Through a single genetic transformation in onion (Allium cepa), a crop recalcitrant to genetic transformation, we suppressed the lachrymatory factor synthase gene using RNA interference silencing in six plants. This reduced lachrymatory synthase activity by up to 1,544-fold, so that when wounded the onions produced significantly reduced levels of tear-inducing lachrymatory factor. We then confirmed, through a novel colorimetric assay, that this silencing had shifted the trans-S-1-propenyl-L-cysteine sulfoxide breakdown pathway so that more 1-propenyl sulfenic acid was converted into di-1-propenyl thiosulfinate. A consequence of this raised thiosulfinate level was a marked increase in the downstream production of a nonenzymatically produced zwiebelane isomer and other volatile sulfur compounds, di-1-propenyl disulfide and 2-mercapto-3,4-dimethyl-2,3-dihydrothiophene, which had previously been reported in trace amounts or had not been detected in onion. The consequences of this dramatic simultaneous down-and up-regulation of secondary sulfur products on the health and flavor attributes of the onion are discussed.

In normal onion (Allium cepa), trans-S-1-propenyl-L-cysteine sulfoxide is transformed via 1-propenesulfenic acid into propanethial S-oxide, a lachrymatory factor, through successive reactions catalyzed by alliinase and lachrymatory factor synthase (LFS). A recent report showed that suppression of the LFS activity caused a dramatic increase in thiosulfinates previously reported as "zwiebelane isomers". After purification by recycle high-performance liquid chromatography and subsequent analyses, we established the planar structure of the putative "zwiebelane isomers" as S-3,4-dimethyl-5-hydroxythiolane-2-yl 1-propenethiosulfinate, in which two of the three molecules of 1-propenesulfenic acid involved in the formation gave the thiolane backbone, and the third molecule gave the thiosulfinate structure. Of at least three stereoisomers observed, one in the (2'R,3'R,4'R,5'R)-configuration was collected as an isolated fraction, and the other isomers were collected as a combined fraction because spontaneous tautomerization prevented further purification. Both fractions showed inhibitory activities against cyclooxygenase-1 and α-glucosidase in vitro.

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The ability to discriminate between such geometric isomers will be extremely useful for the chemical assignment of unknown metabolites in MS-based metabolomics.

Lachrymatory factor synthase (LFS) is responsible for the natural production of syn-propanethial S-oxide. Because its substrate and product are both transient, the mechanistic insight is currently poor. Here, we decipher the catalytic cycle of onion LFS (AcLFS) by employing crystallography, biochemical assays, and molecular simulations. Crystal structures complexed with solute compounds demonstrate not only the properties of the catalytic site but also a potent binding mode of the labile substrate, a mode which can fulfill the "syn-effect" of the reaction. Mutagenesis analyses identified the key residues in the active site, and the kinetic and energetic backgrounds were specified with computational approaches. We present a rational catalytic mechanism based on the intramolecular proton shuttling that is distinct from the canonical [1,4]-sigmatropic rearrangement reaction. Our investigations both in vitro and in silico provide the mechanistic basis explaining how AcLFS generates the lachrymatory agent in nature and provide insights into the molecular machinery concerning organic labile sulfur species.