Oh what a difference a single carbon atom makes!
It can be the difference between life and death! Just two tablespoons of ethylene glycol can kill a child while the same amount of propylene glycol is practically harmless. When it comes to molecular structure the difference would appear to be minor; propylene glycol contains one more carbon than ethylene glycol. These compounds are not esoteric substances that are only of theoretical interest; they are commonly encountered in various antifreeze preparations. Ethylene glycol is found in virtually every automobile radiator and is responsible for a couple of dozen deaths every year in North America along with thousands of cases of poisoning. Since it can cause inebriation just like ethanol, some desperate alcoholics resort to ethylene glycol with the mistaken belief that it is no more toxic than the usual alcohol they drink. But it is! Other poisoning cases involve suicides or the accidental ingestion of the sweet-tasting glycol by children. There are also innumerable cases of poisonings of pets that love to lap up the little puddles of fluid that leak from a car’s radiator.
Antifreeze formulated with propylene glycol (Sierra brand) is available, albeit it is somewhat more expensive. Many zoos have converted to propylene glycol in the radiators of their vehicles to reduce the risk to animals that ingest the fluid after radiator leaks or boil-overs. The switch to the safer antifreeze received a big boost when a rare California condor died after drinking from an ethylene glycol spill. So how can just one little extra carbon atom in a molecule cause such a difference in toxicity? It all comes down to the manner in which ethylene glycol and propylene glycol are metabolized in the body.
The liver is the body’s main detoxicating organ and it is here that an enzyme, alcohol dehydrogenase, reacts with either glycol. But the products of the reaction are quite different! Propylene glycol is converted into lactic acid, while ethylene glycol mainly forms glycolic and oxalic acids. Any excess acid in the blood is a problem but oxalic acid presents a further complication. It reacts with calcium ions in the blood to form microscopic crystals of calcium oxalate which can interfere with blood flow and cause damage to the brain, heart and lungs. But the most serious effect is blockage of the tiny tubes that make up the blood-filtering structure of the kidneys. Death from ethylene glycol poisoning is usually due to kidney failure.
If ethylene glycol poisoning is diagnosed quickly, it can be treated effectively. A stomach wash with saline solution and activated carbon (a form of charcoal that absorbs toxins) can remove some of the glycol before it is absorbed. Intravenous sodium bicarbonate can counter the metabolic acidosis. And then comes the most interesting part of the treatment. Oral and intravenous administration of alcohol! Since alcohol dehydrogenase has a greater affinity for alcohol than for ethylene glycol, the enzyme goes to work on alcohol and leaves the glycol alone. By the time the alcohol has been metabolized, the ethylene glycol hopefully has been eliminated in the urine. The patient ends up very drunk, but very much alive!
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