(LA), also known as α-lipoic acid and alpha lipoic acid (ALA) is an organosulfur compound derived from octanoic acid. LA contains two sulfur atoms (at C6 and C8) connected by a disulfide bond and is thus considered to be oxidized although either sulfur atom can exist in higher oxidation states. The carbon atom at C6 is chiral and the molecule exists as two enantiomers ®-(+)-lipoic acid (RLA) and (S)-(-)-lipoic acid (SLA) and as a racemic mixture (R/S)-lipoic acid (R/S-LA). Only the ®-(+)-enantiomer exists in nature and is an essential cofactor of four mitochondrial enzyme complexes. Endogenously synthesized RLA is essential for aerobic metabolism. Though de novo synthesis supplies all LA needed for its function in intermediary metabolism, it can also be absorbed from foods (leafy green vegetables and meats) and dietary supplements
Oxidative damage has been associated with various neurodegenerative diseases including Parkinson's disease, amyotrophic lateral sclerosis (ALS), and Alzheimer's disease, as well as non-neurodegenerative conditions such as cancer and heart disease
The lipoic acid, also thanks to its ability to overcome the blood-brain barrier, can protect neurons from oxidative damage through the Keap1 - Nrf2 system and, consequently, through the synthesis of glutathione(GSH).
The induction of many cytoprotective enzymes in response to reactive chemical stress is regulated primarily at the transcriptional level. This transcriptional response is mediated by a cis-acting element termed ARE (antioxidant response elements), initially found in the promoters of genes encoding the one major detoxication enzymes, GSTA2 (glutathione S-transferase A2). The ARE possesses structural and biological features that characterize its unique responsiveness to oxidative stress. It is activated not only in response to H2O2 but specifically by chemical compounds with the capacity to either undergo redox cycling or be metabolically transformed to a reactive or electrophilic intermediate.
Activation of gene transcription through the ARE is mediated primarily by Nrf2 .
Nrf2 is a key factor capable of binding to the sequence ARE and thus mediate the expression of antioxidant and detoxifying genes including GSH.
The Nrf2 transcription function and its degradation by the proteasomal pathway (Keap1-Nrf2-Cul3-Roc1 ) are regulated by the cytoplasmic repressor protein, Keap1 which possesses BTB, BACK (IVR region) and Kelch domains. The BTB-BACK domains are important for Keap1 homo-dimerization as well as to interact with Cul3 for Nrf2 degradation. The Keap1 - Nrf2 system is one of the most important cytoprotective system which has been developed over the course of evolution. Nrf2 (nuclear factor (erythroid-derived 2)-like 2) is a basic region-leucine zipper (bZIP) transcription factor that plays essential role to express many cytoprotective genes in response to oxidative and electrophilic stresses.
, the cellular concentration of Nrf2 remains low. Cul3 based E3 ubiquitin ligase brings about ubiquitination of the substrate molecule Nrf2 via the adaptor protein, Keap1. Keap1 dimerization requires two β- propeller domains to interact with two distinct epitopes in Nrf2 simultaneously. The Cul3 binds to the BTB-BACK domains of Keap1, to form a ternary complex of a core E3 ubiquitin ligase complex, which helps Nrf2 to undergo proteasomal (26S) degradation.
, such as exposure to electrophiles or ROS , Keap1 loses repression activity and hence Nrf2 dissociates from Keap1 and translocate into the nucleus, and subsequently coordinately activates cytoprotective genes and exerts a protective function against xenobiotic and oxidative stress.
LA seems to modulate the redox-sensitive cysteine groups on Keap1 preventing the degradation of Nrf2 and consequently, increasing the transcription of ARE- containing genes. It has also been shown that LA treatment increases hepatic nuclear Nrf2 -mediated gene transcription.
LA ultimately increased intracellular GSH by inducing the transcription of both the catalytic and regulatory subunits of c-glutamylcysteine ligase, which is the rate-controlling enzyme for GSH synthesis. In addition to elevating the expression of enzymes involved in GSH synthesis.
MECHANISM OF ACTION OF GLUTATHIONE.
Glutathione exists in both reduced (GSH) and oxidized (GSSG) states. In the reduced state, the thiol group of cysteine is able to donate a reducing equivalent (H++ e−) to other unstable molecules, such as reactive oxygen species. In donating an electron, glutathione itself becomes reactive, but readily reacts with another reactive glutathione to form glutathione disulfide (GSSG). Such a reaction is probable due to the relatively high concentration of glutathione in cells (up to 5 mM in the liver).
GSH can be regenerated from GSSG by the enzyme glutathione reductase (GSR): NADPH reduces FAD present in GSR to produce a transient FADH-anion. This anion then quickly breaks a disulfide bond (Cys58 - Cys63) and leads to Cys63's nucleophilically attacking the nearest sulfide unit in the GSSG molecule (promoted by His467), which creates a mixed disulfide bond (GS-Cys58) and a GS-anion. His467 of GSR then protonates the GS-anion to form the first GSH. Next, Cys63 nucleophilically attacks the sulfide of Cys58, releasing a GS-anion, which, in turn, picks up a solvent proton and is released from the enzyme, thereby creating the second GSH. So, for every GSSG and NADPH, two reduced GSH molecules are gained, which can again act as antioxidants scavenging reactive oxygen species in the cell.
When oxidative stress in the body produces peroxides is determined by the reaction:
2 GSH + ROOH ----> GSSG + ROH + H2O (ROOH :peroxide).
From the reaction you get water, alcohol (ROH) and a molecule of glutathione disulfide (GSSG).
If the peroxide is hydrogen peroxide, the reaction will produce:
2 GSH + H2O2 ----> GSSG + 2 H2O
In healthy cells and tissue, more than 90% of the total glutathione pool is in the reduced form (GSH) and less than 10% exists in the disulfide form (GSSG). An increased GSSG-to-GSH ratio is considered indicative of oxidative stress.
In many neuro-degenerative diseases and certain diseases mental abnormalities were observed in the concentration of glutathione. In subjects suffering from Parkinson for example there is a low level of glutathione conjugated to a decrease in the level and to a greater oxidation of dopamine produced by neurons in the substantia nigra.
.Tissues of the central nervous system may be especially vulnerable to oxidative stress because of their constant high rate of oxygen consumption and high mitochondrial density. Mitochondria inevitably produce free radicals as “byproducts” of normal oxidative metabolism, and these free radicals damage the mitochondrial DNA. The defective proteins coded for by the damaged DNA can lead to synthesis of mitochondria in which components of the electron transport chain preceding the damaged protein become reduced, leading to greater free radical production and more mitochondrial damage, in a vicious cycle. Such a vicious cycle may be responsible, in part for neurodegenerative diseases. Support for this view comes from the observation of high degrees of oxidative damage as well as damaged mitochondria in tissues form patients with neurodegenerative disease. Logical therapy or prevention would therefore involve antioxidant treatment.
α- Lipoic acid is a good candidate as an antioxidant agent in neurodegenerative diseases. It can interrupt the chain at several points: by competing for free transition metals as a chelator, by scavenging hydroxyl or superoxide radicals, and by scavenging peroxyl radicals..
Studies are underway in animals. For example, a study examined the effect of α-lipoic acid on memory loss in aging mice. Many species, including man, monkeys, rats, and mice, exhibit aging – related cognitive deficits that may be caused, at least in part, by oxidative stress. In mice, α-lipoic acid ( 100 mg/Kg body weight for 15 days) improved performance in an open-field memory test --- in fact, the α-lipoic acid –treated animals performed better than young animals 24 h after the first test (though the difference was not significant). Treatment with α-lipoic acid did not improve memory in young animals exhibited decreased age-related N-methyl-D-aspartate (NMDA) receptor deficits compared to controls, but showed no improvement in muscarinic, benzodiazepine, or a2-adrenergic receptor deficits. The authors concluded that α-lipoic acid’s free radical scavenging ability may improve NMDA receptor density, leading to improved memory. These intriguing results suggest further tests in other species, including humans.
Greenamyre et al. observed that the intraperitoneal administration of α-lipoic acid or DHLA reduced the rat striatum lesion induced by excitotoxins which affect NMDA receptors, which may lead to calcium influx and generation of nitric oxide and other free radicals. Both α-lipoic acid and DHLA treatment decreased the area of lesion by 50%.
Another way in which mitochondria may be important in neurodegeneration is through alterations in their effects on calcium homeostasis. In this regard, the recent report of Christof Richter ( personal communication) that α-lipoic acid inhibits mitochondirial calcium transport may be relevant to its beneficial effects noted in neurodegenerative disorders noted by Greenamyre.
In addition, α-lipoic acid treatment of rats exposed to inhalation of nhexane (constant exposure to 700 ppm) delayed the onset of severe neuropathy by 3 weeks:control rats displayed severe motor neuropathy by 6 weeks, while in rats which also received α-lipoic acid, severe neuropathy did not appear until the ninth week.
( Neuroprotection by the methabolic antioxidant lipoic acid- 1996. )
- Analysis of dimerization of BTB-IVR domains of Keap1 and its interaction with Cul3, by molecular modeling - maggio 2013
- The Nrf2-Antioxidant Response Element Signaling Pathway and Its Activation by Oxidative Stressmaggio - 2008
- CUL3 cullin 3
- I PRINCIPI DI BIOCHIMICA DI LEHNINGER David Lee Nelson, Albert L. Lehninger, Michael M. Cox Zanichelli editore.
Schematic diagram of the Keap1-Nrf2 pathway.
Is a-Lipoic Acid a Scavenger of Reactive Oxygen Species in vivo? - 2008
Neuroprotection by the methabolic antioxidant lipoic acid- 1996