TET Enzymes — Active DNA Demethylation

The only pathway for actively removing aberrant DNA methylation marks. Requires four simultaneous inputs — the absence of any one prevents the reaction.

What TET Enzymes Do

TET1, TET2, TET3 are dioxygenases that oxidize 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC):

5mC + O₂ + α-ketoglutarate → 5hmC + CO₂ + succinate

5hmC is then further oxidized to 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC), which are recognized by TDG (thymine-DNA glycosylase) and replaced with unmodified cytosine through base excision repair. This is true active demethylation — removal of the methyl group without DNA replication.

Why this matters: DNMT-mediated methylation can be passively diluted by cell division (if maintenance methylation is impaired), but active removal requires TET enzyme function. Aberrant methylation at anti-inflammatory gene promoters written by 4-HNE/MDA persists until TET enzymes remove it.

The Four Requirements

1. Alpha-Ketoglutarate (α-KG) — Primary Cosubstrate

Consumed in the reaction (converted to succinate). Must be continuously supplied from the TCA cycle.

TCA cycle dependence: α-KG is produced from isocitrate by isocitrate dehydrogenase. This reaction requires functional mitochondria running the TCA cycle. 4-HNE adducts on isocitrate dehydrogenase impair this step. Mitochondrial restoration is a prerequisite for TET enzyme function — another reason Phase 0 must come first.

Direct supplementation: Calcium alpha-ketoglutarate can supply TET cosubstrate without requiring complete mitochondrial restoration first — a practical bridge.

2. Ferrous Iron (Fe²⁺) — Metal Cofactor

Iron must be in the ferrous (Fe²⁺) state in the enzyme's active site. During the catalytic cycle, Fe²⁺ is oxidized to Fe³⁺ and must be reduced back to Fe²⁺ before the next reaction can occur. Without this reduction, TET becomes permanently oxidized and inactive.

3. Ascorbate (Vitamin C) — The Iron Reducer

Ascorbate specifically reduces Fe³⁺ back to Fe²⁺ in TET's active site, completing the catalytic cycle. Without ascorbate:

TET runs 1–2 catalytic cycles
Iron oxidizes to Fe³⁺
TET becomes inactive (uncompetitively inhibited)
Active demethylation stops entirely

The functional deficiency problem: Under chronic oxidative stress, ascorbate is consumed as an antioxidant faster than diet supplies it. This produces functional vitamin C deficiency for TET enzyme purposes even with normal dietary intake. Liposomal vitamin C specifically addresses this by achieving higher plasma concentrations than gut-limited absorption allows.

4. Molecular Oxygen

Required as the oxygen source in the dioxygenase reaction. Not a practical limiting factor in normal physiology but relevant under severe tissue hypoxia (relevant for understanding adipose remodeling where hypoxic adipose has impaired TET function).

The 4-HNE Deadlock

The same 4-HNE that writes aberrant methylation also disables the repair machinery:

4-HNE writes inappropriate methylation at anti-inflammatory gene promoters (histone adducts altering chromatin accessibility)
4-HNE impairs Complex I/II → mitochondrial dysfunction → reduced TCA cycle flux → reduced α-KG
4-HNE impairs isocitrate dehydrogenase directly (documented adduct target)
Oxidative stress from mitochondrial dysfunction consumes ascorbate (reducing TET Fe²⁺ maintenance capacity)
Net result: TET enzymes have no cosubstrate (α-KG) and no iron reducer (ascorbate) → demethylation stalls

Breaking the deadlock: Must address both α-KG supply (calcium α-KG supplement + mitochondrial restoration) AND ascorbate supply (liposomal vitamin C) simultaneously. Either alone is insufficient.

Connections

[[One Carbon Metabolism]] — the methylation being reversed
[[Phase 0 - Source Control and Redox]] — ascorbate and mitochondrial requirements
[[Phase 3 - Epigenetic Reprogramming]] — the phase using TET
[[Epigenetic Transmission]] — what TET is cleaning up
[[NAD Pool]] — parallel epigenetic system (sirtuins handle histone acetylation; TET handles DNA methylation)