Figure 2.

Schematic diagram of different tryptophan metabolism pathways. According to results found in T cells lacking tetrahydrobiopterin (BH4) (see text), it is conceivable that the conversion of tryptophan by tryptophan mono-oxygenase results in high iron levels and efficient mitochondrial activity, whereas the kynurenine pathway initiated by IDO1 activity leads to the formation of the iron chelator picolinic acid.