Wikipedia
Coprine is a mycotoxin. It was first isolated from common inkcap (Coprinopsis atramentaria). It also occurs in other mushrooms in the genus Coprinopsis and in the brawny bolete (Imperator torsus).
When combined with the consumption of alcohol, it causes "Coprinus syndrome". Symptoms include facial reddening, nausea, vomiting, malaise, agitation, palpitations and tingling in limbs, and arise five to ten minutes after consumption of alcohol. The signs are similar to those induced by disulfiram (Antabuse) and subsequent alcohol consumption. If no more alcohol is consumed, the symptoms will generally subside over two or three hours. Symptom severity is proportional to the amount of alcohol consumed.
Coprine is metabolized into glutamic acid and 1-aminocyclopropanol, the biologically active substance which inhibits the enzyme acetaldehyde dehydrogenase. This enzyme is involved in the metabolism of alcohol. When it is blocked, acetaldehyde accumulates causing the adverse reaction.