Bioavailability

When taken orally as a supplement or administered in vitro, a-lipoic acid is rapidly absorbed and taken up into cells where it is reduced to the more active form, dihydro-lipoic acid. The uptake of exogenously supplied a-lipoic acid has been studied in the perfused rat liver and isolated hepatocytes (4). Two different transport mechanisms were reported: carrier-mediated uptake, which is prominent below 75 /¿M, and passive diffusion, which is prominent at higher concentrations. This study also showed that the carrier-mediated uptake can be blocked by medium-chain fatty acids, suggesting that the same translocator was in use (4). In vitro studies have investigated the various cellular reduction pathways of lipoic acid (5). Two mechanisms exist that are both tissue- and stereo-specific. The natural R enantiomer is preferentially reduced in the mitochondria by lipoamide dehydrogenase, a nicotinamide adenine nucleotide (NADH) dependent enzyme. The S form is predominantly reduced in the cytosol by glutathione and/or thioredoxin reductase, which are NADPH dependent. The various contributions of each pathway are dependent upon the tissue: heart muscle almost entirely reduces the R form, while in the liver there is an equal distribution between the two pathways.

Reduction contributes one mechanism of lipoate metabolism. Other routes have been demonstrated using radiolabeled lipoic acid administered either orally (6) or via injection (7) into the rat. Several components were identified, including the short-chain homologues bisnorlipoic and tetranorlipoic acids (7). Interestingly, no evidence for the oxidation of the dithiolane ring was observed. Administration of [1,6-14C] lipoic acid has shown that lipoic acid is rapidly absorbed in the gut and passed to various tissues for catabolism (6). The localization of administered lipoate was greatest in the liver, other intestinal organs, and

36. D. E. Pszczola, "Natural Colors: Pigments of Imagination," Food Technol. 52, 70-76 (1998).

37. FDA, Title 21.73.95, April, Food and Drug Administration, Washington, D.C., 1986.

38. E. Karmas and R. S. Harris, Nutritional Evaluation of Food Processing, 3rd ed., AVI Books, Van Nostrand Reihold Co., New York, 1988.

39. P. M. Bluestein and T. P. Labuza, "Effects of Moisture Removal on Nutrients," in E. Karmas and R. S. Harris, eds., Nutritional Evaluation of Food Processing, 3rd ed., AVI Books, Van Nostrand Reihold Co., New York, 1988, pp. 393-422.

40. M. J. Minguez-Mosquera, M. Jaren-Galan, and J. Garrido-Fernandez, "Effect of Processing of Paprika on the Main Carotenes and Esterified Xanthophylls Present in the Fresh Fruit," J. Agrie. Food Chem. 41, 2120-2124 (1993).

41. W. J. Lessin, G. L. Catigani, and S. J. Schwartz, "Quantification of cis-trans Isomers of Provitamin A Carotenoids in Fresh and Processed Fruits and Vegetables," J. Agrie. Food Chem. 45, 3728-3732 (1997).

Robert Parker Cornell University Ithaca, New York

Lipoic acid was first isolated in 1951 by Reed and colleagues (2) and was tentatively described as a vitamin until it was later discovered to be synthesized in both plant and animal tissues. Recently, the levels of naturally occurring lipoic acid (lipoyllysine) in both plant and animal tissues have been determined (3). In animal tissues, the content was found to be tissue-specific, the highest levels being found in the kidney, heart, and muscular tissues (2.6, ~1, and ~1 fJg/g dry weight, respectively), while lower amounts were found in the brain and lung (0.2 fig/g dry weight). Since natural source lipoic acid is present in mitochondrial enzymes, it is highly likely that lipoic acid content will correlate with metabolic activity of the tissue. In plant tissues, the highest concentrations were found in green tissues, especially spinach (3.6 fig/g dry weight). Plants are unique in that they possess a chloroplastic form of lipoic acid; therefore, lipoic acid content, dependent on the type of tissue, will be determined by the density of mitochondria and chloroplasts. Such low levels of lipoic acid (—0.1% total weight of a mitochondrion) express the need for supplementation if the antioxidant properties of lipoic acid are to be gained. The antioxidant properties of lipoic acid can rarely be achieved through a normal diet.

See also Carotenoid pigments; Colorants:

carotenoids.

PHYTOCHEMICALS: LIPOIC ACID

a-Lipoic acid is a naturally occurring compound present as a cofactor in a number of mitochondrial enzymes that are involved in metabolism and energy production. In its free form, lipoic acid is a powerful antioxidant, functioning as a free-radical scavenger and an antioxidant "protector." Recently, attention has focused on lipoic acid as a cellular redox regulator, having the capacity to interact at various stages in signal transduction pathways. Such properties make this compound of potential therapeutic importance in conditions in which oxidative stress is involved. These properties and their implications are discussed in the article.

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