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I find this particular chart about constituents of blood very informative and interesting. Looking through the different components, I see metals such as copper, zinc, etc. Are these only carried as ions or conjugated to proteins, or does blood carry nanoparticles of these elements as well?

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It is very unlikely to have metallic nanoparticles in blood under normal circumstances.

Reasoning:

  • There are specific proteins that strongly bind metal ions: transferrin and ferritin for iron, zinc transporters (Zrt) for zinc, metallothioneins as a catch all for most heavy metal ions etc. These tend to be intracellular so most of the metal would be bound inside cells. Any particles would be endocytosed and oxidized to the ionic form (probably in peroxisomes), then the metal would be bound to proteins.

  • Metal nanoparticles as well as free ions (of metals with more than one oxidation state) have considerable catalytic activity which is non-specific (and therefore quite toxic). They can serve as a redox catalyst as well as generate ROS from oxygen. That's probably why metal-binding proteins have evolved.

  • Lastly, iron in particular is a limiting nutrient for many pathogens. Extracellular iron is very tightly controlled as part of non-specific pathogen defenses. There is only exactly as little iron in blood plasma (bound to transferrin) as absolutely necessary to move it to the cells that take it up. It's unlikely there would be any free iron, because there wouldn't be any fast, specific way to take it up the way the transferrin receptor does.

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  • $\begingroup$ 'll ask a follow up question: What happens when metallo-proteins get denatured(say by thermal degradation), do they drop their metal payloads as elemental metals? Or as ions that are then taken up by other constituents? $\endgroup$ – user1155386 Dec 1 '18 at 18:57
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    $\begingroup$ @user1155386 Eventually, I suppose they must drop they payload at some stage of being recycled, but simple denaturation may not do it for many metalloproteins. At least one reference I checked (Campbell PG, Metal Ions in Life Sciences. 2009 Jan 19;5:239-77) says that heavy metals don't leak out of the denatured protein fraction of cells. Most of them would drop as ions if they do drop (the exceptions being metal in heme and in iron-sulfur clusters) $\endgroup$ – Alex I Dec 1 '18 at 20:13

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