Rapid steady-state analysis of blood-brain transfer of l-Trp in rat, with special reference to the plasma protein binding

We estimated constants for the binding of tryptophan (Trp) to plasma proteins, and for the transfer of Trp from plasma to brain in rat. The measurements were made under conditions in which the plasma and brain concentrations of Trp were raised to new steady-states for at least 10 min before being measured. The concentration of other competing amino acids were also at a steady-state. The plasma Trp concentration was elevated by i.p. injection of different doses of l-tryptophan methyl ester 60 min before the measurement of the plasma-brain transfer. We simultaneously measured blood flow with [¹⁴C]-butanol, and the brain tissue Trp uptake with [³H]Trp. The maximal velocity (V_max), apparent half-saturation Michaelis-Menten constant (K_m^app), and diffusion constant (P_dS) for Trp transport from plasma into brain were found to be 7.0±2.1 nmol g^-1 min^-1, 36 ± 17 μM, and 0.065 ± 0.006 ml g^-1 min^-1, respectively. The maximum plasma protein binding (B_max) and dissociation constant (K_D) for Trp were estimated at 360 ± 16 nmol/ml-plasma and 81 ± 10 μM, respectively. We conclude that the plasma protein binding of Trp inhibits the blood-brain transfer in inverse proportion to the plasma free Trp concentration.