Albumin binding proteins

This post is exploring the Bridges hypothesis that heavy metals like Hg and Cd bind to serum albumin, get taken up in the liver, get metabolized, and then the products of said metabolism gunking up the kidneys. This is explored in the GSH Hg Detox post. This post takes the list of albumin binding and transporting proteins from the Merlot [1] and Schnitzer [2] and and expands on the Bridges hypothesis in the Renal Heavy Metals post. The featured image of this post are three crystal structures of human albumin binding to common drugs found on the Protein Database website rcsb.org. This post introduces the concept that some albumin binding proteins bind to native albumin. Albumin binds to just about everything. binding to some things appears to cause changes in the structure of albumin that causes albumin to bind to different receptors that cause albumin to bind to mysterious proteins that appear to be part of the scavenger pathway.

Denatured vs native albumin [1,2]

albondin/gp60, SPARC, hnRNPs, calreticulin, and DcRn bind to native albumin. Cubilin/Megalin may bind to both. gp18 and gp30 seem to be restricted to binding to denatured albumin. The receptors for native albumin will be mentioned briefly.[1] A barrier cell like an endothelial might want to transport albumin from the blood to the interstitial spaces. The liver might want to remove damaged albumin from the blood, digest it, and release amino acids and peptides back into the blood. Vascular endothelial cells of the skin and skeletal muscle are thought to remove albumin via ndocytosis involving scavenger receptors gp18, gp30 and gp60 (albondin). The liver is involved in this process to a lesser extent (approximately 15%). In addition to vascular endothelial cells, Albumin’s catabolism in parenchymal cells of the liver also occurs through structures associated with caveolin of endocytic vesicles. Albumin is degraded via lysosomes. [2]

Albondin/gp60

Merlot [1] and Schnitzer [2] SEEM to offer different renderings as to whether or not caveolin mediated endocyutosis of albumin via albondin results in transcytosis or lysomal fusion of endosomes. The general consensus is that albumin comes in at one end of the cell and exits the other. There is no data on Protein protein or mRNA expression on the ProteinAtlas.org website. The protein expression data from GeneCards.org was only so-so.

Secreted Protein Acidic and Cysteine Rich

SPARC is getting mentioned in that it has the theoretical possibility of receiving Hg and other heavy metals from albumin before the albumin undergoes some sort of confrontational change that makes it no longer a recognizable albumin molecule. SPARC has other alias including basement membrane protein 40 (BM40) and osteonectin. SPARC is known for binding to to the collagen component of the basement membrane.

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The implications for almost native albumin transferring Hg from Cy34 to they cysteine rich domain of SPARC as well as transferring divalent transition metals like Cd²⁺ to the Ca²⁺ site are hypothetical. at this point.

gp18 and gp30, damaged albumin, Schnitzer Lab 1992-3

How do you find a protein receptor for albumin when you don’t have any antibodies? The Schnitzer Lab separated proteins by size via a process called SDS polyacrylamide gel electrophoresis. These proteins were then transfered to a membrane. Albumin was labeled with colloidal gold. (A-Au). [3] Au-albumin is where we see dark bands. These experiments are riddled with caveats. SDS is a detergent that denatures proteins to that they might be sieved through the gel. In my experience, they refold when they are transferred to the filter and the SDS is washed away. Another caveat is that attaching gold to albumin causes conformational changes.

This was addressed in Figure 3 (not shown) and Figure . BSA is bovine serum albumin. Judging by the dark bands at 18 and 30 kDa, native BSA does not compete with Au-albumin lending strength to the concern that labeling albumin with Au causes it to have higher affinity for gp18 andgp30. Fixing the albumin with formaldehyde and maleic anhydride allows them to complete with Au labeled albumin strengthening the argument that gp18 and pg30 are receptors for denatured albumin. Figure 4 established maleimide and formaldehyde modified albumin as as high affinity competitive inhibitors of Au-albumin binding to both gp18 and gp30. [3] Figure presented ¹²⁵I-modified albumin as the reporter. Maximal binding was shown to occur at pH 6 or higher. Note that this has implications for lysosomes. Binding is maximal at physiological ionic strength. [3] ¹²⁵I was used to label otherwise unmodified BSA, mal-BSA, and Au-albumin. These mixes were added to RFK monolayers. The less native forms of albumin were quicker to be degraded by the cells. [3] A year later the Schnitzer group continued to explore modified albumin degradation in three different cell lines. Chlroquin was used to establish that degradation occurred n the lysosomes.

Pb and other metals change structure of Albumin ?

A study by Belatik used three different techniques that are sensitive to conformational changes to calculate the affinity of lead for bovine serum albunin. We can speculate that gp18 and gp30 may recognize the Pb bound albumins even if the affinities are not that high K_Pb-HSA = 8.2 (±0.8)×10⁴ M⁻¹ and K_Pb-BSA = 7.5 (±0.7)×10⁴ M⁻¹.

The only Cys residues not taking part in -S-S- bridge formation and containing free mercapto-group is located at position 34 (Cys₃₄).[5] Cys34 , about half is in the reduced form in native albumin is in a crevice of albumin helps to improve its specificity for binding metal in linear coordination: Hg²⁺, Au⁺, Ag⁺ and Pt²⁺ but not Cd²⁺ or Zn² [6]

The N-terminal binding site (NTS) involves the Nitrogen of His2 and the backbone nitogen of Asp1 and Ala2. The authors claim that this site is selective for Cu²⁺. This site is even named ‘ATCUN’, for the Amino Terminal Cu(II) and Ni(II) binding motif The NTS motif is thought to have high conformational flexibility in the absence of bound metal, [6]

Concluding remarks

Thirty years after the Schnitzer studies [3,4] the identity of p18 and p30 binders of denatured albumin are not known, or easy to find on the Internet. We can speculate from the number of drug/albumin structures on the Protein Data Base that the possibility of drugs binding to albumin enough to alter the structure and target it for lysosomal degradation is relevant. All of the metal binding to albumin studies involved measuring structural changes. We do not know if these structural changes enough to target the albumin for gp18/30 lysosomal degradation? What happens to the metal binding when the pH of the lysosome drops below the pKa of the side chains involved in binding the metal?

References

1. Merlot AM, Kalinowski DS, Richardson DR. Unraveling the mysteries of serum albumin-more than just a serum protein. Front Physiol. 2014 Aug 12;5:299 PMC free article
2. Schnitzer JE, Oh P. Albondin-mediated capillary permeability to albumin. Differential role of receptors in endothelial transcytosis and endocytosis of native and modified albumins. J Biol Chem. 1994 Feb 25;269(8):6072-82. PMC free article
3. Schnitzer JE, Sung A, Horvat R, Bravo J. Preferential interaction of albumin-binding proteins, gp30 and gp18, with conformationally modified albumins. Presence in many cells and tissues with a possible role in catabolism. J Biol Chem. 1992 Dec 5;267(34):24544-53. PMC free article
4. Schnitzer JE, Bravo J. High affinity binding, endocytosis, and degradation of conformationally modified albumins. Potential role of gp30 and gp18 as novel scavenger receptors. J Biol Chem. 1993 Apr 5;268(10):7562-70. free article
5. Belatik A, Hotchandani S, Carpentier R, Tajmir-Riahi HA. Locating the binding sites of Pb(II) ion with human and bovine serum albumins. PLoS One. 2012;7(5):e36723. PMC free article
6. Coverdale JPC, Katundu KGH, Sobczak AIS, Arya S, Blindauer CA, Stewart AJ. Ischemia-modified albumin: Crosstalk between fatty acid and cobalt binding. Prostaglandins Leukot Essent Fatty Acids. 2018 Aug;135:147-157. PMC free article