Searching for Acrolein-Scavenging Compounds in Plants: Isoalliin from Onion as an Excellent Scavenger

Reactive carbonyl species (RCS), such as acrolein (Acr), are generated via the degradation of lipid peroxides and function both as signaling molecules and as cytotoxins downstream of reactive oxygen species (ROS). The aim of this study was to identify RCS-scavenging compounds, acting as secondary antioxidants, in plants. We established an HPLC-based method to sensitively evaluate the RCS-scavenging ability of compounds, using Acr as a representative RCS. Among 80% ethanol extracts of 46 angiosperm species, the highest scavenging activities were observed in garlic, spinach, avocado, broccoli, and lotus, representing a diverse range of plant families. The active ingredient from garlic cloves was identified as S-allyl cysteine sulfoxide (alliin), an amino acid typical of Allium species, which trapped up to two Acr molecules at its amino group. Notably, S-(1E)-propenyl cysteine sulfoxide (isoalliin), a structural analog of alliin and a major constituent of onion, showed even stronger Acr-scavenging activity. Isoalliin outperformed known Acr scavengers such as carnosine, epigallocatechin gallate, resveratrol, and hesperetin. These findings highlight isoalliin as a representative plant-derived RCS scavenger and emphasize the importance of S-alk(en)yl cysteine sulfoxides as potent Acr-scavenging compounds in plants. This study further suggests that structurally diverse RCS scavengers remain to be discovered among specialized metabolites in the plant kingdom.