This post started out exploring a model of mercury (Hg) toxicity first proposed by the Bridges Laboratory. A request was made to look at some of the transport proteins, not necessarily only in the renal tubules. The conclusion is that these transporters are rather promiscuous. The Bridges model may work for transition metal complexes as well. The highlights the importance of glutathione, GSH. One transporter of GSH-Hg complexes also brings in stable cystine, an oxidized for of cysteine. Cystine may be reduced in the cell for the production of GSH. Note: The Bridges Lab model relies of hepatic digestion of Hg-albumin. a sister post covers other heavy metal binding sites on albumin and albumin receptor proteins throughout the circulation. The Bridges model may be far more universal though out the circulation
Bridges Lab Model 
- Mercury (Hg) is a toxin with high affinity for proteins and small molecules containing thiols.
- Most Hg found in the blood is bound to albumin.
- Hepatocytes take up Hg bound albumin and release Hg bound to GSH as GSH-Hg and GSH-Hg-HSG
- Subsequently, GSH-Hg-GSH conjugates are exported from hepatocytes into blood via multidrug resistance transporters (MRP) 3 and 5.
- The portal vein and hepatic artery in Wistar rats were ligated to prevent delivery of Hg to the liver.
- Ligated and control rats were injected with HgCl2 or GSH-Hg-GSH (containing radioactive Hg) and the disposition of Hg was assessed in various organs.
- Renal accumulation of Hg was reduced significantly in ligated rats exposed to HgCl2.
- In contrast, when rats were exposed to GSH-Hg-GSH, the renal accumulation of Hg was similar in control and ligated rats.
- Experiments using HepG2 cells indicate that Hg-albumin conjugates are taken up by hepatocytes and additional experiments using inside-out membrane vesicles showed that MRP3 and MRP5 mediate the export of GSH-Hg-GSH from hepatocytes.
- These data are the first to show that Hg-albumin complexes are processed within hepatocytes to form GSH-Hg-GSH, which is, in part, exported back into blood via MRP3 and MRP5 for eventual excretion in urine.
TRPML the missing links [2,3]
TRPML, TRPML1 specifically, is a member of the transient receptor potential cation channel family that transports Fe2+, Ca2+ and Zn2+ out of the lumen of the lysosome and into the cytosol  Another thought is that release of divalent cation in proximity of the SNARE complex can facilitate lysososmal membrane fusion and release of contents into extra cellular spaces. Also note that PAT1 may transport peptides out of the lysosome.
Instead of an autophagossome from a macrophage, imagine we have a hepatocyte phagocytosing Hg2+ bound albumin. This autophagosome fuses with a lysosome that digests things. Instead of Ca2+ coming out of the phagosome that serves the function of membrane fusion, we have Hg2+ coming out. Is this where the Hg2+ gets conjugated with GSH glutathione?
The chemistry of Hg and thiols 
The Ajsuvakova review quickly became intense. The following bullet points are comments deemed part of the GSH detox of Hg2+ narrative..
- Hg is claimed to have a higher affinity for thiol groups than Cd, As, and Pb. 
- Hg is rapidly passed from one thiol to another bucket brigade at a rate approximating that of diffusion.
- Hg0 vapor is rapidly oxidized to Hg2+ which binds to plasma proteins such as albumin and then transferred to lower molecular weight thiols. 
- Most Hg is associated with protein thiols. 
- “It is known that Hg predominantly binds inorganic and organic S (HS−, sulfide or S2−, and thiol/thiolate) and Se (HSe−, selenide or Se2−, and selenol/selenolate) compounds.
- Thiol affinity: (Hg(II) > Cu(II) > Pb(II) 
- GSH and Hg(II) are found at pH of 1–13 These GSH complexes with Hg2+ and CH3Hg+ are more stable than GSH complexes with Cd(II), Zn(II), Pb(II), or Sn(IV).
- Albumin accounts for 90% of all protein-bound Hg in the form of Hg2+, CH3Hg+, C2H5Hg+, C6H5Hg
The review is overwhelming because there is something innate about the interaction of thiols, sometimes called mercaptans., a German shortening of the Latin phrase “phrase corpus mercurium captāns” or quick silver (mercury) capturing!
The 3D and 2D structures of the mercury glutathione structure from PubChem. A search for the perfect image to beter understand what is so special about mercury and thiols/mercaptans yielded nothing that really said it all. That Hg is only binding to S (sulfur) in the glutathione tripeptide says a lot.
This list came from the mercury review, but probably applies to transition metal small peptides conjugates. It appears that these peptide transporters are pretty non discriminate.
- LAT1.The MeHg-S-Cys complex structurally closely resembles L-methionine and a substrate for the neutral amino acid carrier transport (LAT1) system found in the BBB. GS-CH3Hg may also be transported by The large neutral amino acid transporter 1 is a heterodimerSlc7A5 and Slc3A2. SLC7A5 belongs to the APC superfamily and forms a disulfide linked heterodimer with the glycoprotein CD98 (SLC3A2).
- Hypothetically, Cys-S-Hg-S-Cys may also be trafficked by the cystine-glutamate transporter SLC7A11, playing a significant role in brain functioning Note, bringing in oxidized GSH may be a key factor for maintaining redox balance and removing Hg and transition metals like Cd and Pb.
- . It is also noteworthy that the uptake rate of GSH-S-CH3Hg is twofold higher as compared to Cys-S-CH3Hg, although both forms are effectively transported intracellularly.
- GSH-S-CH3Hg may be transported by amino acid transporters.
- MRP2 also transports Cys-S-Hg-S-Cys
- Renal GGT may also be involved in renal transport of the Hg-SG complex. Clinicians and toxicologists may be focused on GGT as a marker of liver injury when GGT is an enzyme in the synthesis and degradation of glutathione
- Oat1 (organic anion transposrter) substrates include para-aminohippurate (PAH), dicarboxylates, prostaglandins, cyclic nucleotides, urate, folate, diuretics, ACE inhibitors, antiviral agents, beta-lactam antibiotics, antineoplastics, mycotoxins, sulfate conjugates, glucuronide conjugates, cysteine conjugates, ochratoxin A, NSAIDs, mercapturic acids and uremic toxins. Homocysteine Hg conjugates may also be on the list of substrates. Organic anion transporter 1 (hOAT1) may transport Cys-S-Hg-SCys but not G-S-Hg-S-G.
Searching for more images of these transporters brings home the point that none of the transporters mentioned in the Ajsuvakova Hg review are really that specific. This brings up the notion that these transporters may just as easily be transporting small peptides conjugated to transition metals.
Scalise and coauthors have depicted the LAT1 hetero dimer in blue and red to illustrate its two faced functioning The blue face represents the transporter of essential amino acids (dark blue) and light blue non essential amino acids. The size is suggestive of the higher or the lower specificity toward the amino acids. The red face is the exporter of non-essential amino acids and a myriad of drugs while remaining an import of the essential amino acid and heavy metal binder histidine. Scalise and coauthors added known specificity toward non-amino acid substrates: hormones, drugs and inhibitors. The publication discused requirements for transport. Glycosylation of CD98 is depicted as “antennas”.Scalise and coauthors remind us Thiols at the physical heterodimer interface are sensitive to Hg…. and probably transition metals too.
aNote the pairing of the MRP exporters with OAT antiporters taing organic anions from the interstitium, through the renal p;roximal tubule cell, and out into the lumen (urine).
Does cystine make sense as supplement to boost GSH levels? If Hg and transition metal GSH chelates, GSH/GSSG may need to be replenished. Slc7A11, right, brings in thiols for not only removing reactive oxygen species but also for conjugating Hg and transition metals.
Immunocal® and Cheaper versions
Immunocal is a systine and whey protein supplement that has been through a few peer reviewed scientific studies. Going back to the Bridges model, there is a certain amount of simplicity in SLC7A11 bringing in GSH production substrate. Cystine is nice in that one does not have to worry about it oxidizing as it is already oxidized. It does not have the “rotten egg” odor either.
Bulk Supplements sells an L-cystine powder that can be added to smoothies and so on. If one wants to replicate the Immunocal concept, the same company sells a whey protein isolate.
- Barfuss DW, Buchanan JT, Joshee L, Pittman EH, D’Souza N, Matta KE, Brownlee RT, Bridges CC. Hepatic processing of mercuric ions facilitates delivery to renal proximal tubules. Toxicol Lett. 2022 Apr 15;359:1-9. PubMed
- Wang W, Zhang X, Gao Q, Xu H. TRPML1: an ion channel in the lysosome. Handb Exp Pharmacol. 2014;222:631-45. PubMed
- Kiselyov K, Colletti GA, Terwilliger A, Ketchum K, Lyons CW, Quinn J, Muallem S. TRPML: transporters of metals in lysosomes essential for cell survival? Cell Calcium. 2011 Sep;50(3):288-94 PMC free article
- Ajsuvakova OP, Tinkov AA, Aschner M, Rocha JBT, Michalke B, Skalnaya MG, Skalny AV, Butnariu M, Dadar M, Sarac I, Aaseth J, Bjørklund G. Sulfhydryl groups as targets of mercury toxicity. Coord Chem Rev. 2020 Aug 15;417:213343. PMC free article
- Scalise M, Galluccio M, Console L, Pochini L, Indiveri C. The Human SLC7A5 (LAT1): The Intriguing Histidine/Large Neutral Amino Acid Transporter and Its Relevance to Human Health. Front Chem. 2018 Jun 22;6:243. PMC free article
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