Spirulina and Polyamine Metabolism.

Polyamine Biosynthesis: ODC and the Antizyme Gate

Polyamine flux begins at ornithine decarboxylase (ODC1; EC 4.1.1.17), a pyridoxal-5′-phosphate (PLP)-dependent homodimer converting L-ornithine to putrescine. ODC is among the most rapidly turned-over mammalian enzymes (t½ ~10–30 min) and is degraded by a unique ubiquitin-independent 26S proteasome pathway: antizyme-1 (OAZ1) binds and targets ODC for proteasomal destruction, while antizyme inhibitor (AZIN1) competes with ODC for OAZ1 binding as a pseudo-substrate that titrates antizyme away and stabilises ODC. OAZ1 mRNA contains a +1 programmed ribosomal frameshift stimulated by spermidine itself: rising spermidine → OAZ1 translation → ODC degradation → putrescine ↓ (negative feedback). Putrescine is converted to spermidine by spermidine synthase (SRM; EC 2.5.1.16) transferring an aminopropyl group from decarboxylated S-adenosylmethionine (dcSAM) generated by AMD1 (EC 4.1.1.50); spermine synthase (SMS; EC 2.5.1.22) adds a second aminopropyl group. AMD1 activity is allosterically enhanced by putrescine (Km ~10–30 μM).

Catabolism: SSAT, PAOX/SMOX, and the H&sub2;O&sub2;-NF-κB Loop

Catabolism proceeds via spermidine/spermine N¹-acetyltransferase (SSAT; SAT1; EC 2.3.1.57), which acetylates the N¹-position of spermidine and spermine using acetyl-CoA. Acetylated intermediates are oxidised by polyamine oxidase (PAOX; EC 1.5.3.11) regenerating shorter polyamines plus H&sub2;O&sub2; and 3-acetamidopropanal. Spermine oxidase (SMOX; EC 1.5.3.3) directly back-converts spermine to spermidine, also generating H&sub2;O&sub2; and 3-aminopropanal. The H&sub2;O&sub2; produced by PAOX/SMOX activates NF-κB (IKKβ Ser177 autophosphorylation via ROS), creating a pro-inflammatory positive feedback when catabolism is dysregulated. SSAT is transcriptionally induced by NF-κB (κB sites in the SAT1 promoter at −195 and −60 bp) and rapidly degraded by CRL1-SKP2 (K48-Ub), so NF-κB activation → SSAT ↑ → spermidine/spermine ↓ → H&sub2;O&sub2; ↑ → NF-κB re-activation (cycle relevant to IBD and cancer cachexia).

NF-κB → ODC and eIF5A Hypusination

The ODC promoter contains two functional NF-κB response elements at −330 and −2,300 bp; p65/p50 occupancy drives ODC transcription in LPS-stimulated macrophages, cancer cells, and intestinal epithelium. Elevated spermidine flux supports hypusination of eIF5A (eukaryotic initiation factor 5A) — catalysed by deoxyhypusine synthase (DHPS) using spermidine as aminobutyl donor — required for efficient translation of a subset of mRNAs including HIF-1α, MMP-9, and cyclin D1. Thus NF-κB → ODC → spermidine → DHPS → eIF5A hypusination forms a post-transcriptional amplification module for pro-inflammatory gene expression.

AMPK → Polyamine Flux Control

AMPK suppresses ODC indirectly: AMPK → Raptor Ser792 → mTORC1↓ → 4E-BP1 dephosphorylation → cap-dependent translation ↓ reduces ODC protein (ODC mRNA preferentially uses cap-dependent translation under growth conditions). AMPK→TSC2 Thr1387 reinforces mTORC1 suppression. AMPK→SIRT1 deacetylates PGC-1α and reduces mitochondrial ROS, lowering the oxidative trigger for SMOX induction. Independent of mTORC1, AMPK is proposed to stabilise OAZ1 mRNA-binding proteins that enhance OAZ1 frameshifting efficiency (mechanism documented in yeast orthologues).

Spirulina’s Mechanistic Actions

• NF-κB ↓ → ODC ↓: Phycocyanobilin (PCB) inhibits Nox2 Rac1-p47phox assembly → IKKβ Ser177 ↓ 30–50% → IKBα re-accumulation → p65/p50 nuclear ↓ → ODC promoter occupancy ↓ 20–35% → ODC mRNA ↓ 20–30%.
• AMPK → mTORC1↓ → ODC translation ↓: AMPK Thr172 ↑ → Raptor Ser792 + TSC2 Thr1387 → mTORC1 ↓ 30–50% → 4E-BP1 hypophosphorylation → ODC protein ↓ 20–35%.
• Nrf2 → GSH → SMOX H&sub2;O&sub2; ↓: Nrf2→GCLC/GSH reduces intracellular oxidative stress → SMOX activity ↓ → H&sub2;O&sub2; ↓ → NF-κB re-activation loop dampened.
• eIF5A hypusination ↓: Spermidine ↓ (via ODC↓ + AMPK flux shift) → DHPS substrate ↓ → eIF5A hypusination ↓ 10–20% → HIF-1α, MMP-9, cyclin-D1 mRNA translation selectively reduced.
• Autophagy-spermidine balance: Physiological spermidine supports autophagy (via EP300 acetyltransferase inhibition); spirulina’s AMPK activation may preserve basal spermidine-driven autophagy while suppressing excess putrescine flux in inflammatory tissue.

Clinical Correlates and Dosing

In CRC and prostate models, spirulina extract (50–200 mg/kg) reduced tumour ODC activity 30–45% and putrescine 20–35%, with spermidine relatively preserved. In LPS-challenged rodents, SAT1 mRNA ↓ 25–40% correlated with NF-κB suppression. Human data remain indirect: 4–8 g/day spirulina lowers CRP and IL-6 in controlled trials, consistent with ODC suppression in inflammatory tissue. Interactions: DFMO (α-difluoromethylornithine; irreversible ODC inhibitor used in cancer chemoprevention) — additive ODC suppression possible; monitor polyamine depletion symptoms (mucositis, thrombocytopenia) at high doses.