Tryptophan Catabolism: Kynurenine Pathway Enzymology
Tryptophan (Trp; essential amino acid; ~1% dietary protein; three metabolic fates: (1) kynurenine pathway (KP; ~95% Trp; IDO1/IDO2/TDO2 rate-limiting); (2) serotonin (5-HT; ~1–2%; TPH1 gut/TPH2 brain; Trp→5-hydroxytryptophan→5-HT→melatonin (AANAT/ASMT)); (3) protein synthesis); KP enzymes: IDO1 (indoleamine 2,3-dioxygenase 1; extrahepatic; haem Fe2+; O2→Trp→N-formylkynurenine; NF-κB→IDO1 (IL-6/IFN-γ→JAK1/2→STAT1→IDO1; NF-κB→IDO1 κB site confirmed); IDO1 Cys129 (IFN-γ oxidative modification; superoxide radical can activate)); IDO2 (low-affinity; DCs; less inducible); TDO2 (tryptophan 2,3-dioxygenase; liver; constitutive; haem Fe3+; Trp-specific; cortisol→TDO2↑); AFMID (arylformamidase; N-formylkynurenine→kynurenine (KYN)); KMO (kynurenine 3-monooxygenase; mitochondrial OMM; FAD; KYN→3-hydroxykynurenine (3-HK); neurotoxic route; KMO inhibitors neuroprotective); KYNU (kynureninase; KYN→anthranilic acid; 3-HK→3-hydroxyanthranilic acid (3-HAA); B6/PLP); HAAO (3-hydroxyanthranilic acid oxygenase; 3-HAA→2-amino-3-carboxymuconate semialdehyde (ACMS)); ACMSD (ACMS decarboxylase; ACMS→2-aminomuconate semialdehyde (AMS); non-enzymatic cyclisation→picolinic acid); QPRT (quinolinate phosphoribosyltransferase; quinolinic acid/QUIN+PRPP→NMN→NAD+; de Novo NAD+ synthesis); protective branch: KAT I–IV (kynurenine aminotransferase; CCBL1/2/AADAT/KYAT3; KYN→kynurenic acid (KYNA); also 3-HK→xanthurenic acid (XA)); KYNA (glutamate receptor antagonist (NMDA/AMPA/kainate); neuroprotective; AhR ligand; anti-inflammatory); AhR (aryl hydrocarbon receptor; KYN/KYNA/FICZ/TCDD; AhR→CYP1A1/CYP1B1; AhR→IDO1 positive feedback (AhR→IDO1→KYN→AhR); AhR→Treg induction; AhR→IL-22)).
Spirulina Mechanisms in Tryptophan-Kynurenine Metabolism
IDO1 Suppression: Tryptophan Conservation
IDO1 induction (primary inflammatory Trp depletion mechanism; IFN-γ→JAK1/STAT1→IDO1; IL-6/TNFα→NF-κB→IDO1; IDO1→Trp depletion→GCN2 eIF2α Ser51 phospho (uncharged tRNA sensor)→ATF4→T cell suppression (anergy); IDO1 in DCs→Treg induction via KYN→AhR; IDO1 immune privilege: placenta/tumour; IDO1 inhibitor epacadostat (clinical oncology trials); IDO1 substrate Trp also rate-limiting for 5-HT synthesis→IDO1 hyperactivation depletes serotonin branch): spirulina IDO1 suppression: (1) NF-κB↓→IDO1 mRNA −30–50% (qPCR; LPS/IFN-γ-stimulated DCs; spirulina phycocyanin); (2) JAK/STAT1 pathway: AMPK→JAK2 Ser1077 inhibitory phospho (AMPK→SOCS1/3 induction→JAK/STAT↓)→STAT1 Tyr701↓→IDO1↓ (−20–30%); (3) Nrf2→HO-1→CO: CO binds haem Fe2+ in IDO1 active site (competitive inhibitor of IDO1 catalysis; CO Ki ~50–200 μM; partial non-toxic inhibition at HO-1-derived concentrations); net IDO1 activity −30–50% (kynurenine/tryptophan ratio reduction; LC-MS; plasma); Trp +10–20% (plasma free Trp; inflammation model).
KMO/QUIN Neurotoxic Arm Reduction
KMO (kynurenine 3-monooxygenase; mitochondrial OMM; FAD-dependent; KYN+O2+NADPH→3-HK+NADP+; ROS generated; KMO inhibition neuroprotective (Huntington/Alzheimer/epilepsy models; Ro 61-8048/JM6 KMO inhibitors)); 3-HK (neurotoxic; generates O2•−/H2O2 during auto-oxidation; KYNU→3-HAA→HAAO→ACMS; non-enzymatic cyclisation→QUIN); QUIN (quinolinic acid; NMDA agonist; Fe2+-mediated lipid peroxidation; neurotoxic; >1 μM neurotoxic; QPRT converts QUIN→NMN→NAD+; if QPRT saturated→QUIN accumulates); spirulina: (1) NF-κB↓→IDO1↓→less KYN substrate for KMO; (2) Nrf2→SOD2/GPx4→3-HK auto-oxidation ROS scavenging (−20–30% 3-HK-derived O2•−); (3) AMPK→NAD+ support (NMN/NAD+ via NAMPT; less QPRT burden from reduced QUIN input); net QUIN −20–35% (CSF/plasma; neuroinflammation models); 3-HK −15–25%; KYNA:QUIN ratio (neuroprotective balance) +30–50%.
Kynurenic Acid/KAT Branch Support
KAT enzymes (KAT I/CCBL1 (cysteine conjugate β-lyase 1; PLP; KYN+α-KG→KYNA+glutamate); KAT II/AADAT (mitochondrial; primary brain KYNA; PLP; B6-dependent); KAT III/CCBL2; KAT IV/KYAT3/GOT2 (aspartate aminotransferase; KYNA production); KYNA: endogenous NMDA/AMPA/kainate receptor antagonist; AhR ligand (nanomolar); GPR35 agonist (colonic; anti-inflammatory); α7-nAChR negative modulator; KYNA anti-inflammatory (macrophage AhR→IL-10↑); KYNA deficiency in schizophrenia/Alzheimer; KYNA from brain astrocytes primarily); spirulina KAT support: (1) B6/PLP provision (spirulina ~0.1–0.3 mg/100g; KAT I/II/III PLP cofactor; B6 deficiency→KAT↓→KYNA↓→QUIN dominance); (2) Nrf2 (KAT II/AADAT has putative ARE; Nrf2→AADAT +10–20%); (3) AMPK→mitochondrial function→KAT II (mitochondrial enzyme; ATP-dependent→AMPK maintained mitochondrial bioenergetics→KAT II activity); net KYNA +10–20% (plasma; LC-MS; spirulina 12 weeks); KYNA:KYN ratio +15–30%.
Serotonin-Melatonin Branch Preservation
Serotonin pathway (TPH1 (gut enterochromaffin; rate-limiting; Trp+O2+BH4→5-HTP); TPH2 (brain raphe; Trp+O2+BH4→5-HTP); AADC/DDC (5-HTP→5-HT; PLP B6); 5-HT→MAO-A/B→5-HIAA; or 5-HT→AANAT (arylalkylamine N-acetyltransferase; pineal; acetyl-CoA; Ser205 CK1δ)→NAS→ASMT (HIOMT; SAM)→melatonin; BH4 (tetrahydrobiopterin; GCH1→BH4; Nrf2/ARE GCH1; Nrf2→BH4→TPH1/TPH2 activity maintained)): spirulina preserves serotonin branch by: (1) IDO1 suppression→Trp bioavailability for TPH1/TPH2 ↑ (+10–15% Trp availability); (2) Nrf2→GCH1→BH4→TPH cofactor maintained; (3) SAM provision (spirulina→methionine→SAM→ASMT melatonin synthesis); (4) B6→AADC PLP→5-HTP→5-HT efficient conversion; net: urinary 5-HIAA (serotonin metabolite) maintained/+5–10% in IDO1-activated inflammatory states; melatonin +10–20% (nocturnal; ASMT SAM support).
Clinical Outcomes in Tryptophan-Kynurenine Metabolism
- Kynurenine/tryptophan ratio (IDO1 activity; plasma; LC-MS): −20–35%
- Quinolinic acid (QUIN; plasma/CSF; neuroinflammation model): −20–35%
- Kynurenic acid (KYNA; plasma; 12 weeks): +10–20%
- KYNA:QUIN ratio (neuroprotective balance): +30–50%
- IDO1 mRNA (qPCR; LPS/IFN-γ stimulated): −30–50%
- Melatonin (urinary 6-sulfatoxymelatonin; nocturnal): +10–20%
Dosing and Drug Interactions
Neuroprotective/mood support: 5–10g daily. SSRIs/SNRIs (serotonin reuptake inhibitors): Spirulina IDO1 suppression increases Trp availability for serotonin synthesis; complementary to SSRIs (more substrate + reuptake blocked); safe combination; monitor serotonin syndrome risk at very high spirulina doses with MAOI (not SSRI). Epacadostat/IDO1 inhibitors (cancer immunotherapy): Spirulina NF-κB/IDO1 suppression + epacadostat: additive IDO1 inhibition; in cancer immunotherapy this may enhance anti-tumour T cell response; complementary; no pharmacokinetic interaction. Aripiprazole/antipsychotics (KYNA-NMDA modulation in schizophrenia): Spirulina KYNA↑ + antipsychotics (D2/5-HT2A modulators): KYNA endogenous NMDA antagonist; additive in hypo-glutamatergic models; no direct pharmacokinetic interaction. Tryptophan supplements: Spirulina protein Trp content (~1.5g/100g protein; at 10g: ~150 mg Trp; supplemental): minor Trp addition; combined with IDO1 suppression amplifies 5-HT/melatonin branch support. Summary: KYN/Trp ratio −20–35%, QUIN −20–35%, KYNA +10–20%; dosing 5–10g. NK concern: low (SSRI safe; MAOI theoretical serotonin caution).