α-Lipoic acid (ALA)

α-硫辛酸 ALA

Place · Level 3 · Supplement

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Connected to · 4

cofactor · 4

Niacin
On PDH/KGDH the lipoyl arm passes reducing equivalents through E3-FAD to NAD⁺ — NAD⁺ is the relay's terminal acceptor.
Thiamin
PDH/KGDH/BCKD all carry TPP (B1) and lipoyl (ALA): B1 cleaves the C-C bond, lipoyl carries reducing equivalents.
Riboflavin
E3 (dihydrolipoyl dehydrogenase) hands lipoyl's reducing equivalents to FAD then NAD⁺ — B2 is the downstream relay.
Pantothenic Acid
PDH carries both lipoyl and CoA: pyruvate's acetyl is handed by the lipoyl arm to CoA-SH forming acetyl-CoA. Five cofactors, no shortcuts.

Dual-soluble

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身份 · 水脂双溶罕见特性

α-lipoic acid (ALA / thioctic acid) is an 8-carbon fatty acid containing a disulfide (-S-S-) ring, first isolated in 1951 by Reed's lab.

What's special about the chemical structure:

8-carbon fatty acid backbone → lipid-solubleDisulfide bond + carboxyl group → water-solubleDual water/lipid solubility, quite rare among vitamins + antioxidantsVit C is water-only, vit E / A lipid-only, GSH water-only — most antioxidants work in one phase onlyALA can enter cytosol, membranes, mitochondria, and nucleus simultaneously
Two stereoisomers (chirality):

R-ALA (dextrorotatory): the naturally occurring form, also what the body synthesizes and usesS-ALA (levorotatory): a synthetic byproduct, weak biological activityCommercial ALA is usually racemic (50% R + 50% S) — low industrial-synthesis costR-ALA-only products advertise 'natural form', but oral R-ALA has poor stability (polymerizes); only salt forms like R-ALA-Na are actually stable
Endogenous synthesis:

ALA is synthesized inside mitochondria from octanoic acid + sulfur supplyNot an essential nutrient (unlike vitamins, which must come from food)But synthesis may decline under aging / pathology, so supplementation has some theoretical rationale
Food content:

Red meat (organ): 5–30 µg/100 gSpinach / broccoli: ~5 µg/100 gYeast / tomato: traceFood content is extremely low — reaching supplemental doses (300–600 mg) from food is impossible
Key fact: 'ALA is an essential nutrient' is supplement-marketing misdirection. It's not a vitamin and not a 'deficiency-causes-disease' molecule — it's an endogenous metabolic cofactor + an optional therapeutic drug (IV in diabetic neuropathy).

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Educational content, not medical advice — consult a clinician.

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