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 skeletal muscle (6). The presence of both oxidized and reduced forms has also been demonstrated in various tissues of vitamin-E-deficient mice after lipoic acid supplementation (8).

Vegetarian Food and Cooking

Vegetarian Food and Cooking

Switch To A Vegetarian Diet And Live To Be A 100. Are You Suffering From Weak Bones And Digestive Disorders? Did You Ever Wonder About Why You Feel Restless, Listless Or Anxious Without Any Plausible Reason? The Answer Probably Lies In Your Dietary Habits.

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