Induction of lysine imbalance in rats: relation to competition for lysine transport into the brain in vitro.

The ability of amino acids inhibitory of lysine transport into brain slices to induce lysine imbalance was determined by feeding wheat gluten or casein diets with additions of such amino acids. Lysine transport was only moderately inhibited by amino acids; the most effective were basic amino acids or mixtures of indispensable (IAA) or branched chain amino acid (BCAA). Only mild depressions in growth and food intake occurred during a 10-day period when male, 60--65 g rats of the Sprague-Dawley strain were fed lysine-limiting, 18% wheat gluten diets with additions of these amino acids. The effects were prevented by added lysine. Rats allowed a choice between the lysine-imbalanced or non-protein diets selected the imbalanced, wheat gluten diets (in severe imbalances rats will choose the non-protein diet). Growth depression, prevented by added lysine, occurred in rats fed a 6% casein diet supplemented with IAA; individual amino acids were ineffective. Growth depressions also occurred when rats were fed a basal diet containing 6% case in + 5% of an equimolar mixture of nine IAA and supplemented with arginine or more IAA; BCAA were less effective. Additional lysine completely prevented the growth depressions, but growth of rats fed the diets containing arginine and BCAA was greater than that of those fed the extra IAA. It is difficult to induce a severe lysine imbalance; this is consistent with the failure of amino acids to cause under our conditions strong inhibition of lysine transport into brain.