Expression patterns of two tobacco isoflavone reductase-like genes and their possible roles in secondary metabolism in tobacco

Plants contain a large number of proteins homologous to isoflavone reductase, an NADPH-dependent reductase involved in the biosynthesis of isoflavonoid phytoalexins in legumes. Although some are bona fide isoflavone reductases, others may catalyze distinct reductase reactions. Two tobacco genes, TP7 and A622, encoding isoflavone reductase-like proteins, had been previously identified from their unique expression patterns, but their functions were not known. We show here that TP7 is a tobacco phenylcoumaran benzylic ether reductase involved in lignan biosynthesis, but that A622 is not. To gain insight into the possible function of A622, we analyzed in detail the expression patterns of the A622 gene by RNA and protein blots, immunohistochemistry, and its promoter expression in transgenic Nicotiana sylvestris roots. The A622 expression patterns were qualitatively similar to those of putrescine N-methyltransferase, the first enzyme in nicotine biosynthesis, suggesting that A622 may function in the metabolism of nicotine or related alkaloids.     

Analysis of protochlorophyllide reaccumulation in the phytochrome chromophore-deficient aurea and yg-2 mutants of tomato by in vivo fluorescence spectroscopy

The aurea and yellow-green-2 (yg-2) mutants of tomato (Solanum lycopersicum) are unable to synthesize the phytochrome chromophore from heme resulting in a block of this branch of the tetrapyrrole pathway. We have previously shown that these mutants also exhibit an inhibition of protochlorophyllide (Pchlide) synthesis and it has been hypothesised that this is due to feedback inhibition by heme on the synthesis of 5-aminolevulinic acid (ALA). In this study we have investigated Pchlide reaccumulation in cotyledons from etiolated wild-type (WT), aurea and yg-2 seedlings using low-temperature fluorescence spectroscopy. WT cotyledons showed two characteristic Pchlide emission maxima at 630 nm (F630) and 655 nm (F655) respectively, while the aurea and yg-2 mutants contained only phototransformable Pchlide F655. Following a white-light flash to WT cotyledons, reaccumulation of phototransformable Pchlide F655 in the first 30 min was absolutely dependent on the presence of Pchlide F630 before the flash. Reaccumulation of Pchlide F630 was not apparent until at least 2 h after the phototransformation. In contrast, Pchlide F630 never accumulated in aurea cotyledons. The relative rates of both Pchlide F655 and total Pchlide synthesis were approximately twice as high in WT compared to aurea. Measurement of ALA synthesis capacity during this period showed that the reduced rate of Pchlide reaccumulation in aurea was due to an inhibition at this step of the pathway. In addition, feeding of ALA resulted in a substantial and equal increase of non-phototransformable Pchlide in both WT and aurea indicating that aurea cotyledons are capable of accumulating high levels of Pchlide that is not associated to the active site of NADPH:Pchlide oxidoreductase (POR). The implications of these results for the mechanism of inhibition of Pchlide synthesis in phytochrome chromophore-deficient mutants and the role of non-phototransformable Pchlide F630 during plastid development are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Sulfite-oxido-reductase is involved in the oxidation of sulfite in Desulfocapsa sulfoexigens during disproportionation of thiosulfate and elemental sulfur

The enzymatic pathways of elemental sulfur and thiosulfate disproportionation were investigated using cell-free extract of Desulfocapsa sulfoexigens. Sulfite was observed to be an intermediate in the metabolism of both compounds. Two distinct pathways for the oxidation of sulfite have been identified. One pathway involves APS reductase and ATP sulfurylase and can be described as the reversion of the initial steps of the dissimilatory sulfate reduction pathway. The second pathway is the direct oxidation of sulfite to sulfate by sulfite oxidoreductase. This enzyme has not been reported from sulfate reducers before. Thiosulfate reductase, which cleaves thiosulfate into sulfite and sulfide, was only present in cell-free extract from thiosulfate disproportionating cultures. We propose that this enzyme catalyzes the first step in thiosulfate disproportionation. The initial step in sulfur disproportionation was not identified. Dissimilatory sulfite reductase was present in sulfur and thiosulfate disproportionating cultures. The metabolic function of this enzyme in relation to elemental sulfur or thiosulfate disproportionation was not identified. The presence of the uncouplers HQNO and CCCP in growing cultures had negative effects on both thiosulfate and sulfur disproportionation. CCCP totally inhibited sulfur disproportionation and reduced thiosulfate disproportionation by 80% compared to an unamended control. HQNO reduced thiosulfate disproportionation by 80% and sulfur disproportionation by 90%.     

Plant molybdoenzymes and their response to stress

Molybdenum-containing enzymes catalyse basic reactions in the nitrogen, sulphur and carbon metabolism. Mo-enzymes contain at their catalytic sites an organometallic structure termed the molybdenum cofactor or Moco. In higher plants, Moco is incorporated into the apoproteins of four enzymes: nitrate reductase (EC 1.6.6.1-3; NR), xanthine dehydrogenase (EC 1.1.1.204; XDH), aldehyde oxidase (EC 1.2.3.1; AO) and sulphite oxidase (EC1.8.3.1; SO). Molybdoenzymes in plants are key enzymes in nitrate assimilation, purine metabolism, hormone biosynthesis, and most probably in sulphite detoxification. They are considered to be involved in stress acclimation processes and, therefore, elucidation of the mechanisms of their response to environmental stress conditions is of agricultural importance for the improvement of plant stress tolerance. Here we would like to give a brief functional and biochemical characteristic of the four plant molybdoenzymes and to focus mainly on their sensitivity to environmental stress factors.     

Effect of the side chain structure of coenzyme Q on the steady state kinetics of bovine heart NADH: coenzyme Q oxidoreductase

Steady state kinetics of bovine heart NADH: coenzyme Q oxidoreductase using coenzyme Q with two isoprenoid unit (Q₂) or with a decyl group (DQ) show an ordered sequential mechanism in which the order of substrate binding and product release is NADH-Q₂ (DQ) -Q₂H₂ (DQH₂)-NAD⁺ in contrast to the order determined using Q₁ (Q₁-NADH-NAD⁺-Q₁H₂) (Nakashima et al., J. Bioenerg. Biomembr. 34, 11–19, 2002). The effect of the side chain structure of coenzyme Q suggests that NADH binding to the enzyme results in a conformational change, in the coenzyme Q binding site, which enables the site to accept coenzyme Q with a side chain significantly larger than one isoprenoid unit. The side chains of Q₂ and DQ bound to the enzyme induce a conformational change in the binding site to stabilize the substrate binding, while the side chain of Q₁ (one isoprenoid unit) is too short to induce the conformational change.     

Cyanidin and cyanidin 3-O-β-D-glucoside as DNA cleavage protectors and antioxidants

Anthocyanins, colored flavonoids, are water-soluble pigments present in the plant kingdom; in fact they are secondary plant metabolites responsible for the blue, purple, and red color of many plant tissues. Present in beans, fruits, vegetables and red wines, considerable amounts of anthocyanins are ingested as constituents of the human diet (180-215 mg daily). There is now increasing interest in the in vivo protective function of natural antioxidants contained in dietary plants against oxidative damage caused by free radical species. Recently, the antioxidant activity of phenolic phytochemicals, has been investigated. Since the antioxidant mechanism of anthocyanin pigments is still controversial, in the present study we evaluated the effects of cyanidin and cyanidin 3-O-beta-D-glucoside on DNA cleavage, on their free radical scavenging capacity and on xanthine oxidase activity. Cyanidin and cyanidin 3-O-beta-D-glucoside showed a protective effect on DNA cleavage, a dose-dependent free radical scavenging activity and significant inhibition of XO activity. These effects suggest that anthocyanins exhibit interesting antioxidant properties, and could therefore represent a promising class of compounds useful in the treatment of pathologies where free radical production plays a key role.     

Cytotoxicity of RH1 and related aziridinylbenzoquinones: involvement of activation by NAD(P)H:quinone oxidoreductase (NQO1) and oxidative stress

It is supposed that the main cytotoxicity mechanism of antitumour aziridinyl-substituted benzoquinones is their two-electron reduction to alkylating products by NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2). However, other possible cytotoxicity mechanisms, e.g., oxidative stress, are studied insufficiently. In the single-electron reduction of quinones including a novel compound RH1 (2,5-diaziridinyl- 3-(hydroxymethyl)-6-methyl-1,4-benzoquinone), by NADPH:cytochrome P-450 reductase (EC 1.6.2.4, P-450R), their reactivity increased with an increase in the redox potential of quinone/semiquinone couple (E(1)7), reaching a limiting value at E(1)7> or =-0.1V. The reactivity of quinones towards NQO1 did not depend on their E(1)7. The cytotoxicity of aziridinyl-unsubstituted quinones in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) mimics their reactivity in P-450R-catalyzed reactions, exhibiting a parabolic dependence on their E(1)7. The toxicity of aziridinyl-benzoquinones, although being higher, also followed this trend and did not depend on their reactivity towards NQO1. The action of aziridinylbenzoquinones in FLK cells was accompanied by an increase in lipid peroxidation, their toxicity decreased by desferrioxamine and the antioxidant N,N'-diphenyl-p-phenylene diamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea. The inhibitor of NQO1, dicumarol, protected against the toxicity of aziridinyl-benzoquinones except of 2,5-bis-(2'-hydroxyethylamino)-3,6-diaziridinyl-1,4-benzoquinone (BZQ), which was almost inactive as NQO1 substrate. The same events except the absence of pronounced effect of dicumarol were characteristic in the cytotoxicity of aziridinyl-unsubstituted quinones. These findings indicate that in addition to the activation by NQO1, the oxidative stress presumably initiated by single-electron transferring enzymes may be an important factor in the cytotoxicity of aziridinylbenzoquinones. The information obtained may contribute to the understanding of the molecular mechanisms of aziridinylquinone cytotoxicity and may be useful in the design of future bioreductive drugs.     

Involvement of the electrophile responsive element and p53 in the activation of hepatic stellate cells as a response to electrophile menadione

The cytotoxic effects of menadione and hydrogen peroxide were examined in two hepatic stellate cell lines derived from normal or cirrhotic rat liver. The cirrhotic fat-storing cells (CFSC) were found more resistant than the normal fat-storing cells (NFSC) to menadione cytotoxicity. No significant differences were observed in hydrogen peroxide toxicity in these two cell lines. Although protein levels and enzymatic activities of catalase, Cu,Zn-SOD, Mn-SOD, and NADPH cytochrome c reductase were similar in these cell lines, 20-fold increases of NAD(P)H:quinone oxidoreductase 1 (NQO1) enzymatic activity and protein levels were detected in CFSC compared to those of NFSC. Gel mobility shift assays and functional analysis using transient transfection experiments indicated the involvement of the electrophile responsive element (EPRE) in the up-regulation of the NQO1 expression. Antibody supershift analysis revealed that, although Nrf2 is a member of the EPRE-binding complex in both NFSC and CFSC, Nrf1 was identified as a part of the protein/DNA complex only in CFSC. Expression of p53 tumor suppressor gene was found in higher levels in CFSC than in NFSC. We conclude that activation of the EPRE-signaling pathway, which up-regulates several phase II genes and affects p53 stabilization, may offer resistance to hepatic stellate cells against oxidative damage during hepatic injury. This resistance may be a part of the activation process of the hepatic stellate cells and could contribute to their increased proliferation and production of extracellular matrix.     

Conformation-driven and semiquinone-gated proton-pump mechanism in the NADH-ubiquinone oxidoreductase (complex I)

A novel mechanism for proton/electron transfer is proposed for NADH-quinone oxidoreductase (complex I) based on the following findings: (1) EPR signals of the protein-bound fast-relaxing semiquinone anion radicals (abbreviated as Q(Nf)-) are observable only in the presence of proton-transmembrane electrochemical potential; (2) Iron-sulfur cluster N2 and Q(Nf)- are directly spin-coupled; and (3) The projection of the interspin vector extends only 5A along the membrane normal [Yano, T., Dunham, W.R. and Ohnishi, T. (2005) Biochemistry, 44, 1744-1754]. We propose that the proton pump is operated by redox-driven conformational changes of the quinone binding protein. In the input state, semiquinone is reduced to quinol, acquiring two protons from the N (matrix) side of the mitochondrial inner membrane and an electron from the low potential (NADH) side of the respiratory chain. A conformational change brings the protons into position for release at the P (inter-membrane space) side of the membrane via a proton-well. Concomitantly, an electron is donated to the quinone pool at the high potential side of the coupling site. The system then returns to the original state to repeat the cycle. This hypothesis provides a useful frame work for further investigation of the mechanism of proton translocation in complex I.     

Apoptosis repressor with caspase recruitment domain (ARC) is expressed in cancer cells and localizes to nuclei

Apoptosis repressor with caspase recruitment domain is expressed at high levels in brain and myogenic tissues, consistent with a role to inhibit apoptosis in the terminally differentiated cells. Expression of ARC in cancers is not known. In this study, we reported that ARC was highly expressed in various non-myogenic and non-neurogenic human and rat cancer cell lines. Unexpectedly, ARC was localized almost exclusively to the nuclei of cancer cells, which was unlike the cytoplasmic localization of ARC in non-cancer cells. Furthermore, nuclear ARC in cancer cells did not co-localize with nucleolus protein of 30 kDa, an alternatively spliced ARC isoform. These findings indicate that ARC is distributed differently in cancer cells than non-cancer cells and thus might play a role in neoplastic transformation.