Competitive Inhibition - Practical Examples

A few years ago a European wine producer added ethylene glycol to his wines in order to give the appearance of more expensive wines. As a result a large number of unsuspecting people ingested this toxic compound. They are also ingested accidentally by humans. Animals, mainly pet dogs and cats, sometimes lap up spilled antifreeze because of its sweet taste. There is a danger of children doing the same thing.

In these cases the parent alcohol is not toxic but metabolic products are. Alcohol dehydrogenase catalyzes the formation of the corresponding aldehyde. This is then converted to the acid by aldehyde dehydrogenase. The result is formaldehyde and formic acid from methanol and glyoxal and oxalic acid from ehtylene glycol. The aldehydes are probably the main toxins, but the acids have other undesireable effects. Firstly, they can cause acidosis. Oxalic acid can crystalize out as the calcium salt in the kidney microtubules, causing physical damage.

Thus, if conversion to these products can be avoided the methanol and/or ethylene glycol can be excreted unchanged. In some cases this can be accomplished by use of a competitive inhibitor. Ethanol is such an inhibitor. By adding 2 mg/dl of ethanol to alcohol dehydrogenase, the enzyme is 50% saturated. It takes 326 mg/dl of ethylene glycol to get the same saturation. That is, the enzyme prefers to bind the ethanol. The treatment then is to raise the blood ethanol level to 0.1% or 0.1 mg/l00 ml. This is the equivalent of drinking 5 shots in one hour, and allows for the excretion of unmetabolized ethylene glycol in the urine.

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