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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:



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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