Question

working as a houseman in the medical ward, awoken by the nurse at 3 am, informing you that a patient has been admitted for ingesting methanol. As you walk hurriedly to see the patient, you briefly recall that the toxicity of methanol is caused by its conversion by alcohol dehydrogenase leading to the formation of the ultimate toxicant formic acid.

Examination, the patient appears stable and this affords you some time to find out more about methanol poisoning. Looking up the acute poisoning handbook in the ward, it mentions that the metabolism of metnanol can be inhibited by ethanol.
a) What type of inhibition could this be?
b) Explain why?

Answer 1

When the methanol is ingested, with the help of alcohol dehydrogenase it is converted into formaldehyde and further to formic acid which is a very toxic metabolite. Therefore to retard the metabolism of methanol, ethanol is given. Ethanol competes with methanol for alcohol dehydrogenase. This type of inhibition is called COMPETITIVE INHIBITION. Alcohol dehydrogenase enzyme has more affinity for ethanol as compared to methanol. Therefore the metabolism of methanol is stopped and ethanol is now converted into acetaldehyde and further to acetic acid which is not toxic. In this way, the symptoms due to methanol poisoning are decreased.