Saturation of a shared mechanism which transports L-arginine and L-lysine into the brain of the living rat.

1. The rate of entry of L-arginine and L-lysine into the brain of the rat was measured in vivo by a direct method in which the amino acid concentration was at a constant level in the blood plasma over the period of the experiment.2. Both L-arginine and L-lysine enter the brain by a transport mechanism which can be saturated by a high concentration of the same amino acid in the bloodstream. The rate of entry can be explained by Michaelis-Menten kinetics.3. The entry into the brain of L-arginine can be inhibited by raised plasma concentrations of L-lysine or L-ornithine and the entry of L-lysine by raised concentrations of L-arginine.4. The inhibition of entry of an amino acid is most severe when its own concentration in the blood plasma is low and that of the inhibitor is high. The inhibition appears to be basically competitive in type, suggesting that common transport systems are shared by three dibasic amino acids.5. It is suggested that the raised levels of amino acids found in various disorders of amino acid metabolism are likely to reduce the rate of entry into the brain of other amino acids and a way is suggested in which the dietary treatment of hyperlysinaemia may be made more effective.