The role of L-tryptophan transport in L-tryptophan degradation by indoleamine 2,3-dioxygenase in human placental explants.

The physiological importance of L-tryptophan transport for placental indoleamine 2,3-dioxygenase-mediated degradation of L-tryptophan has been studied using human placental chorionic villous explants. L-Tryptophan influx into villous explants is supported exclusively by transport system L and is substantially inhibited by the L-system-specific substrate 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) and also by 1-methyl-tryptophan which is also an inhibitor of indoleamine 2,3-dioxygenase. L-Tryptophan influx is enhanced 2.3-fold following in vitro culture of the villous explant. Interferon-gamma, which increases villous explant indoleamine 2,3-dioxygenase expression, has no effect on L-tryptophan influx. In explants both BCH and 1-methyl-tryptophan inhibit indoleamine 2,3-dioxygenase-mediated L-tryptophan degradation. This also applies when L-tryptophan degradation has been stimulated by interferon-gamma. These findings show transport of L-tryptophan into the trophoblast to be a rate-limiting step for indoleamine 2,3-dioxygenase-mediated L-tryptophan degradation and therefore for the normal physiology of mammalian pregnancy.