Tryptophan metabolites, kynurenines

The amino acid tryptophan (Trp) is catabolized mainly through the kynurenine pathway, forming metabolites collectively referred to as kynurenines. Some kynurenines are neuroactive compounds with immunomodulatory effects.
The intial step in this pathway, the conversion of Trp to formylkynurenine (fKyn), is catalyzed by indoleamine 2,3-dioxygenase (IDO), which is activated by pro-inflammatory cytokines, like INFγ and TNFα. fKyn is converted to Kyn, wich in turn is metabolized to HK by the FAD-dependent kynurenine mono-oxygenase (KMO), and then cleaved to HAA by the pyridoxal 5´-phosphate (PLP)-dependent enzyme, kynureninase (KYNU), which also catalyzes the conversion of Kyn to AA. The PLP-dependent enzyme kynurenine transaminase (KAT) catalyzes the formation of two end-stage metabolites, KA (from Kyn) and XA (from HK) (2) (Figure 1).
The downstream metabolite, QA, is converted to nicotinic acid mononucleotide, which is a precursor of niacin and nicotinamide dinucleotide (NAD), i.e. B3 vitamers. N1-methylnicotinamide (mNAM) in urine is used as a marker of niacin status, and administration of NAM resulted in increased plasma concentrations of both NAM and mNAM.
Kynurenic acid (KA) and picolinic acid (Pic) are considered as neuroprotective, whereas 3-hydroxykynurenine (HK) and in particular quinolinic acid (QA) have neuroexcitatory effects (Figure 1). These kynurenines are suggested to be involved in the pathogenesis of some neurodegenerative and neuropsychiatric diseases (2).
The CSF levels of neuroactive kynurenines, KA and HK, show no change (KA) or decrease (HK) with age, whereas the neurotoxic kynurenine, QA, in CSF shows a remarkably strong positive associations with age, but also with markers of neuroinflammation, like neopterin and KTR. These age-related changes may be associated with increasing incidence of neurodegenerative diseases with age (3).
A composite index involving five kynurenines, HKr (HK:(KA+AA+XA+HAA)), is a functional marker of B6 status.