Inhibition by albumin of tryptophan uptake by rat brain

Abstract

CHANGES in the tryptophan content of the brain influence the rate of synthesis of brain 5-hydroxytryptamine (5-HT). Therefore, factors that affect the delivery of tryptophan to the brain may affect its 5-HT content. Tryptophan in plasma can be displaced from its binding site on albumin by non-esterified fatty acids (NEFA)¹ and by certain drugs². Although in some circumstances changes in non-albumin bound (NAB) plasma tryptophan, brought about by drugs and changes in plasma NEFA, parallel those of brain tryptophan^2,3, this relationship does not apply in all conditions. For example, after administration of glucose plasma NEFA decline, as does NAB tryptophan, but the brain tryptophan content increases⁴. Thus, it is uncertain whether binding of tryptophan to albumin is important in controlling brain tryptophan and hence brain 5-HT. To resolve this controversy we looked at the influence of albumin on the uptake of tryptophan by brain in vivo. We found that albumin inhibits tryptophan uptake, and that diffusion, a process that will depend only on the NAB plasma tryptophan concentration, has a small but significant role in the uptake of tryptophan by brain.