The exclusion of L-isoleucine or of L-leucine from the brain of the rat, caused by raised levels of L-valine in the circulation, and the manner in which this exclusion can be partially overcome.

The amino acids L-isoleucine, L-leucine and L-valine appear to compete for a shared transport carrier for entry into the brain of the living rat, since high concentrations of L-valine in the plasma reduced the influx both of L-isoleucine and of L-leucine into the brain to 10-15% of the normal. The exclusion of L-isoleucine or of L-leucine from the brain by a raised level of L-valine in the circulation can be partially overcome by simultaneously raising the concentration of L-isoleucine or of L-leucine in the circulation. These results indicate the important role played by a saturable shared carrier-mediated transport system in ensuring that an adequate supply of these amino acids reaches the brain from the circulation during life. The bearing of our findings on two inborn errors of amino acid metabolism, maple-syrup-urine disease and hypervalinaemia, is discussed.