Transport of amino acids across the blood-brain barrier: implications for treatment of maternal phenylketonuria.

Amino acid transport at the mammalian blood-brain barrier has been extensively characterized. Transport of L-phenylalanine and related neutral amino acids is known to be mediated by a stereospecific, sodium independent, saturable carrier. The affinity of this carrier is much higher than that of comparable systems in other tissues. This feature renders it susceptible to inhibition. It has been suggested that inhibition of neutral amino acid influx into the brain by hyperphenylalaninaemia contributes to the pathophysiology of brain damage in this condition. Methods for investigation of amino acid transport at the blood-brain barrier are discussed, and current knowledge of blood-brain barrier amino acid transport at the blood-brain barrier is reviewed. Developmental changes are delineated, with particular reference to recent work on the ovine blood-brain barrier. There is insufficient information concerning blood-brain barrier transport of amino acids in the fetal brain to allow firm conclusions to be drawn concerning implications for treatment of maternal PKU. Reasonable extrapolation from animal data suggests that transport inhibition may contribute to impaired fetal brain growth in maternal PKU, and can be minimized by attempts to maintain a normal milieu from the time of conception.