Why do onions make us cry?

[shkrobius]

When you cut onions they make you "cry." Why? Is it making YOU cry (a defensive measure against herbivores) or the lachrymatory (tear-producing) effect is purely coincidental

Comments

[wsobchak]

>the NADP+ dependent alcohol dehydrogenase and the NAD+ dependent alcohol dehydrogenase

как это влияет на лук в качестве закуски?

[shkrobius]

I am not sure I got your question right. Alcohol definitely stops the synthesis of thiopropanal S-oxide; it also extracts allicin, so that's good. If it's cold it may extract alliin w/o decomposition, which is not doing anything. The lachrymatory factor undergoes rapid solvolysis in aqueous ethanol, so only the amino acid precursor is extracted. It is not doing anything. So the tear-making is reduced but there is not much benefit in terms of health. Garlic + room temperature ethanol, on the other hand, is beneficial: more allicin is extracted.

[wsobchak]

I mean, does the enzyme in question affect the metabolization of alcohol, and which way? Do people who eat onions with their alcohol get drunker faster, or slower?

[shkrobius]

It should not, directly. Ethanol is metabolized in five ways, three oxidative and two nonoxidative. Alcohol (ADH) and aldehyde dehydrogenases (ALDH)s, cytochromes P450, catalases and phosphlipases should not be affected directly. I think people use raw onions mainly to mask the unpleasant taste of poor quality alcohol. In principle, also, the byproduct of thiopropanal S-oxide is the propanal, which may prime the ALDHs. This is something that my father taught me; you may profit from this bit of biochemistry, too. The dynamics of alcohol metabolism is such that if you drink a lot there is a lag after ADH converts ethanol to ethanal and ALDH converts the latter to acetate: making ALDH2 takes time. So if you drink 50-100 g of alcohol 3-4 hours before going to a party then you'd have elevated concentration of ALDH during serious drinking, the acetylaldehyde is more rapidly removed, and poisoning is minimized. Another reason pre-drinking works might be that it activates cytochrome P450 isozyme in the brain, which is deficient in the ADHs.