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Garlic has been demonstrated to exert protective effects against oxidative damage using numerous experimental models. The antioxidant effects of garlic are associated with the activation of Nrf2-dependent gene expression. S-1-Propenylcysteine (S1PC) and S-allylcysteine (SAC) are two predominant sulfur amino acids present in aged garlic extract; however, the exact roles of these amino acids within the Keap1/Nrf2 system remain unknown. We hypothesized that sulfur-containing amino acids derived from garlic could activate Nrf2 in the presence of nitric oxide (NO). Neither S1PC nor SAC affected gene expression of either heme oxygenase-1 (HMOX1) or the glutamate-cysteine ligase modifier subunit (GCLM) in human umbilical vein endothelial cells (HUVECs) or human aorta endothelial cells (HAECs). Interestingly, S1PC augmented expression levels induced by nitric oxide donors (NO-donors) such as NOR3 and GSNO. NO-donors were found to induce nuclear accumulation of NRF2 and activation of the eIF2α/ATF4 pathway, whereas S1PC did not further amplify the NO-induced effects on NRF2 or eIF2α/ATF4. Additionally, NO-donors induced the degradation of BTB domain and CNC homolog 1 (BACH1), a transcriptional repressor that can compete with NRF2. In addition, S1PC enhanced BACH1 downregulation within the nucleus. Pretreatment with deferoxamine, an inhibitor of heme synthesis, upregulated BACH1 protein levels and abolished the effect of NO-donors and S1PC on HMOX1 expression. The above results indicate that S1PC could modulate antioxidant gene expression via the NO/heme/BACH1 signaling pathway, thereby suggesting that S1PC-induced degradation of BACH1 may provide a basis for therapeutic applications., (Copyright © 2019 Elsevier Inc. All rights reserved.)

S-Alk(en)yl-l-cysteine sulfoxides are cysteine-derived secondary metabolites highly accumulated in the genus Allium. Despite pharmaceutical importance, the enzymes that contribute to the biosynthesis of S-alk-(en)yl-l-cysteine sulfoxides in Allium plants remain largely unknown. Here, we report the identification of a flavin-containing monooxygenase, AsFMO1, in garlic (Allium sativum), which is responsible for the S-oxygenation reaction in the biosynthesis of S-allyl-l-cysteine sulfoxide (alliin). Recombinant AsFMO1 protein catalyzed the stereoselective S-oxygenation of S-allyl-l-cysteine to nearly exclusively yield (RC SS )-S-allylcysteine sulfoxide, which has identical stereochemistry to the major natural form of alliin in garlic. The S-oxygenation reaction catalyzed by AsFMO1 was dependent on the presence of nicotinamide adenine dinucleotide phosphate (NADPH) and flavin adenine dinucleotide (FAD), consistent with other known flavin-containing monooxygenases. AsFMO1 preferred S-allyl-l-cysteine to γ-glutamyl-S-allyl-l-cysteine as the S-oxygenation substrate, suggesting that in garlic, the S-oxygenation of alliin biosynthetic intermediates primarily occurs after deglutamylation. The transient expression of green fluorescent protein (GFP) fusion proteins indicated that AsFMO1 is localized in the cytosol. AsFMO1 mRNA was accumulated in storage leaves of pre-emergent nearly sprouting bulbs, and in various tissues of sprouted bulbs with green foliage leaves. Taken together, our results suggest that AsFMO1 functions as an S-allyl-l-cysteine S-oxygenase, and contributes to the production of alliin both through the conversion of stored γ-glutamyl-S-allyl-l-cysteine to alliin in storage leaves during sprouting and through the de novo biosynthesis of alliin in green foliage leaves., (© 2015 The Authors The Plant Journal © 2015 John Wiley & Sons Ltd.)