Tryptophan is interesting in that it is the sole precursor of the inhibitory neurotransmitter serotonin. The relative percentage of tryptophan in protein sources is small in comparison to the other amino acids (roughly 1%). Additionally, the degradation of tryptophan occurs via two pathways: the kynurenine pathway that is responsible for 90-95% of tryptophan degradation and the minor pathways that lead to serotonin and melatonin (serotonin synthesis). The kynurerine pathway l