Lipoic acid (LA) is a small molecule with only two kinds of functional groups. It is a naturally-occurring co-factor present in many multi-enzyme complexes regulating metabolism. It is present in human body in two forms (Fig. 1): lipoic acid, which is an oxidized form, or as the reduced form dihydrolipoic acid. Lipoic acid or more specifically α-lipoic acid (alpha-lipoic acid) is chemically 5-(1,2-dithiolan-3- yl)pentanoic acid (Fig. 1A, C8H14S2O2). The other name of LA is thioctic acid and some other name were in use, i.e. 6,8- thioctic acid, 6,8-dithioctane acid, 1,2-tithiol-3-valeric acid or 1,2-ditiol-3-pentanoic acid . The formula of its reduced form, dihydrolipoic acid or 6,8-disulfanyloctanoic acid. (Fig. 1B) is C8H16S2O2. LA exists in the form of two enantiomers, R or S. In physiological condition, LA is present in the form of lipoate with the proton of the hydroxyl functional group substituted by remains of an organic alcohol or with an inorganic ion.
LA (in the form of lipoate) acts as a cofactor in reactions of aerobic metabolism. It participates in transfers of acyl and methylamine groups. It is essential for aerobic processes of life and serves as a coenzyme in the Krebs cycle . LA was discovered as a growth factor in some microorganisms . Presence of LA was then confirmed in mammals where it participates in many reduction-oxidation reactions catalyzed by cellular dehydrogenases: pyruvate dehydrogenase complex (PDG) and α-ketoglutarate dehydrogenase complex (KGDG) . Both enzyme complexes are necessary for proper functioning of the citrate cycle. The presence of LA as a cofactor was also shown in H-protein glycine-utilizing system [5,6]. It was shown that LA helps in protecting the body against free radicals [7,8].
Physico-chemical properties and toxicity. Alpha-LA is a sulfurous fatty acid. It would be recognized as a vitamin. However, the human body is able to synthesize it. As a potent antioxidant, LA quenches free radicals, inhibits reactive oxygen- generators and regenerates other antioxidants . Elucidation of the specific cellular target(s) for LA is in need of more research for better identification. However, it is well established that on cellular level, R-LA is the most efficient oxidative-stress protector  and is stimulating increase in mitochondrial metabolism.
In the organism, LA is bonded to various proteins and, consequently, there is not possible to extract it by water or non-polar solvents. However, LA is soluble in both water and lipids . LA is highly reactive due to the tension of the S-S-C bond in the heterocyclic disulfide circle. LA is relatively stable as a solid but it polymerizes when heated above its melting point (47.5 oC) or, under the influence of a light, when it is dissolved in a neutral solution. Photolysis in an acidic environment does not result in polymerization but in an opening of the disulfide ring . To possess a freeradical scavenging activity, LA has to be a redox-active molecule. Consequently, its ΔE = -0.288 makes it possible to undergo thiol-disulfide exchange  and participate in maintaining optimal cellular redox status . As an antioxidant, LA is able of bringing other antioxidants to their active antioxidant state. These effects were reported for an interaction with ascorbic acid, vitamin E, coenzyme Q10 and glutathione [13-16].
Toxicity of LA is low but very large doses are toxic. When applied orally to rats, the LD50 value established for LA was >2000 mg/kg of body weight and LA had no acute toxicity to the experimental animals . The high doses were associated with small changes of liver enzymes activities. Additionally, some histopathological effects were observed on the liver and mammary glands. LA does not possess any mutagenic or genotoxic activities . On the other hand, LA exhibits antimutagenic and anticlastogenic activities and as such belongs to the group of natural antimutagens . The no-observed-adverse-effect level (NOAEL) dose reported is 61.9 mg/kg of body weight per day .
Biochemical bases of LA biological activities and mechanisms of action. The interaction of LA with various biological molecules and macromolecules and with various cellular targets was investigated on different levels. It demonstrated that LA or lipoate participates in many biochemical reactions affecting cellular processes.
LA as an essential cofactor of various enzymes is requiring formation of covalent bonding to the enzyme for demonstration of biological activity. The main lipoylated proteins are enzymes of central metabolism. These are the pyruvate and alpha-ketoglutarate dehydrogenase complexes. Normally, the lipoate ligase activates the LA carboxyl group using ATP and this activation is followed by attachment of LA. However, there are some exceptions to this . In Escherichia coli lipoylated proteins are formed in the absence of ATP-dependent ligase activity . The cleavage of LA from proteins and small molecules is accomplished through the action of lipoamidase . Lipoamidase belongs among amidohydrolases and it removes not only LA but also biotin from 2-oxoacid dehydrogenases .
LA’s R-isomer lowers glucose and lactase levels in diabetic subjects. This is probably due to LA ability to inhibit mammalian pyruvate dehydrogenase kinase. Out of 4 pyruvate dehydrogenase kinase (PDK and two pyruvate dehydrogenase phosphatase (PDP) isoenzymes, LA affects only PDKs but not PDPs. It was shown in purified proteins system that LA directly regulates activity of the pyruvate dehydrogenase complex (PDC) through phosphorylation/ dephosphorylation of its pyruvate dehydrogenase (E1) component. The inhibition of PDKs by LA leads to decreased phosphorylation and, consequently, toward increased activity of PDC .
LA takes part in complex enzymatic reactions/systems, i.e. the 2-oxo acid dehydrogenase multienzyme complexes. In some cases, the LA catalytic site is very similar to the site for biotin . It was shown experimentally that cellular dihydro-LA catalyzes the denitrosation of S-nitrosoglutathione, S-nitrosocaspase 3, S-nitrosoalbumin, and S-nitrosometallothionenin to their reduced state with concomitant generation of nitroxyl (HNO) . In this sense, this activity is the same as of other cellular dithiol thioredoxin. The denitrosation of S-nitrosocaspase 3 is in an agreement with findings in HT-29 human colon cancer cells that exposure to LA dose dependently increases caspase-3-like activity associated with DNA-fragmentation . These effects were accompanied by cytosolic oxygen-radical scavenging and by increase oxygen-radical generation inside mitochondria resulting in the down-regulation of the anti-apoptotic protein bcl-X(L). However, when LA was examined in non-transformed human colonocytes, it did not induce any apoptotic processes. The induction of apoptosis in colon cancer cells relates to an increased uptake of oxidizable substances/substrates into mitochondria. Additionally, when cellular antioxidant status of cultured HL-60 cell was improved through pre-incubation with LA , the protection against caspase-3 activation and apoptosis induced by an addition of 200 μM hydrogen peroxide was observed.
LA in in vitro experiments. In vitro experiments involving LA are usually reporting on the free-radical scavenging effects or on the processes that are mediated by free radicals or on the processes related to glutathione synthesis. Additionally, some authors speculate that cytotoxic effects of LA occur because of the similarity of the LA structure to the structure of fatty acids, i.e. octanoic acid . This makes it possible for LA to correct the deficient thiol status of cells, which can be used to the advantage of patients in clinical situations and therefore highlights clinical relevance of LA applications. When tested in vitro in human Jurkat T cells, human erythrocytes, C6 glial cells, NB41A3 neuroblastoma cells and peripheral blood lymphocytes , LA mediated an increase in reduced cellular glutathione. This was caused through reduction of LA to its dihydro form that, after being released into the medium, reduced cystine to cysteine. Cysteine was than transported back into cells and used in the synthesis of glutathione. LA is essential in helping cystine to overcome the problem with its low transport as xc-transport system. It is expressed weakly in some cells and it is also inhibited by glutamate. Consequently, LA makes it possible for gamma-glutamylcysteine synthetase to work as needed. By this mechanism LA normalizes cell subpopulations with compromised thiol status. It was reported that, by an unknown mechanism, combination of vitamin D3 with LA be useful in overcoming the differentiation block present in acute promyelocytic leukemia cells . Additionally, some other vitamin D-related substances, i.e. calcitrol, are capable of increasing the activity of tumor necrosis factor alpha (TNFalpha) [30,31]. However, LA was demonstrated to cause significant reduction of this enhancing effect of calcitrol on TNF-alpha-induced caspase activation [30,31].
It was shown that the human tumor cell lines FaDu and Jurkat and in the Ki-v-Ras-transformed Balb/c-3T3 murine mesenchymal cell line that LA induces hyperacetylation of histones in cultured cells. The presence of LA in medium has different effect on the growth and viability of normal and transformed cells . Non-transformed cell lines treated with LA responded to the treatment only through reversible arrest of cell cycle in G0/G1. LA induces a post-translational increase of the cyclin-dependent kinase inhibitor p27Kip1 levels. This inhibitor is necessary for the LA-mediated arrest of cell cycle. On the other hand, the pro-apoptotic effect of LA in transformed cells seems to beneficial only for the LA-use in cancer chemotherapy.
Cultivation of HT-29 cells with either form of LA (oxidized or reduced) leads to the dose dependent increase of caspase-3-like activity. The LA effects were associated with DNA fragmentation  and dihydro-LA was acting as a scavenger of cytosolic oxygen radicals. Interestingly, both forms of LA were reported to increase formation of oxygen radicals in mitochondria. However, this is preceded by an increased influx of lactate or pyruvate into mitochondria and, consequently, the anti-apoptotic protein bcl-X(L) is downregulated. Apoptosis that is induced by LA (or its dihydro form) can be prevented by the free radical scavenger benzoquinone. In the contrast, this apoptotic effect of LA was not seen in normal human colonocytes . This seems to be an additional indication of the benefits of LA inclusion in cancer therapy, especially because some other reports  indicate that LA also mediates upregulation of phase II detoxication enzymes, i.e. NAD(P)H:quinone oxidoreductase (NQO1) and glutathione-S-transferase (GST). A preferential cytotoxicity of LA toward the leukemic cell lines compared to mitogen-stimulated normal peripheral blood lymphocytes was also reported . This is in the line of the work including normal human colonocytes . LA was found to induce apoptosis in leukemia cells but not in normal cells. This was documented as substantial but opposite changes of interleukin-2 (IL-2) concentrations in normal and leukemia cells . Favorable effects of LA on some important T-cell functions (in vitro) were also reported in patients in advanced-stage of the disease . LA induced generation of reactive oxygen species (ROS) is accompanied by increase of apoptotic cells in human lung epithelial cancer H460 . On the other hand, inhibition of ROS formation or overexpression of glutathione peroxidase and superoxide dismutase that in fact functions as antioxidant enzyme inhibits this LA-induced apoptosis. LA-induced apoptosis is the result of the activation of the mitochondrial death pathways. This requires caspase-9 activation. Consequently, this apoptosis is fully inhibited by caspase inhibitors . Additionally, LA induces down-regulation of mitochondrial Bcl-2 protein while its overexpression prevents the apoptotic effect of LA . LA in experimental cancer therapy and cancer chemotherapy in human. In experimental cancer therapy, LA was tested in combination with doxorubicin for its effect on experimental murine leukemia L1210. These experiments tested the combination of the drug (doxorubicin) that is known for high rate of free-radical formation and free-radicals scavenging antioxidant LA . The experiments were performed with the idea that LA would decrease the toxicities of doxorubicin. However, the selection of an appropriate dose of the antioxidant was shown to be crucial in designing therapeutic protocols. At low concentration of 1 μmol/l, LA acted as a growth factor while it functioned as an antiproliferation agent at concentration 100 μmol/l . It was observed (in vitro) at the majority of doxorubicin and LA combination that the effect of LA on doxorubicin was antagonistic. Synergistic effect of doxorubicin and LA was observed only at the relatively high concentrations of both drugs. However, the use of doxorubicin and LA in vivo produced an increase in survival of experimental mice. The combination of a single dose of 5mg/ kg of doxorubicin and 16 mg/kg of LA resulted in super-additive effect on survival of leukemia-bearing mice thus confirming that the proper scheduling of anticancer drugs in therapy is highly important for achieving desired therapeutic outcome. On the other hand, it indicates that insufficient dosage may result in adverse effects in patients.
There is only one report on the use of LA in human patients. A rare but interesting case of a patient with pancreatic cancer without any LA toxic or other adverse effect during a long-time survival was reported . The patient was on the intravenous LA and low-dose naltrexone (LA-N) protocol. This together with a healthy lifestyle possibly extended the life of the otherwise terminal patient for several years.and they were reported to benefit from this treatment . LA prevention of cancer-chemotherapy toxicity. LA’s ability to prevent toxicities related to cancer therapy is arising from its ability to be a scavenger of free-radicals. Consequently, any toxicity caused by a formation of free radical during cancer therapy can be potentially alleviated by an administration of LA as already shown in several scientific publications. LA in the form of a racemate demonstrated protective effect on cyclophosphamide induced hyperlipidemic cardiomyopathy . It was shown that 200 mg cyclophosphamide per kg of body weight induces abnormal elevation of serum lipids, levels of free and esterified cholesterol and triglycerides in cardiac muscle and in serum. This mirrored the abnormal distortion in the activity of lipid-metabolizing enzymes that followed cyclophosphamide administration. The addition of racemic lipoic acid in the dose of 25 mg/kg of body weight for 10 days resulted in normalization of the lipid levels and activity of the lipid-metabolizing enzymes. LA was also shown to induce lysosomal changes in cases oxidative cardiotoxicity . As previously reported , LA is also capable of influencing the nephrotoxicity potential of adriamycin. This study was performed with Wistar strain adult male albino rats receiving intravenous adriamycin (1 mg/kg of body weight. once a week. for the period of 12 weeks) without or with LA (35 mg/kg of body weight per day for the period of 12 weeks, once a week, 24 h prior to the administration of adriamycin). An administration of adriamycin led to the decrease in activities of the glycolytic enzymes in the rat renal tissue. The gluconeogenic enzymes, glucose-6-phosphatase and fructose-1,6-diphosphatase, also showed a decline in their activities on adriamycin administration. Decreased activities of the TCA cycle enzymes isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, suggested a loss in mitochondrial function and integrity. Nephrotoxicity was evident from the increased excretions of Nacetyl- beta-D-glucosaminidase and gamma-glutamyl transferase in the urine of adriamycin treated rats. These biochemical disturbances were effectively counteracted by a pre-treatment with lipoic acid that resulted in an increase in activities of glycolytic enzymes, ATPases and the TCA cycle enzymes. Other reports from the same research team deal with the influence of LA on adriamycin-induced hyperlipidemic nephrotoxicity  and adriamycin-induced lipid peroxidation [43,44] in rat kidney, and also with protective role of racemic LA against adriamycin-induced cardiac lipid peroxidation . All these studies report on the beneficial effects of LA when applied in situations when free-radical related toxicity appears. Consequently, LA administration leads to an improvement in parameters reflecting oxidative stress induced by an anticancer agent. Similar findings were reported when LA was used to prevent cardiotoxicity induced by doxorubicin (15 mg/kg, i.p.) . Doxorubicin cardiotoxicity was reflected by a significant elevation of serum creatine phosphokinase and lactate dehydrogenase and by the significant increase in lipid peroxides 48 hours after doxorubicin administration. Protein thiols in heart muscle were decreased. Orally administered LA (100 mg/kg, 5 days prior doxorubicin and 2 days after doxorubicin administration) resulted in a significant protection against cardiotoxicity mirrored by an improvement of the biochemical parameters.
The results suggest that LA may be useful in increasing therapeutic index of doxorubicin. The same therapeutic- index improvement would probably be observed when LA is included in therapeutic regimes of any free radicalproducing anticancer agent, for which the dose administration is limited by toxicity based on the oxidative stress in human tissue. Clinical experience shows that the LA can be used to treat effectively oxaliplatin-induced cumulative polyneuropathy  that is, very probably, also caused by excessive production of free radicals. The free-radical scavenging is not the only way, by which LA decreases toxicity. It was shown that LA can be used to prevent glutamate cytotoxicity . This kind of toxicity occurs through an inhibition of cystine transport as glutamate and cystine share the same transporter. Consequently, elevated extracellular glutamate competitively inhibits cystine transport and, therefore causes depletion of intracellular glutathione. An impairment of cellular antioxidant defenses and oxidative stress occur. The addition of LA increases supply of cysteine to cells from their surrounding environment. As cysteine is the reduced form of cystine that is transported into the cell by a glutamate-insensitive transport mechanism, the glutamate cytotoxicity can be prevented. The LA protection corresponds with the intensity of glutathione protection. However, it was shown experimentally  that doses of LA smaller than 100 μM do not protect cells against glutamate-induced cytotoxicity and that protection against glutamate cytotoxicity even in glutathione synthesis-arrested cells occurs only at concentrations of LA above 500 μM. This indicates that the primary mechanism of LA protection at low concentrations is mediated by a pro-glutathione quality rather than direct scavenging of reactive oxygen. A direct antioxidant effect of LA takes place rather at high concentrations. An interesting open, non-randomized phase II study was performed  that included patients with advanced solid tumors. The patients received a maintenance treatment that included recombinant interleukin-2 (rIL-2; 1.8 MIU; 3 times/ week subcutaneously on alternate days for the first two weeks of every month), medroxyprogesterone acetate (MPA; 500 mg/day at alternate days without interruption; orally) and two antioxidants, LA (300 mg/day orally; continuously) and Nacetyl cysteine (NAC; 1800 mg/day orally; continuously). The median duration of this maintenance treatment in 28 patients was 10 months (6-30+). Eleven patients achieved complete remission. No significant toxicity was reported. The conclusion of this study is that this protocol has “a very low toxicity and results in the improvement of biological markers which are predictive for patient outcome .
LA is a very interesting substance that is involved in many important biological and biochemical cellular processes. It has the ability to influence activities of some specific enzymes with acting role in those processes. LA with its free-radicalscavenging capacity has the potential to become a very useful substance for interfering with processes within malignant cells. However, as many clinically-used anticancer drugs act through generating various radical species, it is quite possible that some of them (i.e. alkylating agents) may demonstrate decreased therapeutic effect as their active radicals may be eliminated by LA. On the other hand, it was already demonstrated that toxicity of some anticancer drugs is related to the formation of free radicals and may be decreased by LA. Because of this, inclusion of LA in therapeutic protocols or its use in chemoprevention of cancer may be beneficial. Furthermore, the research findings published so far warranty future investigations on this very interesting compound
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