Thirteen amino acids labeled with 14C were rapidly injected individually into rat common carotid artery along with tritiated water as a diffusable reference substance. All amino acid brain uptakes studied were reduced by a control serum vehicle pooled from nonphenylketonuric (PKU) retardeds, relative to a vehicle of buffered Ringer's solution. Two vehicles, sera pooled from phenylketonurics and a control serum vehicle to which unlabeled phenylalanine had been added and thus reconstituted to bring the serum phenylalanine level up to that of the PKU serum, each caused a significant reduction of large neutral amino acid uptakes but had an insignificant effect on basic amino acids. Saturation of the large neutral amino acid blood brain barrier transport system may result in a brain deprivation in infancy of those amino acids using the same carrier transport system as phenylalanine and thus be causally related to mental retardation in PKU.
William H. Oldendorf. Saturation of Blood Brain Barrier Transport of Amino Acids in Phenylketonuria. Arch Neurol. 1973;28(1):45–48. doi:10.1001/archneur.1973.00490190063008