The Role of Dihydropyrazines in Accelerated Death of Escherichia coli on Addition of Copper(II)

Identification of dihydropyrazine-glutathione adducts

Dihydropyrazines (DHPs) are glycation intermediates generated both in vivo and in food. DHPs can lead to the formation of a variety of different radical species, which can lead to DNA damage and enzyme inhibition. In addition, the presence of DHPs can lead to a decrease in cellular glutathione (GSH) levels, and induce the expression of antioxidant genes. In this study, the products resulting from the reaction of DHP with GSH have been analyzed in detail, with some of the products being separated by reversed-phase HPLC. The structures of the isolated DHP-GSH adducts were determined by FAB-MS and NMR analyses. These data suggested that the reaction of DHP with a thiol moiety could be involved in oxidative stress, because an increase in the amount of DHP-GSH adducts would result in a decrease in the cellular GSH levels.

Iodine-Mediated Efficient Synthesis of 2,3-Dihydro-Pyrazines

The synthesis of 2,3-dihydro-pyrazines has been developed by an efficient protocol of annulations of 1,2-diketones and ethylenediamine. A variety of 2,3-dihydro-pyrazines were prepared in high yields in the presence of a catalytic amount of iodine.

Effect of dihydropyrazines on human hepatoma HepG2 cells: a comparative study using 2,3-dihydro-5,6-dimethylpyrazine and 3-hydro-2,2,5,6-tetramethylpyrazine

Dihydropyrazines (DHPs) are glycation products that are nonenzymatically generated in vivo and in food. In this study, we compared the effects of 2,3-dihydro-5,6-dimethylpyrazine (DHP-1), a low toxicity DHP, and 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), a high toxicity DHP on the redox indices in HepG2 cells. An apparent increase in intracellular hydrogen peroxide concentration was observed at 24 hr after 1 mM DHP-3 treatment. In addition, DHP-3 exposure significantly increased the mRNA levels of heme oxygenase-1 (HO-1) and glutamate cysteine ligase catalytic subunit (GCLC), which are stress-responsive genes, at 6 hr (HO-1 and GCLC), 12 hr (HO-1 and GCLC) and 24 hr (GCLC) after exposure. These indices, with the exception of the increase in GCLC mRNA after a 6 hr exposure, were not affected by treatment with 1 mM DHP-1. HO-1, GCLC, and nuclear factor erythroid 2-related factor 2 (Nrf2) protein levels also increased at 6 hr (Nrf2), 12 hr (Nrf2, HO-1 and GCLC) and 24 hr (GCLC) after DHP-3 treatment. The increase in HO-1 and Nrf2 protein levels were observed with lower concentration (0.5 mM) of DHP-3, and in agreement with this, antioxidant responsive element-luciferase reporter activity was significantly increased with exposure to at least 0.5 mM DHP-3. These results support our previous report establishing that oxidative stress is in part involved in the effects of DHP on mammalian cells. Additionally, our results suggest that the cell response to DHP-3 exposure was exerted via the activation of the Nrf2-ARE signal pathway.

Possible involvement of glutathione balance disruption in dihydropyrazine-induced cytotoxicity on human hepatoma HepG2 cells

Dihydropyrazines (DHPs), formed by nonenzymatic glycation, are known to exert various effects in vitro and in vivo, such as generation of radical species, DNA strand breakage, enzyme inhibition, and inhibition of bacterial growth. However, their effects on mammalian cells remain elusive. To address this issue, we investigated the effects of a range of DHP concentrations on human hepatoma HepG2 cells using 2,3-dihydro-5,6-dimethylpyrazine (DHP-1), 2,3-dihydro-2,5,6-trimethylpyrazine (DHP-2), and 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3) as model compounds. All of the tested compounds exerted cytotoxic activity against HepG2 cells in the range of 10 µM-1 mM, and significantly so at the highest concentration. DHP-3 was the most effective drug, and it also caused a significant decrease in the ratio of intracellular reduced and oxidized glutathione (GSH/GSSG). In addition, the cytotoxic effect of DHP-3, but not DHP-1 and DHP-2, was enhanced by the inhibition of GSH biosynthesis using 100 µM l-buthionine-(S,R)-sulfoximine (BSO). From these results, it is suggested that the mechanisms of cytotoxicity exerted by DHP-3 are distinct from those exerted DHP-1 and DHP-2. In addition, it is possible that the disruption of intracellular glutathione balance induced by DHP-3 is related to its effect on HepG2 cells.

2-(2-Chlorophenyl)-3-methyl-5,6-diphenyl-2,3-dihydropyrazine

In the title mol­ecule, C₂₃H₁₉ClN₂, the heterocyclic ring adopts a screw-boat conformation, with all substituents equatorial. The benzene ring at position 2 makes dihedral angles of 77.88 (12) and 76.31 (12)° with the phenyl rings at positions 5 and 6, respectively. The dihedral angle between the phenyl rings at positions 5 and 6 is 70.05 (10)°. The Cl atom is disordered over two positions with occupancy factors of 0.946 (5) and 0.054 (5). In the crystal, C—H⋯π inter­actions are found.

Glutathione depression by dihydropyrazine derivative

Dihydropyrazine (DHP), which is formed by nonenzymatic glycation, generates various radical species that lead to DNA damage and enzyme inhibition. In this study, we examined the reaction between DHP derivatives and glutathione (GSH). DHP exposure caused more intense growth inhibition of a GSH-deficient mutant Escherichia coli strain compared with the wild-type strain. DHP-exposed mouse fibroblasts showed a decrease in the cellular GSH level. The obtained data suggested that the reaction of DHP with GSH possibly potentiates cellular stress via the depletion of cellular GSH levels.

Dihydropyrazine-induced inactivation of glyceraldehyde-3-phosphate dehydrogenase

Dihydropyrazine (DHP), which is produced during the Maillard reaction, generates radicals that not only cause breakage of chromosomal DNA leading to mutagenic lesions but also induce oxidative damage to cellular proteins. In the present study, we show that three DHP derivatives, which generated superoxide anions, caused inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). SH-compounds, such as cysteine, dithiothreitol (DTT), 2-mercaptoethanol, 2-mercaptoethylamine, and N-acetyl-cysteine, suppressed the inhibition of GAPDH by DHP in vitro, although the effect of DHP on GAPDH was not reversed by DTT. In addition, DHP-exposed Escherichia coli showed almost unaffected growth on plates containing a rich medium, but poor growth on plates containing M9 synthetic medium with glucose as the sole carbon source. Furthermore, DHP-exposed E. coli exhibited reduced GAPDH activity. These findings indicate that DHP disturbs the glycolytic pathway by inhibiting GAPDH activity.

Effects of phenyl derivatives of dihydropyrazines with ability to generate radical species on Escherichia coli

Phenyl-substituted dihydropyrazines (Ph-DHPs) are derivatives of 2,3-dihydro-5,6-dimethylpyrazine (Me-DHP). Upon the addition of Cu(2+), Me-DHP inhibits the growth of Escherichia coli by generating hydroxyl and carbon-centered radicals that cause DNA strand breakage. Here, we investigated the toxic effect of Ph-DHPs in several DNA repair-deficient or detoxifying enzyme-deficient mutant strains. Ph-DHPs caused cytotoxic and genotoxic damage, but, in a sodA sodB strain, the effects in the presence or absence of Cu(2+) were different than those of Me-DHP. Our results suggest that the action of the generated superoxide anion in the interior side of the cell is remarkable.

Mutation spectrum induced by dihydropyrazines in Escherichia coli

Dihydropyrazine (DHP), which induces mutagenesis in E. coli, was investigated. From analyzing mutations in the chromosomal rpoB gene, the mutation spectrum in uvrB strain revealed the different behavior on exposure to two DHP derivatives 3-hydro-2,2,5,6-tetramethylpyrazine (HTMP), and 2,3-dihydro-5,6-dimethylpyrazine (DHDMP). A higher level of DHP-induced mutation was observed, with base substitutions at G : C pairs predominant. HTMP and DHDMP increased the frequency of G : C to T : A transversions. HTMP increased the frequency of G : C to A : T transitions, than did DHDMP. These findings suggest that DHPs prefer to attack the G : C pair and that different DHP derivatives may prefer distinct mutagenic base pairs; and further, that nucleotide excision repair may be involved in the repair of DHP-induced mutations.