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This Enzyme Is Why Onions Make You Cry

Figuring out the how the tear-inducing fumes form could give surprising insights into our own human proteins

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smithsonian.com

Onions are one of humanity’s oldest friends—providing sustenance (and tears) for at least 5,000 years.

It wasn't until the last couple decades that researchers began to figure out just why slicing into raw onions makes us cry. And a recent paper published in the journal ACS Chemical Biology presents the last piece of the puzzle, teasing out how the onion's primary eye-irritating chemical is formed.

The tear-producing—or lachrymatory—effect of onions is a type of chemical warfare used by the plants to ward off would-be predators and knife-wielding chefs alike. But the steps that take place upon that first slice into the onion's thin skin is complex.

As Ashton Yoon reports for Discover, onion cells contain pockets of fluid filled with an enzyme called allinase. When an onion is sliced, these sacs rupture, releasing the allinase which then reacts with the onion's amino acids to create sulfenic acid.

That sulfenic acid then enters in to another reaction, helped along by an enzyme known as lachrymatory factor synthase (LFS), which creates a volatile compound known as lachrymatory factor (LF) that wafts into the air and reacts with the nerves of your cornea, causing uncontrollable tears. 

It took decades to figure that process out, say Marcin Golczak, an author of the study and professor of pharmacology at Case Western University. While researchers discovered LF was the primary eye-irritant in the 1970s, they did not discover the LFS enzyme until 2002. But figuring out how LFS helps produce LF proved even more challenging due to LF's instability and tendency to vaporize.

To tackle this question, Golczak and his team had to get creative. They were able to make stable crystals of the LFS enzyme by binding it to a more stable compound, crotyl alcohol. They then examined the structure of the enzyme, which revealed that the compound is similar to a well-studied superfamily of proteins called START. By comparing sections of the LFS enzyme where other compounds attach to similar sites on START proteins, the researchers teased apart the steps of how sulfenic acid becomes LF.

While solving the onion puzzle is a feather in their cap, Golczak’s team is more interested in what the enzyme can tell them about human proteins. “We are not working with plants at all, we’re part of a medical school,” he tells Smithsonian.com. “Our lab studies proteins involved in the transport of metabolites. But the structure of LFS and its shape is similar to what we have in humans, so we decided to look into it.”

Even so, the discovery could have implications for agriculture. In 2015, the Japanese researchers that discovered LFS found that they could produce less tearful onions by bombarding the bulbs with ions, which break down the enzymes involved in the chain reaction that leads to LF. In 2008, another group of researchers in New Zealand produced a tear-free onion by snipping out the gene that produces LFS from the onion genome. Neither of those techniques, however, has yet brought tear-free onions to the marketplace.

Golczak says that knowing exactly how LFS creates LF could help researchers design an inhibitor to prevent the formation of the tear-inducing compound, which might be less controversial than genetic modification. “You could design a solution or a spray with the inhibitor. I don’t know if that’s a good approach,” he says. “We’re not pursuing it. We’ll let the Japanese guys look into it.”

Until then, try chilling your onions before you slice to slow down the release of those noxious fumes.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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