Kei Ohkubo

A Key Role for Old Yellow Enzyme in the Metabolism of Drugs by Trypanosoma cruzi

Trypanosoma cruzi is the etiological agent of Chagas disease. So far, first choice anti-chagasic drugs in use have been shown to have undesirable side effects in addition to the emergence of parasite resistance and the lack of prospect for vaccine against T. cruzi infection. Thus, the isolation and characterization of molecules essential in parasite metabolism of the anti-chagasic drugs are fundamental for the development of new strategies for rational drug design and/or the improvement of the current chemotherapy. While searching for a prostaglandin (PG) F2α synthase homologue, we have identified a novel “old yellow enzyme” from T. cruzi (TcOYE), cloned its cDNA, and overexpressed the recombinant enzyme. Here, we show that TcOYE reduced 9,11-endoperoxide PGH2 to PGF2α as well as a variety of trypanocidal drugs. By electron spin resonance experiments, we found that TcOYE specifically catalyzed one-electron reduction of menadione and β-lapachone to semiquinone-free radicals with concomitant generation of superoxide radical anions, while catalyzing solely the two-electron reduction of nifurtimox and 4-nitroquinoline-N-oxide drugs without free radical production. Interestingly, immunoprecipitation experiments revealed that anti-TcOYE polyclonal antibody abolished major reductase activities of the lysates toward these drugs, identifying TcOYE as a key drug-metabolizing enzyme by which quinone drugs have their mechanism of action.

Efficient radical scavenging ability of artepillin C, a major component of Brazilian propolis, and the mechanism

Hydrogen transfer from artepillin C to cumylperoxyl radical proceeds via one-step hydrogen atom transfer rather than via electron transfer, the rate constant of which is comparable to that of (+)-catechin, indicating that artepillin C can act as an efficient antioxidant.

Formation of superoxide–metal ion complexes and the electron transfer catalysis

Abstract The formation of a variety of superoxide–metal complexes and their catalytic functions have been reviewed. The g zz -values of the EPR spectra of superoxide–metal ion complexes vary significantly depending on the type of metal ions. From the deviation of the g zz -value from the free spin value are determined the energy splitting values (Δ E ) of π g levels due to complex formation. The Δ E values correlate well with the catalytic reactivities of the metal ions in the electron transfer reduction of oxygen. Such a complex formation between superoxide ion and metal ions has been shown to play an essential role in the enzymatic function of superoxide dismutase, formation of active oxygen–copper complexes and the novel catalytic effect of O 2 in back electron transfer of zinc porphyrin–fullerene linked molecules.

Driving Force Dependence of Photoinduced Electron Transfer Dynamics of Intercalated Molecules in DNA

A series of acridinium, quinolinium, and phenanthridinium ions (9-substituted-10-methylacridinium (AcrR+, R = H, Pri, and CH2Ph), 3-substituted-1-methylquinolinium (RQuH+, R = CN and Br), and 5-methylphenanthridinium (5-MePhen+) perchlorate salts) are shown to be intercalated into the DNA double helix from calf thymus. The one-electron reduction potentials (E0red) of these intercalators have been determined in the absence and presence of DNA by both cyclic voltammetry and second harmonic ac voltammetry. The E0red values of intercalators are shifted in a positive direction by intercalation into the DNA double helix. The one-electron oxidation potential (E0ox) of ethidium bromide, which is known to be intercalated into DNA, is also shifted in a positive direction by the intercalation. The wide range of E0red values of intercalators thus determined in the presence of DNA allows us to examine the exact driving force dependence of the rates of photoinduced electron transfer from the singlet excited state of et...

Structure and photoelectrochemical properties of ITO electrodes modified with self-assembled monolayers of meso, meso-linked porphyrin oligomers

A systematic series of ITO electrodes modified chemically with self-assembled monolayers (SAMs) of meso, meso-linked porphyrin oligomers have been designed to provide valuable insight into the development of artificial photosynthetic devices. Photoelectrochemical measurements were carried out in an argon-saturated 0.1 M Na2SO4 aqueous solution containing methyl viologen (MV2+) as an electron acceptor or triethanolamine (TEA) as a sacrificial electron donor in a three electrode system including the modified ITO electrodes. The action spectrum of porphyrin oligomer SAMs exhibits photocurrent generation in a wide wavelength region, demonstrating the enhancement of light-harvesting properties in the visible region. The relative integrated value of the action spectrum as well as the quantum yield of photocurrent generation of the porphyrin oligomer SAMs increases with increasing the number of the porphyrins. These results clearly show that our strategy is highly promising for improving light energy conversion efficiency.

Evidence that gold(III) porphyrins are not electrochemically inert: facile generation of gold(II) 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin

Gold(III) porphyrin 1 is shown to undergo reduction at the central metal ion to give the first known gold(II) porphyrin overturning the long held assumption that reduction of such complexes only occurs at the macrocycle.

Direct Detection of Superoxide Anion Generated in C60-Photosensitized Oxidation of NADH and an Analogue by Molecular Oxygen

Visible-light irradiation of poly(vinylpyrrolidone) n(PVP)-solubilized C60 in water in the presence of NADH (dihydronicotinamide adenine dinucleotide) and molecular oxygen (O2) results in formation of superoxide anion (O2˙−). Formation of O2˙− having a characteristic g‖ value of 2.18 was evidenced by the direct observation with use of a low-temperature EPR technique at 77 K. Photoinduced O2˙− formation was also observed for an N-methyl-2-pyrrolidone (NMP) solution of C60 and 1-benzyl-1,4-dihydronicotinamide (BNAH) in the presence of O2, whereas C60 radical anion (C60˙−) was formed in the absence of O2 under otherwise the same experimental conditions. These results suggest that C602− formed in the photoinduced electron-transfer reduction of C60 by BNAH acts as an electron donor to O2 to give O2˙− in NMP.

Fast self-exchange electron transfer and delocalization of unpaired electron between zinc porphyrin radical cations and zinc porphyrins

Self-exchange electron transfer rates between π-radical cations of zinc porphyrins and the neutral metalloporphyrins have been determined from the line-width broadening in the ESR spectra in different solvents at various temperatures. Fine tuning of the substituent on the porphyrin ring and the proper choice of the solvent have enabled us to observe negative activation enthalpies for the self-exchange electron transfer reactions. The observation of negative activation enthalpies indicates that the self-exchange electron transfer occurs via the charge-transfer π-complexes formed between zinc porphyrin radical cations and the neutral zinc porphyrins. The complete delocalization of the unpaired electron over two porphyrin moieties is observed in the radical cation of a zinc porphyrin dimer, 5,5-bis(10,20-bis(3,5-di-tert-butylphenyl)porphyrinatozinc(II)). This is regarded as the extreme limit of the rapid self-exchange electron transfer between zinc porphyrin radical cation and the neutral form.

Effects of magnesium ion on kinetic stability and spin distribution of phenoxyl radical derived from a vitamin E analogue: mechanistic insight into antioxidative hydrogen-transfer reaction of vitamin E

The phenoxyl radical 1˙ of a vitamin E analogue, generated by the reaction of 2,2,5,7,8-pentamethylchroman-6-ol (1H) with 2,2-di(4-tert-octylphenyl)-1-picrylhydrazyl (DPPH˙) or galvinoxyl (G˙), was significantly stabilized by the presence of Mg2+. Addition of Mg2+ into a solution of 1˙ resulted in a red shift of the absorption band of 1˙ as well as a decrease in the g value of the EPR spectrum of 1˙, indicating a complex formation between 1˙ and Mg2+. The complexation between the phenoxyl radical and Mg2+ significantly retards the disproportionation reaction of 1˙ nby electronic repulsion between the metal cation and a generated organic cation (1+), leading to stabilization of the organic radical species. No effect of Mg2+ on the rate of hydrogen atom transfer from 1H to DPPH˙ or to G˙ was observed, suggesting that the hydrogen-transfer reaction between 1H and DPPH˙ or G˙ proceeds via a one-step hydrogen atom transfer mechanism rather than electron-transfer followed by proton transfer.

Remarkable effects of counter ions on scandium ion-promoted electron transfer reactions

Scandium ion-promoted electron transfer reactions of p-benzoquinone are remarkably accelerated when tetrakis(pentafluorophenyl)borate anion is used instead of trifluoromethanesulfonate anion as the counter anion of scandium ion. Only a catalytic amount of scandium borate salt (Sc[B(C6F5)4]3) accelerates significantly the Diels-Alder reaction of 9,10-dimethylanthracene with p-benzoquinone, which proceeds via Sc(3+)-promoted electron transfer from the anthracene to p-benzoquinone.