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===Biological sources and degradation=== Lipoic acid is present in many foods in which it is bound to lysine in proteins,<ref name=lpi/> but slightly more so in kidney, heart, liver, spinach, broccoli, and yeast extract.<ref>{{cite journal |last1= Durrani |first1= AI |last2= Schwartz |first2= H |last3= Nagl |first3= M |last4= Sontag |first4= G |title= Determination of free [alpha]-lipoic acid in foodstuffs by HPLC coupled with CEAD and ESI-MS |journal= [[Food Chemistry (journal)|Food Chemistry]] |date= October 2010 |pages= 38329β36 |volume= 120 |issue= 4 |doi= 10.1016/j.foodchem.2009.11.045}}</ref> Naturally occurring lipoic acid is always covalently bound and not readily available from dietary sources.<ref name=lpi/> In addition, the amount of lipoic acid present in dietary sources is low. For instance, the purification of lipoic acid to determine its structure used an estimated 10 tons of liver residue, which yielded 30 mg of lipoic acid.<ref>{{cite journal |last= Reed |first= LJ |title= A trail of research from lipoic acid to alpha-keto acid dehydrogenase complexes |journal= [[Journal of Biological Chemistry]] |date= October 2001 |pages= 38329β36 |volume= 276 |issue= 42 |pmid= 11477096 |doi= 10.1074/jbc.R100026200 |doi-access= free }}</ref> As a result, all lipoic acid available as a supplement is chemically synthesized.{{cn|date=November 2024}} Baseline levels (prior to supplementation) of RLA and R-DHLA have not been detected in human plasma.<ref>{{cite journal | doi = 10.1016/0928-0987(95)00045-3 |last1= Hermann |first1= R |year= 1996 |title= Enantioselective pharmacokinetics and bioavailability of different racemic formulations in healthy volunteers |journal= [[European Journal of Pharmaceutical Sciences]] |volume= 4 |issue= 3 |pages= 167β74 |last2= Niebch |first2= G |last3= Borbe |first3= HO |last4= Fieger |first4= H |last5= Ruus |first5= P |last6= Nowak |first6= H |last7= Riethmuller-Winzen |first7= H |last8= Peukert |first8= M |last9= Blume |first9= H |display-authors= 4}}</ref> RLA has been detected at 12.3β43.1 ng/mL following acid hydrolysis, which releases protein-bound lipoic acid. Enzymatic hydrolysis of protein bound lipoic acid released 1.4β11.6 ng/mL and <1-38.2 ng/mL using [[subtilisin]] and [[alcalase]], respectively.<ref>{{cite book |doi= 10.1016/S0076-6879(97)79019-0 |pmid= 9211267 |last1= Teichert |first1= J |last2= Preiss |first2= R |chapter= High-performance liquid chromatography methods for determination of lipoic and dihydrolipoic acid in human plasma |title= Vitamins and Coenzymes Part I |volume= 279 |year= 1997 |pages= 159β66 |series= [[Methods in Enzymology]] |isbn= 9780121821807}}</ref><ref>{{cite journal |doi= 10.1016/0378-4347(95)00225-8 |pmid= 8581134 |last1= Teichert |first1= J |last2= Preiss |first2= R |title= Determination of lipoic acid in human plasma by high-performance liquid chromatography with electrochemical detection |journal= [[Journal of Chromatography B]] |volume= 672 |issue= 2 |date= October 1995 |pages=277β81}}</ref><ref>{{cite journal |pmid= 1490813 |last1= Teichert |first1= J |last2= Preiss |first2= R |title= HPLC-methods for determination of lipoic acid and its reduced form in human plasma |journal= International Journal of Clinical Pharmacology, Therapy, and Toxicology |volume= 30 |issue= 11 |date= November 1992 |pages= 511β2}}</ref> Digestive proteolytic enzymes cleave the R-lipoyllysine residue from the mitochondrial enzyme complexes derived from food but are unable to cleave the lipoic acid-<small>L</small>-[[lysine]] amide bond.<ref>{{cite journal |pmid= 9378235 |last1= Biewenga |first1= GP |last2= Haenen |first2= GR |last3= Bast |first3= A |title= The pharmacology of the antioxidant lipoic acid |journal= General Pharmacology |volume= 29 |issue= 3 |date= September 1997 |pages=315β31 |doi= 10.1016/S0306-3623(96)00474-0}}</ref> Both synthetic lipoamide and (''R'')-lipoyl-<small>L</small>-lysine are rapidly cleaved by serum lipoamidases, which release free (''R'')-lipoic acid and either <small>L</small>-lysine or ammonia.<ref name=lpi/> Little is known about the degradation and utilization of aliphatic sulfides such as lipoic acid, except for [[cysteine]].<ref name=lpi/> Lipoic acid is metabolized in a variety of ways when given as a dietary supplement in mammals.<ref name=lpi/><ref name="ReferenceA">{{Cite journal |last1= Schupke |first1= H |last2= Hempel |first2= R |last3= Peter |first3= G |last4= Hermann |first4= R |last5= Wessel |first5= K |last6= Engel |first6= J |last7= Kronbach |first7= T |display-authors= 4 |title= New metabolic pathways of alpha-lipoic acid |journal= [[Drug Metabolism and Disposition]] |volume= 29 |issue= 6 |pages= 855β62 |date= June 2001 |pmid= 11353754}}</ref> Degradation to tetranorlipoic acid, oxidation of one or both of the sulfur atoms to the sulfoxide, and S-methylation of the sulfide were observed. Conjugation of unmodified lipoic acid to glycine was detected especially in mice.<ref name="ReferenceA"/> Degradation of lipoic acid is similar in humans, although it is not clear if the sulfur atoms become significantly oxidized.<ref name=lpi/><ref>{{Cite journal |last1= Teichert |first1= J |last2= Hermann |first2= R |last3= Ruus |first3= P |last4= Preiss |first4= R |title= Plasma kinetics, metabolism, and urinary excretion of alpha-lipoic acid following oral administration in healthy volunteers |journal= [[The Journal of Clinical Pharmacology|Journal of Clinical Pharmacology]] |volume= 43 |issue= 11 |pages= 1257β67 |date= November 2003 |doi= 10.1177/0091270003258654 |pmid= 14551180|s2cid= 30589232 }}</ref> Apparently mammals are not capable of utilizing lipoic acid as a sulfur source.
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