Why do onions make us cry?
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?
To wit: the offensive smell, pungent taste, and tears do not stop us from eating garlic and onions. It is the opposite: precisely these qualities made these Alliaeae one of the first domesticated species, most likely as medicinal plants. Squirrels, rabbits, hamsters, chipmunks, sheep just love them. In fact, most herbivores can eat raw onions without any problem. The problematic toxin, N-propyl disulfide, has nothing to do with the smell, taste, or lachrymatory action of these raw onions. This alkaloid intereferes with hexose monophosphate pathway that is not targeted by the molecules responsible for what makes an onion an onion. In fact, the sheep have been shown to subsist indefinitely on the diet that is 90-100% cull onions. The inescapable conclusion is that large herbivores simply do not care. Neither do the insects; the commercial farming has been plagued by onion thrips for many years. Not only the thrips are resistant to these onion toxins, they are resistant to all known pesticides.
http://www.royalsociety.org.nz/Site/publish/Journals/nzjchs/2003/015.aspx
So, why do we cry? Before going further, here is a brief account of HOW the onions make you cry. For more detail, see
http://www.garlicworld.co.uk/flavour/page2.html
http://www.chm.bris.ac.uk/webprojects2001/gray/props.htm
http://www.uky.edu/~dhild/biochem/25/lect25.html
Pure Appl Chem 65 (1993)625; Nature 419(2002)685
The plants of the onion family practice the most advanced sulfur chemistry. In particular, garlic makes alliin, a 2-propenyl S-oxide derivative of amino acid cystein. Large quantities of alliin are stored inside (large, multinuclear) cells in the bulb. A C-S lyase called alliinase is stored in the membranes of these cells. If the bulb is damaged (e.g., cut by a knife) the alliinases deploy their prosthetic group (peridoxal-5'-phosphate) to cleave alliin into peruvic acid, ammonia, and 2-propanesulfuric acid, CH2=CHCH2SOH. The two molecules of the acid dimerize yielding a stable compound, allicin, that is CH2=CHCH2SS(O)CH2CH=CH2. The allicin has malodorous smell, but it does not make one cry. In onions, the alliin is 1-propenyl, and so the lyases make 1-propenesulfenic acid, CH3CH=CHSOH. Until 2002, it was thought that this sulfenic acid isomerizes to the lacrymatory factor, thiopropanal S-oxide, Et-CH=S(O), quite on its own. Had that been so, there would be a competition between the dimerization, yielding an allicin-like molecule, and this isomerization. The onion takes no chances: it has an enzyme (lachrymatory factor synthase) whose only function is to isomerize the precursor into the factor. The factor is volatile, and so it gets into moist eyes, where it hydrolyzes yielding two highly irritant chemicals: propionic aldehyde and sulfuric acid. The mobility of this factor is important for rapid diffusion of the factor, because the two molecules of the factor can dimerize forming a 4-member C2S2 ring, which is not lachrymatory. So the onion tries hard, at the biochemical level, to get the lachrymator out and to make sure that the alliin is efficiently converted into thiopropanal S-oxide rather than an allicin-like dimer. In 10 seconds, all of the alliin stored in the damaged cell is converted to the lachrymatory factor.
Why is it doing that? Switching from 2- to 1- propenyl derivative of the cystein is a minor challenge. The onion family plants make methyl, propyl, and other cystein derivatives as well, but those end up as disulfides or allicin-like molecules. The reason why that's done so is that such molecules are potent antifungal, antimicrobial, and antiviral agents. The antifungal action is particularly important for the plant (whereas the antibiotic and antiviral functions are more important to us). The allicin presumably works by
...inhibition of thiol containing enzymes by the rapid reaction of thiosulfinates with thiol groups. Recent studies have suggested that the mechanism of action of allicin may be its ability to react with a model thiol compound (cysteine) to form the S-thiolation product S-allylmercaptocysteine. In the amoeba parasite, allicin was found to strongly inhibit the cysteine proteinases, alcohol dehydrogenases as well as the thioredoxin reductases which are critical for maintaining the correct redox state within the parasite. Allicin also irreversibly inhibited the well known thiol-protease papain, the NADP+ dependent alcohol dehydrogenase and the NAD+ dependent alcohol dehydrogenase.
It might appear that the onion trades this "anti-" function in favor of a short-lived, volatile, irritant product which is targeting animals (suggesting that the lachrymatory factor is made for YOU, after all). In fact, this product is much more reactive towards the thiol-containing enzymes than the allicin. The latter works slowly, but surely. The thiopropanal S-oxide reacts very rapidly, shortly after it is generated in a gigantic burst of activity initiated by rapid conversion of the alliicin by alliases and isomerization of the sulfenic acid into the factor by the synthases. The factor is generated extremely rapidly, it reacts very rapidly, and it decays also very rapidly. The intent is to kill the invader as fast as possible, so the infection does not spread. Keeping the herbivores off is not the concern. The volatility is unintentional: a small sulfo-organic molecule is likely to be volatile. The poor stability of the factor (the irritation effect caused by its hydrolysis) is, actually, a detriment rather than an advantage. This all-out approach must be worth the trouble because of the massive initial knock-out of the microbes. It is the alternative strategy to garlic, with its slow deadly stink.
On the strength of these arguments, I think that the lachrymatory effect of the onion is purely accidental. Interestingly (as has been observed by this fast food consumer), the pungent, mouthburning sensation produced by the thiopropanal S-oxide largely disappears when the sliced raw onion is combined with ketchup or mustard. I suspect that the acetic acid in these condiments inhibits the lyases and synthases generating the factor. It is unlikely that the rate of hydrolysis of the factor is greatly affected by these condiments. Any other idea?
Why do onions make us cry?
To wit: the offensive smell, pungent taste, and tears do not stop us from eating garlic and onions. It is the opposite: precisely these qualities made these Alliaeae one of the first domesticated species, most likely as medicinal plants. Squirrels, rabbits, hamsters, chipmunks, sheep just love them. In fact, most herbivores can eat raw onions without any problem. The problematic toxin, N-propyl disulfide, has nothing to do with the smell, taste, or lachrymatory action of these raw onions. This alkaloid intereferes with hexose monophosphate pathway that is not targeted by the molecules responsible for what makes an onion an onion. In fact, the sheep have been shown to subsist indefinitely on the diet that is 90-100% cull onions. The inescapable conclusion is that large herbivores simply do not care. Neither do the insects; the commercial farming has been plagued by onion thrips for many years. Not only the thrips are resistant to these onion toxins, they are resistant to all known pesticides.
http://www.royalsociety.org.nz/Site/publish/Journals/nzjchs/2003/015.aspx
So, why do we cry? Before going further, here is a brief account of HOW the onions make you cry. For more detail, see
http://www.garlicworld.co.uk/flavour/page2.html
http://www.chm.bris.ac.uk/webprojects2001/gray/props.htm
http://www.uky.edu/~dhild/biochem/25/lect25.html
Pure Appl Chem 65 (1993)625; Nature 419(2002)685
The plants of the onion family practice the most advanced sulfur chemistry. In particular, garlic makes alliin, a 2-propenyl S-oxide derivative of amino acid cystein. Large quantities of alliin are stored inside (large, multinuclear) cells in the bulb. A C-S lyase called alliinase is stored in the membranes of these cells. If the bulb is damaged (e.g., cut by a knife) the alliinases deploy their prosthetic group (peridoxal-5'-phosphate) to cleave alliin into peruvic acid, ammonia, and 2-propanesulfuric acid, CH2=CHCH2SOH. The two molecules of the acid dimerize yielding a stable compound, allicin, that is CH2=CHCH2SS(O)CH2CH=CH2. The allicin has malodorous smell, but it does not make one cry. In onions, the alliin is 1-propenyl, and so the lyases make 1-propenesulfenic acid, CH3CH=CHSOH. Until 2002, it was thought that this sulfenic acid isomerizes to the lacrymatory factor, thiopropanal S-oxide, Et-CH=S(O), quite on its own. Had that been so, there would be a competition between the dimerization, yielding an allicin-like molecule, and this isomerization. The onion takes no chances: it has an enzyme (lachrymatory factor synthase) whose only function is to isomerize the precursor into the factor. The factor is volatile, and so it gets into moist eyes, where it hydrolyzes yielding two highly irritant chemicals: propionic aldehyde and sulfuric acid. The mobility of this factor is important for rapid diffusion of the factor, because the two molecules of the factor can dimerize forming a 4-member C2S2 ring, which is not lachrymatory. So the onion tries hard, at the biochemical level, to get the lachrymator out and to make sure that the alliin is efficiently converted into thiopropanal S-oxide rather than an allicin-like dimer. In 10 seconds, all of the alliin stored in the damaged cell is converted to the lachrymatory factor.
Why is it doing that? Switching from 2- to 1- propenyl derivative of the cystein is a minor challenge. The onion family plants make methyl, propyl, and other cystein derivatives as well, but those end up as disulfides or allicin-like molecules. The reason why that's done so is that such molecules are potent antifungal, antimicrobial, and antiviral agents. The antifungal action is particularly important for the plant (whereas the antibiotic and antiviral functions are more important to us). The allicin presumably works by
...inhibition of thiol containing enzymes by the rapid reaction of thiosulfinates with thiol groups. Recent studies have suggested that the mechanism of action of allicin may be its ability to react with a model thiol compound (cysteine) to form the S-thiolation product S-allylmercaptocysteine. In the amoeba parasite, allicin was found to strongly inhibit the cysteine proteinases, alcohol dehydrogenases as well as the thioredoxin reductases which are critical for maintaining the correct redox state within the parasite. Allicin also irreversibly inhibited the well known thiol-protease papain, the NADP+ dependent alcohol dehydrogenase and the NAD+ dependent alcohol dehydrogenase.
It might appear that the onion trades this "anti-" function in favor of a short-lived, volatile, irritant product which is targeting animals (suggesting that the lachrymatory factor is made for YOU, after all). In fact, this product is much more reactive towards the thiol-containing enzymes than the allicin. The latter works slowly, but surely. The thiopropanal S-oxide reacts very rapidly, shortly after it is generated in a gigantic burst of activity initiated by rapid conversion of the alliicin by alliases and isomerization of the sulfenic acid into the factor by the synthases. The factor is generated extremely rapidly, it reacts very rapidly, and it decays also very rapidly. The intent is to kill the invader as fast as possible, so the infection does not spread. Keeping the herbivores off is not the concern. The volatility is unintentional: a small sulfo-organic molecule is likely to be volatile. The poor stability of the factor (the irritation effect caused by its hydrolysis) is, actually, a detriment rather than an advantage. This all-out approach must be worth the trouble because of the massive initial knock-out of the microbes. It is the alternative strategy to garlic, with its slow deadly stink.
On the strength of these arguments, I think that the lachrymatory effect of the onion is purely accidental. Interestingly (as has been observed by this fast food consumer), the pungent, mouthburning sensation produced by the thiopropanal S-oxide largely disappears when the sliced raw onion is combined with ketchup or mustard. I suspect that the acetic acid in these condiments inhibits the lyases and synthases generating the factor. It is unlikely that the rate of hydrolysis of the factor is greatly affected by these condiments. Any other idea?
Why do onions make us cry?