Albumin binding and brain uptake of 6-fluoro-DL-tryptophan: competition with L-tryptophan.

We investigated potential competition between L-tryptophan (TRP) and 6-fluoro-DL-tryptophan (6-F-TRP) for binding to albumin and for passage through the blood-brain barrier (BBB). In experiments based on equilibrium dialysis, albumin (600 microM) bound about 80% of TRP and 50% of 6-F-TRP with affinity constants (Ka) of 3.7 +/- 0.04 x 10(4) and 0.62 +/- 0.01 x 10(4) M-1, respectively. Competitive inhibition was assessed as the decrease in the apparent Ka (K' a) of TRP in the presence of 6-F-TRP, with no modification of the N value. Competition between TRP, 6-F-TRP and L-valine (VAL) for passage across the BBB was demonstrated using two approaches. When administered concomitantly with TRP or 6-F-TRP to rats, VAL decreased brain uptake of TRP and 6-F-TRP and reversed their action on serotonin. In Oldendorf's model, 6-F-TRP and VAL decreased the brain uptake of TRP.