Phenylalanine Transport Across the Blood‐Brain Barrier as Studied with the In Situ Brain Perfusion Technique

Abstract

Unidirectional L-phenylalanine transport into six brain regions of pentobarbital-anesthetized rats was studied using the in situ brain perfusion technique. This technique allows both accurate measurements of cerebrovascular amino acid transport and complete control of perfusate amino acid composition. L-Phenylalanine influx into the brain was sodium independent and could be described by a model with a saturable and a nonsaturable component. Best-fit values for the kinetic constants in the parietal cortex equaled 6.9 X 10(-4) mumol/s/g for Vmax, 0.011 mumol/ml for Km, and 1.8 X 10(-4) ml/s/g for KD during perfusion with fluid that did not contain competing amino acids. D-Phenylalanine competitively inhibited L-phenylalanine transport with a Ki approximately 10-fold greater than the Km for L-phenylalanine. There were no significant regional differences in Km, KD, or Ki, whereas Vmax was significantly greater in the cortical lobes than in the other brain regions. L-Phenylalanine influx during plasma perfusion was only 30% of that predicted in the absence of competing amino acids. Competitive inhibition increased the apparent Km during plasma perfusion by approximately 20-fold, to 0.21 mumol/ml. These data provide accurate new estimates of the kinetic constants that describe L-phenylalanine transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transfer site affinity (1/Km) for L-phenylalanine is three- to 12-fold greater than previously estimated in either awake or anesthetized animals.

Are Neuropsychological Impairments in Children with Early-Treated Phenylketonuria (PKU) Related to White Matter Abnormalities or Elevated Phenylalanine Levels?

“…WMAs have been related to concurrent and lifetime PHE levels (Cleary et al, 1994;Pietz, Kreis et al, 1996), which is consistent with findings that the WMAs are at least partially reversible with re-introduction to a strict low PHE diet with improved biochemical control (Bick et al, 1993;Cleary, Walter, Wraith, & Jenkins, 1995). High PHE availability can also compete and thus inhibit the transportation of large neutral amino acids (LNAAs) such as TYR and tryptophan at the blood-brain barrier (BBB) (McKean, 1972;Momma, Aoyagi, Rapoport, & Smith, 1987). This occurs even with mild to moderate elevations in PHE levels as the BBB transport proteins for LNAAs have a preference or affinity for PHE (Momma et al, 1987).…”

Development of a Computational Approach to Predict Blood-Brain Barrier Permeability

“…We refined the logPS model to predict the BBB permeability for 23 substrates that penetrate BBB primarily through a passive diffusion mechanism. Phenylalanine and levodopa have been reported as good substrates of system L for large neutral amino acids, an uptake transport system at BBB (Wade and Katzman, 1975;Momma et al, 1987;Alexander et al, 1994;Sanchez del Pino et al, 1995). According to BB KO /BB WT ratio Ͼ 2, five compounds in our data set have been reported as possible substrates for P-gp, an Gratton et al (1997) …”

Molecular modeling studies on the active binding site of the blood–brain barrier choline transporter

“…where F is the cerebral blood flow determined for each region of the brain from the uptake of [ 14 C] diazepam according to the method of Momma et al 14 An apparent inhibition constant (K i ) for choline and its choline analogs was determined 12 from the equation: …”