Selenium, the Ides of March, and the Periodic Table of DEATH and Mystery

Wasn’t it the Ides of March (1) when Julius Caesar was stabbed in the back by people he loved and trusted? Et tu, Brute?

 

The Ides of March tend to be used as a harbinger of misfortune, to warn that something bad is going to happen. But it didn’t start off that way. The words “Ides”, along with “Kalends”, and “Nones” are actually ancient Roman calendar words (2) that refer to the phases of the moon. Kalends = new moon, Nones = first quarter moon, while Ides is for the full moon.

 

Which is propitious because the periodic table element we will be exploring is Selenium (Se), discovered in 1817 by Jöns Jacob Berzelius and whose name derives from the Greek seléne, which means moon (3). It’s right underneath sulfur (S) on the periodic table and apparently smells even worse than the sulfurous stench of rotten eggs. In fact, absorbed selenium caused Berzelius to have extraordinarily stinky breath (4).

 

Selenium is very rare in the Earth’s crust (5). It has an atomic number of 34 and an atomic weight of 78.971. When purified, it takes a couple of forms—a pretty red crumble that can be added to glass (6) and a metallic grey form that looks like alien red blood cells (7). By the way, selenium-infused glass is also beautifully fluorescent under ultraviolet light (8).

 

An essential mineral

 

It turns out animals need selenium to live—it’s an essential mineral. Why is it essential? Mostly because of stress. Yep. Selenium is incredibly important for antioxidant detoxification of reactive oxygen and nitrogen species (ROS and NOS) at the cellular level (9). Selenium—in the amino acid selenocysteine—is the 21st amino acid and a vital component of antioxidant enzymes like glutathione peroxidase (10) and iodothyronine deiodinases (11). Glutathione peroxidases deactivate hydrogen peroxide (H2O2), and deiodinases activate or deactivate thyroid hormones and their metabolites.

 

But wait, you ask, how can selenocysteine be the 21st amino acid if our DNA only encodes 20 standard amino acids? (12)

1)—Because animals contain the necessary selenocysteine enzymes and proteins to convert the amino acid serine to selenocysteine when serine is attached to its transfer RNA. This is very important since no free pool of selenocysteine exists inside the cell. Selenocysteine by itself is too reactive and could damage the cell. It is made only when it is ready to be inserted into selenoproteins (13).

 

And 2)—Because the genes and, ultimately, messenger RNAs (ribonucleic acids) for selenoproteins have a very special RNA sequence that forms a double-stranded stem-loop near the 3’-end of the mRNA called a Selenocysteine Insertion element or SECIS.

 

The importance of Selenocysteine Insertion Elements

 

The SECIS element interacts with proteins necessary for selenocysteine amino acid insertion into the growing selenoprotein using a perfectly placed UGA codon on the selenoprotein mRNA. Normally, the UGA codon is the signal to STOP adding amino acids to a protein. However, the SECIS element, when paired with the UGA codon, binds selenocysteine-linked transfer RNA. The selenocysteine is then inserted into the selenoprotein polypeptide, and—Ta-Da!—important selenocysteine proteins are available to combat dangerous oxygen and nitrogen radicals that damage lipids, nucleic acids, proteins, and even cellular organelles. What’s interesting is that we still don’t know what some of these 25 or so selenoproteins do in our bodies. Research is ongoing.

But why use selenium in these proteins instead of oxygen or sulfur? After all, these three elements have similar properties—they are stacked one right on top of the other—and standard amino acids with oxygen (-O-H, serine) and sulfur (-S-H, cysteine) exist. It turns out that selenium (-Se-H) loses its attached hydrogen (H) and is reduced much more easily under physiological conditions when compared to serine and cysteine, which makes it better to pick up and dispose of those nasty ROSs and NOSs.

 

How much selenium do we need? Not that much, really. For adults, around 55 micrograms, or 55 millionth of a gram, is the recommended daily requirement. If you like Brazil nuts, that means eating one nut per day will give you twice your daily requirement. A lot of foods contain selenium, so a well-rounded diet means you have nothing to worry about in terms of too much or too little (14).

 

Too little or too much selenium

 

Too little selenium is supposedly associated with cancer, cardiovascular disease, cognitive decline and Alzheimer’s, HIV infection, male fertility, and thyroid disease. “Associated” is a wishy-washy word, though, and the research summaries on all these diseases pretty much ended with “not conclusive” or “does not support the use of selenium supplementation,” which is maddening.

 

On the other hand, it is known that too much selenium can kill you. Here’s a cautionary tale about a man who read on the internet about selenium and its “possible” role in preventing and treating prostate cancer. So—because if a little is good, more must be better—he ate 10 grams. That’s 181,000 times the RDA. He suffered cardiac arrest and died within 6 hours of ingestion (15).

 

And don’t get me started on those poor polo ponies. Grazing animals have a relatively restrictive diet and may need selenium supplementation (16). Unfortunately, a compounded supplement containing up to 100X too much selenium was injected into 21 horses at the 2009 U.S. Open Polo Championship (17). The horses collapsed and died within hours of the treatment.

 

Horse Feet

 

Let’s end this on a happier note by (stem)-looping (haha) back to Julius Caesar and his favorite horse, that, as far as anyone knows, didn’t suffer from too much or too little selenium, but instead had polydactyl front feet.

 

“He rode a very remarkable horse, with feet almost like those of a man, the hoofs being divided in such a manner as to have some resemblance to toes. This horse he had bred himself, and the soothsayers, having interpreted these circumstances into an omen that its owner would be master of the world, he brought him up with particular care, and broke him in himself, as the horse would suffer no one else to mount him. A statue of this horse was afterwards erected by Caesar’s order before the Temple of Venus Genetrix.” (18)

 

The Roman culture revered polydactyly, and at the horse’s birth, augurs predicted its rider would rule the world (19). And he did.

 

At least until the Ides of March.