min after completion of the infusion less than 10% of the activity from the [14C]-lysine, and between 10 and 20% of the activity from the [14C]glutamic acid remained in the plasma. 3. In pigs killed 15 min after infusion of either [14C]lysine or [14C]glutamic acid 26% of the activity from the dose of lysine was in the liver of those receiving the LP diet, whereas the corresponding value was only 16% for those which had received the HP diet. The uptake of activity from [14C]glutamic acid by the liver was unaffected by the level of protein in the diet. More activity from [14C]lysine than from [14C]glutamic acid was recovered in the spleen, heart and brain. 4. The recovery of the activity from [14C]lysine as 14CO2 in the 6 h after the infusion of the dose was lower for the pigs on the LP diet (2-7%) than for those on the HP diet (4-8%). The corresponding values for [14C]glutamic acid infusion were 34 and 52% respectively. 5. Supplementing the LP diet with five successive increments of 1-5 g DL-methionine/kg resulted in an increase from 8 to 17% dose in the recovery of activity as 14CO2 from a dose of L-[I-14C]methionine, whereas with the same supplementation of the HP diet the recovery increased from 8 to 24% dose.
