Alpha Lipoic Acid is found in almost all foods, but slightly more so in kidney, heart, liver, spinach, broccoli, and yeast extract.
Lipoic acid is an organosulfur compound, one enantiomer of which is an essential cofactor for many enzyme complexes. This yellow solid is a carboxylic acid and features a cyclic disulfide, or ditholane ring, functional group. The R-enantiomer is biosynthesized and used as a cofactor. It is essential for aerobic life and is a common dietary supplement.
Dihydrolipoic acid is the reduced form which is mostly how the sulfurs exist intracellularly. "Lipoate" is the conjugate base of lipoic acid, and this is the form carboxylic acids take at physiological conditions. So free lipoic acid inside the cell could correctly be called dihydrolipoate. Most intracellular lipoic acid is not free, because it is made and attached to the enzyme complexes that use it. As a cofactor it is covalently bound via and amide bond to a specific lysine residue of lipoyl domains.
One of the most visible roles of lipoic acid is as a cofactor in aerobic metabolism, specifically the pyruvate dehydrogenase complex. Lipoate participates in transfer of acyl or methylamine groups in 2-oxoacid dehydrogenases (2-OADH) and glycine cleavage complexes (GCV), respectively.
Use as a dietary supplement:
Since the early 1990s lipoic acid has been consumed as a dietary supplement, typical doses are 100–200 mg/day.
A chronic/carcinogenicity study in rats reported that racemic lipoic acid was found to be non-carcinogenic and did not show any evidence of target organ toxicity.
Lipoic acid was first postulated to be an effective antioxidant when it was found it prevented the symptoms of vitamin C and vitamin E deficiency. It is able to scavenge reactive species in vitro, though there is little or no evidence that this actually occurs in vivo. The relatively good scavenging activity of lipoic acid is due to the strained conformation of the 5-membered ring in the intramolecular disulfide. In cells, lipoic acid can theoretically be reduced to dihydrolipoic acid (ΔE= -0.288), though significant quantities of dihydrolipoic acid derived from orally-ingested lipoic acid have never been demonstrated. Dihydrolipoic acid is able to regenerate (reduce) antioxidants, such as glutathione, vitamin C and vitamin E. Recent findings suggest that lipoic acid curative effects is due to modulation of regulation in eukaryotes. This occurs due to lipoic acid acting as an oxidant, not a reductant.
Lipoic acid has been shown in cell culture experiments to increase cellular uptake of glucose by recruiting the glucose transporter GLUT4 to the cell membrane, suggesting its use in diabetes. Studies of rat aging have suggested that the use of L-carnitine and lipoic acid results in improved memory performance and delayed structural mitochondrial decay. As a result, it may be helpful for people with Alzheimer's disease or Parkinson's disease.