Cold stress effects on PSI photochemistry in Zea mays: differential increase of FQR-dependent cyclic electron flow and functional implications.

Cold-induced inhibition of CO(2) assimilation in maize (Zea mays L.) is associated with a persistent depression of the photochemical efficiency of PSII. However, very limited information is available on PSI photochemistry and PSI-dependent electron flow in cold-stressed maize. The extent of the absorbance change (ΔA(820)) used for in vivo quantitative estimation of photooxidizable P700(+) indicated a 32% lower steady-state oxidation level of the PSI reaction center P700 (P700(+)) in cold-stressed compared with control maize leaves. This was accompanied by a 2-fold faster re-reduction rate of P700(+) in the dark, indicating a higher capacity for cyclic electron flow (CEF) around PSI in cold-stressed maize leaves. Furthermore, the increased PSI-dependent CEF(s) was associated with a much higher stromal electron pool size and 56% lower capacity for state transitions compared with control plants. To examine NADP(H) dehydrogenase (NDH)- and ferredoxin:plastoquinone oxidoreductase (FQR)-dependent CEF in vivo, the post-illumination transient increase of F(o)' was measured in the presence of electron transport inhibitors. The results indicate that under optimal growth conditions the relatively low CEF in the maize mesophyll cells is mostly due to the NDH-dependent pathway. However, the increased CEF in cold-stressed plants appears to originate from the up-regulated FQR pathway. The physiological role of PSI down-regulation, the increased capacity for CEF and the shift of preferred CEF mode in modulating the photosynthetic electron fluxes and distribution of excitation light energy in maize plants under cold stress conditions are discussed.

Screening of ovarian steroidogenic pathway in Ciona intestinalis and its modulation after tributyltin exposure.

In this study, we have identified several ovarian steroids in Ciona with high similarity to vertebrate steroids and showed that cholesterol, corticosterone, dehydroepiandrosterone, estrone, estradiol-17beta, testosterone, pregnenolone, progesterone, have identical molecular spectra with vertebrate steroids. In addition, we have studied the effects of an endocrine disruptor (tributyltin: TBT) on these sex hormones and their precursors, ovarian morphology, and gene expression of some key enzymes in steroidogenic pathway in the ovary of Ciona. Ovarian specimens were cultured in vitro using different concentrations of TBT (10(-5), 10(-4) and 10(-3)M). Ethanol was used as solvent control. Gene expression analysis was performed for adrenodoxin (ADREN) and adrenodoxin reductase (ADOX) (mediators of acute steroidogenesis) and 17beta-hydroxysteroid dehydrogenase (17beta-HSD). These transcripts were detected and measured by quantitative (real-time) polymerase chain reaction (qPCR). Sex steroids and their precursors were identified and quantified by a gas chromatography-mass spectroscopy (GC-MS) method. Exposure of Ciona ovaries to TBT produced modulations (either increased or decreased) of sterols and sex steroid levels, whereas no significant differences in ADREN, ADOX or 17beta-HSD mRNA expression patterns were observed. Histological analysis shows that TBT produced several modifications on Ciona ovarian morphology that includes irregular outline of nuclear membrane, less compacted cytoplasm, in addition to test and granulosa cells that were detached from the oocyte membrane. Given that the ascidians represent very simple experimental models for the study of endocrine disruption by environmental contaminants, our findings provide excellent models for multiple identification and quantification of sex steroid and their precursors in biological samples exposed to endocrine-disrupting chemicals and for direct extrapolation of such effects across taxonomic groups and phyla. In addition, these results suggest that Cionaintestinalis may be a suitable species for molecular ecotoxicological studies and biomarker model for endocrine-disrupting effects in marine invertebrates.

Direct inhibitions of the activities of steroidogenic cytochrome P-450 mono-oxygenase systems by anticonvulsants.

The effects of anticonvulsants on the activities of cytochromes P-450(17alpha,lyase) (CYP17), P-450arom (CYP19), P-450C21 (CYP21), P-450SCC (CYP11A1), and P-450(11beta) (CYP11B1) mono-oxygenase systems were studied using rat testicular microsomes, human placental microsomes, bovine adrenocortical microsomes, bovine adrenocortical mitochondria and purified cytochrome P-450(11beta). Phenytoin, clonazepam and carbamazepine inhibited the steroidogenesis catalysed by these cytochrome P-450 mono-oxygenase systems and the Ki values for each anticonvulsant were determined. Neither hydantoin nor sodium valproate inhibited the activities of steroidogenic cytochromes P-450. When the activities of cytochromes P-450arom and P-450C21 were measured in the presence of anticonvulsants, the Ki values (0.15 mM) for phenytoin were close to the plasma concentration of phenytoin under therapeutic conditions. Phenytoin, clonazepam and carbamazepine directly inhibited the monooxygenase activities of cytochromes P-450, because they did not affect the activities of NADPH-cytochrome P-450 reductase, NADPH-adrenoferredoxin reductase and adrenoferredoxin.

[Stimulation of the NADPH:adrenodoxin reductase diaphorase reaction by adrenodoxin]. / Stimulirovanie diaforaznoi reaktsii NADPH:adrenodoksinreduktazy adrenodoksinom.

The diaphorase activity of NADPH: adrenodoxin reductase (EC 1.18.1.2) is stimulated by adrenodoxin. The latter prevents the reductase inhibition by NADPH; the Line-weaver-Burk plots are characterized by a biphasic dependence of the reaction rate on the oxidizer concentration. At pH 7.0 the maximal rate of the first phase is 20s-1; that for the second phase at saturating concentrations of adrenodoxin is 5 s-1. Since the second phase rate is equal to that of the adrenodoxin-linked cytochrome c reduction by reductase it is concluded that this phase reflects the reduction of the oxidizers via reduced adrenodoxin. Quinones are reduced by adrenodoxin in an one-electron way; the logarithms of their rate constants depend hyperbolically on their single-electron reduction potentials (E7(1]. The oxidizers interact with a negatively charged domain of adrenodoxin. The depth of the adrenodoxin active center calculated from the Fe(EDTA)- reduction data is 5.9 A.