Isoalliin (Onion Cysteine Sulfoxide · Lachrymatory Factor Precursor · Antimicrobial)
| Compound | Isoalliin (trans-(+)-S-(1-Propenyl)-L-cysteine sulfoxide) |
| Chemical class | Organosulfur — Cysteine Sulfoxide (1-Propenyl cysteine sulfoxide) |
| CAS | 17795-80-1 |
| Primary source | Allium cepa (onion bulb) — primary cysteine sulfoxide of onion |
| Key applications | Lachrymatory factor precursor (onion tears), antimicrobial, antioxidant, anti-inflammatory |
| Claim strength | Moderate |
| Typical form | Onion extract (isoalliin co-constituent); dried onion powder |
| Buy from Herbuno | Onion Extract Powder → |
Name origin: Iso- (indicating the different structural relationship to alliin) + alliin. Isoalliin is the (1-propenyl) cysteine sulfoxide — the onion equivalent of garlic’s alliin (S-allyl cysteine sulfoxide). Instead of the allyl (CH2=CH-CH2-) group of alliin, isoalliin has a 1-propenyl (CH3-CH=CH-) group. This structural difference profoundly alters the chemistry: while alliin → allicin (diallyl thiosulfinate) via alliinase, isoalliin → 1-propenyl sulfenic acid → lachrymatory factor (propanthial S-oxide, the compound that makes onions make you cry) via a different enzyme (LFS, lachrymatory factor synthase). Traditional use: Onion (Allium cepa, Pyaz in Hindi) has been used medicinally for over 5,000 years — documented in the Ebers Papyrus alongside garlic for cardiovascular, antimicrobial, and wound-healing applications. Onion’s pharmacological profile is distinct from garlic due to the isoalliin/lachrymatory factor pathway versus alliin/allicin pathway. Research trajectory: Isoalliin and its hydrolysis products (propyl polysulfides, quercetin, kaempferol from onion) collectively contribute to onion’s cardiovascular, antimicrobial, and antioxidant properties. The lachrymatory factor (propanthial S-oxide) formed from isoalliin has its own antimicrobial and anti-inflammatory activity. Commercial source: Onion Extract Powder and Onion Seed Liquid Extract from Herbuno. See sourcing options below.
Evidence for Isoalliin Applications
Cardiovascular — antiplatelet and antihypertensive: Propyl polysulfides derived from isoalliin hydrolysis (dipropyl disulfide, dipropyl trisulfide) inhibit platelet aggregation and reduce blood pressure by mechanisms similar to garlic polysulfides but with propyl rather than allyl groups. Human studies with onion extract show modest but consistent reductions in platelet aggregation and blood pressure. Claim strength: Moderate.
Lachrymatory factor (antimicrobial/anti-inflammatory): Propanthial S-oxide (the lachrymatory factor from isoalliin) has documented antimicrobial activity against food-borne pathogens and anti-inflammatory activity (NF-κB inhibition) in cell models. Its rapid volatilisation limits systemic exposure from dietary onion consumption. Claim strength: Moderate.
Antioxidant — synergy with onion quercetin: Isoalliin and its hydrolysis products contribute antioxidant capacity to onion extracts alongside quercetin (the primary polyphenol of onion). The combination of organosulfur compounds and quercetin in onion extract addresses oxidative stress via complementary mechanisms. Claim strength: Moderate.
Antimicrobial: Isoalliin and propyl polysulfide hydrolysis products demonstrate antibacterial activity against S. aureus, E. coli, and H. pylori, comparable to but generally slightly weaker than equivalent garlic allicin-based compounds. Claim strength: Moderate.
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Dosage & Formulator Specification
No established isolated isoalliin human dose. Onion extract preparations: 100–400 mg/day dried onion powder equivalent (delivering isoalliin alongside quercetin and other organosulfur compounds). Onion extract standardised to organosulfur content (isoalliin + propyl polysulfides by HPLC) or to quercetin content (more commonly measured) is available. For supplement formulations, specifying total organosulfur content alongside quercetin provides a comprehensive onion extract quality specification. The lachrymatory response from isoalliin during onion processing can be minimised by cold extraction (below 10°C) — slowing lachrymatory factor synthase activity and preserving more isoalliin in the intact form.
Frequently Asked Questions — Isoalliin
Why do onions make you cry but garlic doesn’t?
The answer lies in the different enzymes acting on different cysteine sulfoxides. In garlic, alliin + alliinase → allicin (non-volatile at room temperature — produces garlic aroma). In onion, isoalliin + alliinase + lachrymatory factor synthase (LFS) → propanthial S-oxide (highly volatile, water-soluble, rapidly reaches tear ducts). LFS is unique to onion among Allium species — garlic lacks this enzyme. Lachrymatory factor synthase converts the initial 1-propenyl sulfenic acid (which would form thiosulfinates like garlic) to propanthial S-oxide instead, redirecting the chemistry to the volatile lachrymatory compound.
Do “no-cry” onions have different isoalliin content?
No-cry or low-lachrymatory onion varieties (including “Sunion” cultivars) have been bred or developed to have reduced LFS enzyme activity, not reduced isoalliin content. These onions still contain isoalliin but produce less lachrymatory factor because LFS activity is reduced — the isoalliin is directed toward thiosulfinate formation instead. The health-relevant organosulfur compounds in low-lachrymatory onions are maintained or even enhanced.
Is onion extract the same as garlic extract for cardiovascular applications?
Both contain cardiovascular-relevant organosulfur compounds (isoalliin/propyl polysulfides in onion; alliin/allicin in garlic) with antiplatelet and antihypertensive effects, but they are not interchangeable. Onion additionally provides quercetin (the primary flavonol in Western diets) — a strong antiplatelet and anti-inflammatory compound that garlic lacks. Garlic provides allicin and H2S-generating polysulfides with greater antimicrobial potency. For comprehensive cardiovascular supplement formulation, combining onion extract (quercetin + isoalliin) and garlic extract (alliin + DATS) provides complementary mechanisms.
What is the difference between isoalliin and methiin?
Both are cysteine sulfoxides occurring in Allium and Brassica species respectively. Isoalliin (1-propenyl cysteine sulfoxide) occurs primarily in onion; methiin (S-methyl cysteine sulfoxide) occurs primarily in Brassica vegetables. Isoalliin generates the lachrymatory factor and propyl polysulfides; methiin generates methylsulfinyl compounds. They are related in class (cysteine sulfoxides) but from different plant families with different hydrolysis products.
Related compounds: Alliin, Allicin, Methiin, Diallyl Disulfide
Claim-strength scale – High = multiple human RCTs; Moderate = limited trials or strong preclinical convergence; Emerging = early-stage lab or animal data.
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