This post covers alpha lipoic acid, DMSA, and the transporters that get the heavy metal chelator complex out of our bodies.

These are some comments on Phoenix Rising on alpha lipoic acid for Hg poisoning. Andrew Cutler was part of the discussion. Stas Beckman also discusses use of alpha lipoic acid and DMSA for Hg detox. Dr Cirus has a overview

Alpha lipoic acid (α-LA) and its reduced form dihydrolipoic acid (DHLA) have been historically considered as excellent anti-oxidants and oxidative stress scavengers. Upon oxidation with reactive oxygen species (ROS) and pro-oxidants, α-LA may be reconstituted from DHLA and other reduced forms. Oxidative stress is one of the fundamental causes of functional degeneration, autophagy and apoptosis leading to cytotoxicity and loss of cell survival, often due to exposure to xenobiotics, pollutants, heavy metals, and other environmental and endogenous toxicants. α-LA and DHLA can react with these molecules to strengthen the primary antioxidant defense system during cell injury. The compound α-LA is suggested for heavy metal detoxification, in particular for supporting the mercury (Hg) detoxifying process. Mercury is one of the major environmental toxicant, particularly noxious even upon limited exposure. Oxidative stress pathways have been identified as a key upstream event for Hg-induced toxicity in humans and animals. However, very few existing drugs to date can successfully prevent or reduce Hg toxicity. Although several thiol-based chelators, such as British Anti-Lewisite (2,3-dimercaptopropanol, BAL), meso-2,3-dimercaptosuccinic acid (DMSA), and sodium 2,3-dimercapto-1-propanesulfonate (DMPS), have shown promise for ameliorating Hg intoxication. In this review, the potential role of α-LA and DHLA in scavenging toxic metals and other xenobiotics is discussed, focusing especially on the mechanisms of actions of α-LA and DHLA as potential antioxidants towards Hg-induced toxicity.[1]

The thiols of lipoic acid [2]

My question in all of this is if DHLA is a better of chelator of Hg²⁺ and other heavy metals. The Hossain study looked at much more than just glutahione in response. [2] ALA (250 μM) and DHLA (50 μM) were applied to reduce metal (As, Cd, and Pb)-induced toxicity in PC12 and Caco-2 cells as simultaneous exposure. Both significantly decreased Cd (5 μM)-, As (5 μM)-, and Pb (5 μM) toxicity parameters. This is the SGH data:

Note that DHALA increased the basal level of GSH but ALA did not. Both inhibited the Ph, As, and Cd decreases in GSH. DHALA did a better job at one fifth the concentration. [2] One reason why ALA is preferred might be that DHALA smells bad, or it should smell bad like all reduced thiols.

The esters of DMSA [3]

The Rafati-Rahimzadeh [3] review offers some interesting insights into structure options.

The ester derivatives of DMSA seem like they’d been easy enough to synthesize. My one remark is that Molecular Probes attached lipophillic groups to get their bulky compounds into cells to measure stuff like calcium, pH, reactive oxygen species, etc. Esterases in the cell cleave the ester bonds such that the flourescent probes are trapped inside the cell. If these Hg chelators keep Hg from binding to cellular thiols, we might not care if Hg and the chelator don’t leave the body. The following are some other notes:

• Dimercaprol (BAL) is ineffective and it may even increase mercury levels in the brain and aggravate CNS symptoms in case of organic mercury poisoning. Side effects of dimercaprol include nausea,
• vomiting, hypertension, tachycardia, pain at the injectionsite, headache, diaphoresis and convulsions.
• Meso 2,3-dimercaptosuccinic acid (Succimer,DMSA) is US FDA approved since 1991
• In humans, DMSA is rapidly metabolized and excreted via urine and a small amount via bile and
• lungs .
• In the United States, BAL or DMSA is preferred for treatment of inorganic mercury poisoning
• DMSA has a half-life of 3.2 h .

The DMSA study studied some individuals exposed to Pb in traditional medicine and one as part of his job as a painter.

The arrows indicate the start and finish of DMSA treatments

Heavy Metal Excretion

This should have been the first section presented in this post. The Mrp2, ATP binding cassette multi (drug) resistant protein 2 is involved in renal excretion of DMSA and DMPS from the luminal membrane of renal epithelial cells. [10] Previous work form this group suggested molecular mimicry whereby Hg²⁺ crosslinks cyseine and homosysteine sugh they look like the disulfide versions of these small molecule thiols. These Hg conjugates enter the intracellular space via OAT3 [11] Some of the earlier work of the Bridges Lab was summarized in a 2017 review [12]

The DMPS-mediated extraction of Hg²⁺ has been shown to occur via a direct secretory process whereby mercuric ions move directly into the tubular lumen for subsequent elimination in urine. These are the steps

1. DMPS is taken up at the basolateral membrane of proximal tubular cells via OAT1, OAT3, and the sodium-dependent dicarboxylate transporter (NaC1; SCL13A2)
2. IDMPS and DMSA form complexes with intracellular Hg²⁺ .
3. These complexes appear to be transported across the luminal plasma membrane by luminal membrane MRP2 and confirmed by inside out vesicles

The Bridges and Zallup molecular mimicry review [11] proposes Na⁺-independent transport of Cys-S-Hg-S-Cys is system b0,+ other subunits: b0,+AT, and 4F2hc. This complex has a high affinity for cystine as well as neutral and basic amino acids. [11] Analysis of substrate-specificity indicates that the uptake of Cys-S-Hg-S-Cys and cystine are inhibited by the same amino acids, meaning that these two molecular species are substrates for the same b0+carrier. Hg conjugates of GSH (G-S-Hg-S-G), N-acetylcysteine (NAC-S-Hg-S-NAC), and cysteinylglycine (CysGly; CysGly-S-Hg-S-CysGly are not transported.

Mrp1, the brain version of mrp2

The image on the left is from a Bridges Lab 2004 paper. [11] that describes more how Hg complexes may pass through blood brain barrier endothelial cells. What isn’t clear is how bidirectional this transport is. The work of the Bridges Lab prompted a literature search for a similar mechanism in the brain. This included a search for a brain isoform of mdr and a brain cystine transporter.

In 2012 a study investigated the mechanisms of glutathione-mediated attenuation of MeHg neurotoxicity in primary cortical culture. MeHg (5 μM) caused depletion of GSH and GSSG in neuronal, glial and mixed cultures. Supplementation with exogenous glutathione, specifically glutathione monoethyl ester (GSHME) protected against the MeHg induced neuronal death. MeHg caused increased reactive oxygen species (ROS) formation measured by dichlorodihydrofluorescein (DCF) fluorescence with an early increase at 30 min and a late increase at 6h. This oxidative stress was prevented by the presence of either GSHME or the free radical scavenger, trolox. While trolox was capable of quenching the ROS, it showed no neuroprotection. Exposure to MeHg at subtoxic concentrations (3 μM) caused an increase in system x(c)(-) mediated ¹⁴C-cystine uptake that was blocked by the protein synthesis inhibitor, cycloheximide (CHX). [13] This protein seems to be Slc7A11 also known as the cystine/glutamate transporter that plays a role in many neurological diseases.

Interestingly, blockade of the early ROS burst prevented the functional upregulation of system x(c)(-). Inhibition of multidrug resistance protein-1 (MRP1) potentiated MeHg neurotoxicity and increased cellular MeHg. Taken together, these data suggest glutathione offers neuroprotection against MeHg toxicity in a manner dependent on MRP1-mediated efflux.

• Mdr2 is primarily a liver-gastrointestinal protein. Does this mean that the liver can pump out Hg-thiol conjugates only to have them reabsorbed by the small intestine? This could be what we were talking about concerning zeolite.
• Slc7A11 is really expressed in the brain. This might be why Hg is such a neurotoxin. We were so worried about getting our chelator past the blood brain barrier. Perhaps we really don’t care because…
• Mdr1 is expressed in the brain, particularly the choroid plexus. Hg might not exit the brain as Hg-DMSA or Hg-DMPS, maybe we don’t care as long as these chelators are in the blood to take the Hg from Cys or homocysteine.

References

1. Bjørklund G, Aaseth J, Crisponi G, Rahman MM, Chirumbolo S. Insights on alpha lipoic and dihydrolipoic acids as promising scavengers of oxidative stress and possible chelators in mercury toxicology. J Inorg Biochem. 2019 Jun;195:111-119.
2. Hossain KFB, Akter M, Rahman MM, Sikder MT, Rahaman MS, Yamasaki S, Kimura G, Tomihara T, Kurasaki M, Saito T. Amelioration of Metal-Induced Cellular Stress by α-Lipoic Acid and Dihydrolipoic Acid through Antioxidative Effects in PC12 Cells and Caco-2 Cells. Int J Environ Res Public Health. 2021 Feb 22;18(4):2126. PMC free article
3. Rafati-Rahimzadeh M, Rafati-Rahimzadeh M, Kazemi S, Moghadamnia AA. Current approaches of the management of mercury poisoning: need of the hour. Daru. 2014 Jun 2;22(1):46. PMC free article
4. Jan C. H. van Eijkeren, J. Daniël N. Olie, Sally M. Bradberry, J. Allister Vale, Irma de Vries, Harvey J. Clewell III, Jan Meulenbelt & Claudine C. Hunault (2017) Modeling the effect of succimer (DMSA; dimercaptosuccinic acid) chelation therapy in patients poisoned by lead, Clinical Toxicology, 55:2, 133-141, free article
5. Kosnett MJ. The role of chelation in the treatment of arsenic and mercury poisoning. J Med Toxicol. 2013 Dec;9(4):347-54.PMC free article
6. Patwa J, Thakur A, Flora SJS. Alpha Lipoic Acid and Monoisoamyl-DMSA Combined Treatment Ameliorates Copper-Induced Neurobehavioral Deficits, Oxidative Stress, and Inflammation. Toxics. 2022 Nov 24;10(12):718. PMC free article
7. Flora SJ, Bhadauria S, Pachauri V, Yadav A. Monoisoamyl 2, 3-dimercaptosuccinic acid (MiADMSA) demonstrates higher efficacy by oral route in reversing arsenic toxicity: a pharmacokinetic approach. Basic Clin Pharmacol Toxicol. 2012 May;110(5):449-59. PMC free article
8. Nirmalkar K, Qureshi F, Kang DW, Hahn J, Adams JB, Krajmalnik-Brown R. Shotgun Metagenomics Study Suggests Alteration in Sulfur Metabolism and Oxidative Stress in Children with Autism and Improvement after Microbiota Transfer Therapy. Int J Mol Sci. 2022 Nov 3;23(21):13481.PMC free article
9. Vargason T, Kruger U, Roth E, Delhey LM, Tippett M, Rose S, Bennuri SC, Slattery JC, Melnyk S, James SJ, Frye RE, Hahn J. Comparison of Three Clinical Trial Treatments for Autism Spectrum Disorder Through Multivariate Analysis of Changes in Metabolic Profiles and Adaptive Behavior. Front Cell Neurosci. 2018 Dec 19;12:503 PMC free article
10. Bridges CC, Joshee L, Zalups RK. MRP2 and the DMPS- and DMSA-mediated elimination of mercury in TR(-) and control rats exposed to thiol S-conjugates of inorganic mercury. Toxicol Sci. 2008 Sep;105(1):211-20. PMC free article
11. Bridges CC, Zalups RK. Molecular and ionic mimicry and the transport of toxic metals. Toxicol. Appl. Pharmacol. 2005;204:274–308. [PMC free article]
12. Bridges CC, Zalups RK. Mechanisms involved in the transport of mercuric ions in target tissues. Arch Toxicol. 2017 Jan;91(1):63-81. PMC free article
13. RRush T, Liu X, Nowakowski AB, Petering DH, Lobner D. Glutathione-mediated neuroprotection against methylmercury neurotoxicity in cortical culture is dependent on MRP1. Neurotoxicology. 2012 Jun;33(3):476-81 PubMed abstract
14. Said ES, Ahmed RM, Mohammed RA, Morsi EM, Elmahdi MH, Elsayed HS, Mahmoud RH, Nadwa EH. Ameliorating effect of melatonin on mercuric chloride-induced neurotoxicity in rats. Heliyon. 2021 Jul 6;7(7):e07485 PMC free article