Spirulina and Selenium: Selenoprotein Synthesis and Antioxidant Enzymes

Selenoproteins: A Distinct Antioxidant Family

The human genome encodes 25 selenoproteins, each containing at least one selenocysteine (Sec, the 21st amino acid) co-translationally inserted at UGA codons via the SECIS element–SECISBP2–eEFSec machinery. Selenium status is the primary determinant of selenoprotein hierarchy: when selenium is limiting, GPx1 and GPx3 are sacrificed first, while GPx4, thioredoxin reductase 1 (TrxR1/TXNRD1), and selenoprotein P (SelP/SELENOP) are preferentially maintained—a phenomenon called the "selenoprotein hierarchy."

GPx4: Gatekeeper Against Ferroptosis

Glutathione peroxidase 4 (GPx4) is the only enzyme capable of reducing phospholipid hydroperoxides (PLOOH) in situ within membranes. When GPx4 activity falls, PLOOH accumulate and initiate iron-dependent lipid peroxidation chains—ferroptosis. GPx4 uses GSH as reductant; its active-site Sec147 reacts with PLOOH forming selenic acid, reduced back to selenol by GSH. Spirulina contributes to this axis through multiple nodes:

• GSH substrate: phycocyanobilin (PCB) activates Nrf2→GCLC/GCLM, elevating γ-glutamylcysteine synthetase and GSH synthesis.
• Iron chelation: spirulan polysaccharides sequester labile iron, reducing Fenton-driven •OH that initiates PLOOH chains.
• FSP1/CoQ axis: spirulina's naphthoquinone pigments may support the FSP1 (AIFM2)-CoQ10 parallel ferroptosis defence pathway.

TrxR1 and the Thioredoxin System

Thioredoxin reductase 1 (TrxR1) is a selenoflavoenzyme that reduces oxidised thioredoxin 1 (Trx1) using NADPH. Reduced Trx1 then supplies electrons to peroxiredoxins (Prdx1/2), ribonucleotide reductase, and apoptosis signal-regulating kinase 1 (ASK1)—keeping ASK1 in its reduced, inactive state. Oxidative conditions (H₂O₂) oxidise Trx1 Cys32–Cys35, releasing ASK1 to activate the JNK/p38 stress cascade.

PCB-driven Nrf2 activation induces TrxR1 via the ARE in its promoter (consensus TGAC/GTCA). This creates a feed-forward loop: more TrxR1 → more reduced Trx1 → less ASK1-JNK → less Nrf2 ubiquitination (JNK phosphorylates Nrf2 competitors) → sustained ARE activity.

Selenoprotein P: Selenium Delivery Protein

SelP (SELENOP) contains 10 Sec residues, serves as the primary plasma selenium carrier, and is taken up by ApoER2 (APOER2) in brain neurons and by megalin (LRP2) in kidneys. Hepatic SelP synthesis is transcriptionally co-regulated by Nrf2 and FoxO3a. Interestingly, metformin and AMPK activation suppress SelP in liver (coordinating with glucose output), while peripheral tissues rely on circulating SelP for GPx4/TrxR1 maintenance. Spirulina's AMPK activation (via EGFR kinase inhibition and adenylate changes) may transiently affect hepatic SelP release, a nuance warranting study.

Sec Biosynthesis: The SPS2-SEPSECS Pathway

Selenocysteine is synthesised on its tRNA[Ser]Sec: serine is first loaded by SerRS, then phosphorylated by PSTK, and finally converted to Sec by SepSecS (O-phosphoseryl-tRNA: selenocysteinyl-tRNA synthase) using selenophosphate generated by SPS2 (itself a selenoprotein). Adequate selenium as selenite or selenate enters cells via aquaporins and anion transporters, reduced to H₂Se by TrxR1-dependent or non-enzymatic mechanisms. Organic selenium (selenomethionine in Spirulina platensis biomass, ~0.1–0.5 μg/g DW depending on cultivation medium) is incorporated non-specifically into proteins as Met replacement, providing a slow-release selenium reservoir.

Nrf2 as Master Regulator of Both Selenium and Glutathione

Nrf2 directly transactivates: TXNRD1 (TrxR1), GCLC/GCLM (GSH synthesis), GSR (glutathione reductase), GPx2, SRXN1(sulfiredoxin, reactivates Prdx1 sulfinylation), and SELENOP in hepatocytes. PCB as a Michael acceptor covalently modifies Keap1 Cys151/Cys273/Cys288, causing Nrf2–Keap1 dissociation and nuclear translocation. The result is coordinated upregulation of both selenoprotein-dependent (TrxR1/GPx2) and selenium-independent (GSH, Prdx1) antioxidant defences.

MsrA/B: Methionine Sulfoxide Reductases

Protein oxidation by reactive oxygen/nitrogen species converts Met residues to methionine sulfoxide (MetSO). Methionine sulfoxide reductase A (MsrA) and B (MsrB1, a selenoprotein) regenerate Met from MetSO using Trx as reductant. Loss of MsrB1 leads to accumulation of oxidised proteins associated with neurodegeneration. Spirulina's dual Trx-support (via TrxR1 induction) and iron chelation (reducing •OH-mediated Met oxidation) thus protect the Met-MetSO-MsrB1 repair cycle.