Lipoic acid

Lipoic acid is the organic compound, one enantiomer of which is an essential cofactor for many enzyme complexes. The molecule consists of a carboxylic acid and a cyclic disulfide. Only the R-enantiomer is biologically significant. It is essential for aerobic life and a common dietary supplement. Dihydrolipoic acid is the reduced form of lipoic acid although it is sometimes also called "lipoic acid." "Lipoate" is the conjugate base of lipoic acid, and this form is mainly present at physiological conditions.

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 dehydrogenase (2-OADH) and glycine cleavage complexes (GCV), respectively.^[1]

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History

Lipoate was first called pyruvate oxidation factor (POF) by Irwin C. Gunsalus, the former chair of Biochemistry at the University of Illinois at Urbana-Champaign.^[2][3] This was after the observation by many groups that POF functioned as an essential growth factor for Enterococci, which lack the ability to make lipoate.^[4] The structure was determined in a collaboration of Gunsalus with Lester Reed and Eli Lilly; the synthetic compound designated α-lipoic acid proved to be the correct molecule.^[5] The configuration found in vivo was later found to be the R-enantiomer.^[6]

The first human clinical studies using alpha-lipoic acid (ALA) in the United States were carried out by Fredrick C. Bartter, Burton M. Berkson, and associates from the National Institutes of Health in the 1970’s.^[7][8][9] They administered intravenous ALA to 79 people with acute and severe liver damage at various medical centers across the United States and 75 recovered full liver function. Dr.’s Bartter and Berkson were appointed by the FDA as principal investigators for this therapeutic agent as an investigational drug and Dr. Berkson went on to use it successfully for the treatment of chronic liver disease (viral hepatitis, autoimmune hepatitis, etc).^[10]

In addition, because of ALA’s ability to modify gene expression by stabilizing NF kappa B transcription factor, Berkson started using ALA for the treatment of various cancers for which no effective treatments exist. In a 2006 publication, he and co-authors described the long term survival of a patient with metastatic pancreatic cancer using ALA and various oral antioxidants.^[11]

Lipoic acid-dependent complexes

2-OADH transfer reactions occur by a similar mechanism in the PDH complex, 2-oxoglutarate dehydrogenase (OGDH) complex, branched chain oxoacid dehydrogenase (BCDH) complex, and acetoin dehydrogenase (ADH) complex.

The most studied of these is the PDH complex. These complexes have three central subunits: E1-3, which are the decarboxylase, lipoyl transferase, and dihydrolipoamide dehydrogenase respectively. These complexes have a central E2 core and the other subunits surround this core to form the complex. In the gap between these two subunits, the lipoyl domain ferries intermediates between the active sites.^[12][13] The geometry of the PDH E2 core is cubic in Gram-negative bacteria or dodecahedral in Eukaryotes and Gram-positive bacteria. Interestingly the 2-OGDH and BCDH geometry is always cubic.^[14] The lipoyl domain itself is attached by a flexible linker to the E2 core and the number of lipoyl domains varies from one to three for a given organism. The number of domains has been experimentally varied and seems to have little effect on growth until over nine are added, although more than three decreased activity of the complex.^[15] The lipoyl domains within a given complex are homogenous, while at least two major clusters of lipoyl domains exist in sequenced organisms.^[16]

The glycine cleavage system differs from the other complexes, and has a different nomenclature. In this complex the H protein is a free lipoyl domain with additional helices, the L protein is a dihydrolipoamide dehydrogenase, the P protein is the decarboxylase, and the T protein transfers the methylamine from lipoate to tetrahydrofolate (THF) yielding methylene-THF and ammonia. Methylene-THF is then used by serine hydroxymethyltransferase (SHMT) to synthesize serine from glycine. This system is used by many organisms and plays a crucial role in the photosynthetic carbon cycle.^[17]

Food sources

Lipoic acid is found in a variety of foods, notably kidney, heart and liver meats as well as spinach, broccoli and potatoes. ^[18][19]

Use as a dietary supplement

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. The relatively good scavenging activity of lipoic acid is due to the strained conformation of the 5-membered ring in the intramolecular disulfide.^[20] In cells, lipoic acid can be reduced to dihydrolipoic acid (ΔE= -0.288). Dihydrolipoic acid is able to regenerate (reduce) antioxidants, such as glutathione, vitamin C and vitamin E, maintaining a healthy cellular redox state.^[21][22] 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.^[23][24] 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.^[25] As a result, it may be helpful for people with Alzheimer's disease or Parkinson's disease^{[citation needed]}.

Since the early 1990s lipoic acid has been used as a dietary supplement, typically at doses in the range of 100–200 mg/day. In a chronic/carcinogenicity study in rats, it is reported that racemic lipoic acid was found to be non-carcinogenic and did not show any evidence of target organ toxicity. The NOAEL is considered to be 60 mg/kg bw/day.^[26]

On account of its two thiol groups, dihydrolipoic acid has potential for use as a chelating agent in treatment of mercury intoxication. It is particularly suited to this purpose as it can penetrate both the blood-brain barrier and the cell membrane.^{[citation needed]} However, lipoic acid is not commonly used because dimercaptosuccinic acid (DMSA) and 2,3-dimercapto-1-propanesulfonic acid (DMPS) exhibit greater clinical effectiveness. Lipoic acid has not received approval from the U.S. Food and Drug Administration as a chelating agent and questions remain about the possibility that lipoic acid may re-mobilize mercury from peripheral tissue into the central nervous system during administration.

The S-enantiomer

Normally, only the R-enantiomer of lipoic acid occurs naturally, but the S-enantiomer can assist in the reduction of the R-enantiomer when a racemic mixture is given.^[27] However, some recent studies have suggested that the S-enantiomer in fact has an inhibiting effect on the R-enantiomer, reducing its biological activity substantially and actually adding to oxidative stress rather than reducing it. Furthermore, the S-enantiomer has been found to reduce the expression of GLUT4, responsible for glucose uptake in cells, and hence to reduce insulin sensitivity.^[28]

References

Other Reviews

• Jane Higdon, "Lipoic Acid", Micronutrient Information Center, Linus Pauling Institute