Uptake of tryptophan in the brain

Requirements for a maximum uptake of tryptophan in the brain
 

Tryp competes with other amino acids such as phenylalanine, tyrosine, valine, leucine, and isoleucine for the same transporter in the blood-brain barrier (BBB). These amino acids belong to the group of large neutral amino acids (LNAA) or branched-chain amino acids (BCAA). A low ratio of tryp to the competing amino acids has repeatedly been observed in depressive patients (Möller 1985, Kaneko et al. 1992). Only free tryp that is not bound to protein can cross the BBB. Therefore, to ensure the maximum uptake of tryp in the brain, a high proportion of total tryp in the blood should be in the free state, and there should be a low concentration of competing amino acids (LNAA, BCAA). Pharmacokinetic studies suggest that adequate doses of tryp (at least 1 -1.5 g per single dose) have to be administered in order to achieve the high concentrations of free tryp required at the BBB (Green et al, 1980). In addition, these studies indicate that the concentration of free tryp is only high enough within the first 2 to 3 hours after oral administration to ensure effective tryp uptake by the brain. Furthermore, in order to stimulate central serotonin synthesis it seems to be necessary that a certain threshold in the ratio between tryp and the competing amino acids (tryp/LNAA ratio) in the blood should be exceeded. Accordingly, a 40 – 70% increase in the plasma tryp/LNAA ratio as compared to baseline may be required (Markus et al. 2008).

Link: Tryptophan Literature