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In clinical studies, aged garlic extract (AGE) has been shown to improve endothelial dysfunction. The activation of nuclear factor erythroid 2 like 2 (Nrf2)-dependent gene expression is a proposed mechanism for maintaining vascular homeostasis. S−1-propenylcysteine (S1PC) and S-allylcysteine (SAC) are two predominant sulfur-containing amino acids present in AGE. However, it remains unclear as to whether the two sulfur amino acids activate Nrf2 in cells. Nitric oxide (NO) is an important signaling molecule and one of the activators of the Nrf2 pathway. In a previous study, we examined the effects of the two sulfur amino acids on NO signaling for modulating the Nrf2-dependent antioxidant response. Neither S1PC nor SAC were found to affect the expression of Nrf2-regulated genes, such as heme oxygenase-1 (HMOX1) in human umbilical vein endothelial cells. However, S1PC was found to augment HMOX1 expression, induced by NO donors, such as NOR3. NOR3 was found to induce the nuclear accumulation of NRF2 protein and concomitantly enhance the degradation of BTB domain and CNC homolog 1 (BACH1), a transcriptional repressor that competes with NRF2. Notably, on our previous study, S1PC enhanced the NOR3-induced downregulation of BACH1, but did not further enhance the NOR3-induced accumulation of NRF2. The findings of that study indicated that the S1PC-induced degradation of BACH1 may provide a basis for the antioxidant effects of AGE. Thus, in this review, we aimed to provide a current overview of the antioxidant effects of AGE and sulfur-containing amino acids. [ABSTRACT FROM AUTHOR]

S-Alk(en)yl-L-cysteine sulfoxides are pharmaceutically important secondary metabolites produced by plants that belong to the genus Allium. Biosynthesis of S-alk(en)yl-L-cysteine sulfoxides is initiated by S-alk(en)ylation of glutathione, which is followed by the removal of glycyl and γ-glutamyl groups and S-oxygenation. However, most of the enzymes involved in the biosynthesis of S-alk(en)yl-L-cysteine sulfoxides in Allium plants have not been identified. In this study, we identified three genes, AsGGT1, AsGGT2, and AsGGT3, from garlic (Allium sativum) that encode γ-glutamyl transpeptidases (GGTs) catalyzing the removal of the γ-glutamyl moiety from a putative biosynthetic intermediate of S-allyl-L-cysteine sulfoxide (alliin). The recombinant proteins of AsGGT1, AsGGT2, and AsGGT3 exhibited considerable deglutamylation activity toward a putative alliin biosynthetic intermediate, γ-glutamyl-S-allyl-L-cysteine, whereas these proteins showed very low deglutamylation activity toward another possible alliin biosynthetic intermediate, γ-glutamyl-S-allyl-L-cysteine sulfoxide. The deglutamylation activities of AsGGT1, AsGGT2, and AsGGT3 toward γ-glutamyl-S-allyl-L-cysteine were elevated in the presence of the dipeptide glycylglycine as a γ-glutamyl acceptor substrate, although these proteins can act as hydrolases in the absence of a proper acceptor substrate, except water. The apparent K m values of AsGGT1, AsGGT2, and AsGGT3 for γ-glutamyl-S-allyl-L-cysteine were 86 μM, 1.1 mM, and 9.4 mM, respectively. Subcellular distribution of GFP-fusion proteins transiently expressed in onion cells suggested that AsGGT2 localizes in the vacuole, whereas AsGGT1 and AsGGT3 possess no apparent transit peptide for localization to intracellular organelles. The different kinetic properties and subcellular localizations of AsGGT1, AsGGT2, and AsGGT3 suggest that these three GGTs may contribute differently to the biosynthesis of alliin in garlic.