Poisons of the Day - Part I

I’ve decided to start a little series called “Poisons of the Day.” This series is not to discuss the use of poisons to remove irritating coworkers in the chemical laboratory, but to understand why many elements and simple compounds are poisonous, something I’ve never really thought about before. Ever since I ran into an article describing the mechanism of arsenic poisoning, I’ve been interested in the subject. After doing some research I have come to the conclusion that there appears to be four main ways in which an element can be poisonous.

1. It binds to and/or removes necessary elements or compounds from the body
2. It replaces an element already in the body, but cannot fulfill the same role.
3. It’s a necessary element for life in small amounts but causes trouble when too much is around.
4. It interferes with the chemical reactions in the body.

I’ll start out with four poisons today.

ARSENIC
This is the element which got me started on this series. Arsenic does not attack the body directly, it operates by replacing the phosphorus in the body. Since it sits just below P in the periodic table, it tends to bind to the same compounds that phosphorus does. As you might expect, the arsenic analogues do not work very well, if at all. ATP, the molecule responsible for storing and releasing energy, is particularly susceptible to arsenic poisoning. In the presence of arsenic, cells tend to die from energy starvation.

LEAD
Lead is another poison which works by slowly replacing other metals used by the body, in particular, calcium, iron, and zinc which are cofactors in many enzymatic reactions. Lead tends to bind to the same enzymes as these metals, but the resulting molecules fail to function properly, usually because the shape of the protein is no longer correct. It is known to interfere both with the production of heme (Zn replacement), which leads to anemia, as well as the transmission of electrical impulses in the brain (Ca replacement) which impairs brain function.

BARIUM
Barium apparently causes problems in a lot of ways, but its primary role occurs because it interferes with the sodium-potassium pump used by our muscles. Apparently it reduces the permeability of muscles to potassium entry. This can lead to paralysis, and if the affected muscles include the heart and respiratory muscles, death. Barium does also appear to replace calcium in the body, but this is a minor effect.

I hadn’t really appreciated how toxic barium is, assuming that it’s in the form of a soluble salt. Barium carbonate is a common rat poison. Barium sulfate is so insoluble that you can drink it with no ill effects.

D₂O
I include this compound because I used D₂O (deuterated or heavy water) back as an undergraduate for isotope labeling experiments. My undergraduate advisor used to tell me that if you drank enough of it, it would kill you. Apparently he was correct, although you would have to work really hard to get your body deuterated enough to have an effect. If you can replace between 25 and 50% of the water in your body with D₂O, then you’ll notice the health effects, but this will require the drinking of nothing but large quantities of D₂O for at least a week according to Wikipedia. The poisoning mechanism is pretty simple. Deuterium has slightly different hydrogen bonding properties than hydrogen (which is why we use D₂O in the lab in the first place). Our bodies consist of large number of proteins, including DNA, which depend upon hydrogen bonding to hold them in the rather complicated conformations which are necessary for certain enzymatic reactions to occur. Start changing the hydrogen bonding in our bodies and you can imagine what kind of problems would result.

I’ll have more poisons to discuss at a later date.