May 1, 2019

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 leads to two metabolites, quinolinic and kynurenic acid, which act on the NMDA receptors (glutamate receptor) as either an agonist (neurotoxic) and antagonist (neuroprotective) respectively (1). Interestingly, the kynurenine pathway is involved in UV protection of the retina, which naturally declines with age (2). The kynurenine pathways is also involved in the formation of NAD and NADP necessary for redox reactions. To a lesser degree, tryptophan degradation along the kynurenine pathway is involved in niacin (B3) s...

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