Adaptive response of lysine and threonine degrading enzymes in adult rats.

The mechanism of variation inlysine and threonine conservation was investigated by measuring the activity of the enzyme initiating the amino acid catabolism in adult rats adapted to different diets. Both liver threonine dehydratase and lysine-ketoglutarate reductase activities were increased by high protein intake. Supplementation with 2% L-lysine HCI to a 5% lactalbumin diet increased liver lysine-ketoglutarate reductase activity three-fold. Feeding a lysine-free diet or a 10% wheat gluten diet significantly decreased the activity below the level obtained by feeding a protein-free diet. In contrast, dietary threonine in either excess or deficiency did not change the liver threonine dehydratase activity. At the same level of lysine intake, an excess of dietary wheat gluten seemed to increase the liver lysine-ketoglutarate reductase activity much more than dietary lactalbumin. These data suggest that the activity of lysine-ketoglutarate reductase in rat liver might be influenced by dietary lysine as well as excesses of other amino acids in wheat gluten. The different response of these two catabolic enzymes to their substrate might explain the variation of lysine and threonine conservation during amino acid deficiency.