硫酸铵与盐酸胍对鸡肝二氢叶酸还原酶的激活和失活作用

在５℃、２０℃、３０℃,鸡肝二氢叶酸还原酶的活力随盐酸胍浓度增加,先经历一个激活区,以后则为失活区。温度升高,使最大激活幅度变小,激活区变窄,所需盐酸胍浓度减小,硫酸铵在浓度低于０．２ｍｏｌ／Ｌ时,对天然酶有较小的激活作用,更高浓度的硫酸铵使酶活力下降;胍激活酶的活力随硫酸铵浓度的增加,由天然酶的２１０％单调下降;对失活酶,增加硫酸铵的浓度,则使酶活力逐渐恢复;三种情形的终活力均为天然酶的４０％。     

Chemorepellents in Paramecium and Tetrahymena

Although Paramecium has been widely used as a model sensory cell to study the cellular responses to thermal, mechanical and chemoattractant stimuli, little is known about their responses to chemorepellents. We have used a convenient capillary tube repellent bioassay to describe 4 different compounds that are chemorepellents for Paramecium and compared their response with those of Tetrahymena. The classical Paramecium t-maze chemokinesis test was also used to verify that this is a reliable chemorepellent assay. The first two compounds, GTP and the oxidant NBT, are known to be depolarizing chemorepellents in Paramecium but this is the first report of them as repellents in Tetrahymena. The second two compounds, the secretagogue alcian blue and the dye cibacron blue, have not previously been described as chemorepellents in either of these ciliates. Two other compounds, the secretagogue AED and the oxidant cytochrome c, were found to be repellents to Paramecium but not to Tetrahymena. The repellent nature of each of these compounds is not related to toxicity because cells are completely viable in all of them. More importantly, all of these repellents are effective at micromolar to nanomolar concentrations, providing an opportunity to use them as excitatory ligands in future works concerning their membrane receptors and possible receptor operated ion channels.     

Ammonium and nitrate uptake and nitrate reductase activity of photoautotrophic callus cultures of the fernPlatycerium coronarium (Koenig) DESV

Summary The uptake of nitrate and ammonium by callus ofPlatycerium coronarium from the culture medium was examined. Nitrate reductase activity of photoautotrophic callus cultures under CO₂ enrichment was significantly lower compared to the cultures without CO₂ enrichment, but higher than that of heterotrophic callus cultured on medium with 2% (wt/vol) sucrose. When sucrose concentration of the heterotrophic culture was lowered to 0.2%, nitrate reductase activity increased. The level of nitrate reductase activity increased by about 25% in the heterotrophic callus with an increase in 2,4-D from 2 μM to 10 μM, despite a decline in fresh weight gain. However, photoautotrophic cultures with 1% CO₂ enrichment showed 20% decline in nitrate reductase activity and 45% decline in fresh weight gain with a similar increase in 2,4-D level. The rate of uptake of nitrate from the culture medium was unrelated to the level of nitrate reductase activity in the callus. For photoautotrophic callus under CO₂ enrichment, the presence of 1% (vol/vol) CO₂ generally resulted in the highest rate of nitrate uptake. The rate of uptake of ammonium was higher for callus cultured on 2 μM 2,4-D compared to that on 10 μM 2,4-D.     

Spin-Trapping and Direct Epr Investigations on the Hepatotoxic and Hepatocarcinogenic Actions of Luteoskyrin, An Anthraquinoid Mycotoxin Produced by Penicillium Islandicum Sopp. Generations of Superoxide Anion and Luteoskyrin Semiquinone Radical in the Redox Systems Consisted of Luteoskyrin and Liver Nadph- Or Nadh-Dependent Reductases

Luteoskyrin is a hepatotoxic and hepatocarcinogenic bisdihydroanthraquinone produced by Penicillium islandicum Sopp. By observing the EPR spectra of DMPO-spin adducts and luteoskyrin semiquinone radical, we investigated in vitro whether luteoskyrin is reduced to its semiquinone radical leading to the generation of active oxygen species in redox systems catalyzed by NADPH-dependent cytochrome reductases of the liver. We found (1) the formation of luteoskyrin semiquinone radical in the NADPH-cytochrome P-450 reductase system under anaerobic conditions, (2) the generation of O- in the systems composed of luteoskyrin, NAD(P)H, and either rat liver microsomal NADPH-cytochrome P-450 reductase or submitochondrial particles and (3) dicoumarol showed no effect on the O- generation in the case of submitochondrial particles. From these results we proposed that luteoskyrin liver injuries are induced by the active oxygen species generated in the process of autoxidation of luteoskyrin semiquinone radical which is produced in the one-electron redox systems catalyzed by the liver NAD(P)H-dependent cytochrome reductases.     

Antioxidant systems in insects

Insects possess a suite of antioxidant enzymes and small molecular weight antioxidants that may form a concatenated response to an onslaught of dietary and endogenously produced oxidants. Antioxidant enzymes such as superoxide dismutase, catalase, glutathione transferase, and glutathione reductase have been characterized in insects. Water-soluble and lipid-soluble antioxidants such as ascorbate, glutathione, tocopherols, and carotenoids have not been well studied in insects but may play very important antioxidant roles. Additionally, the peritrophic matrix and trehalose may possess important antioxidant functions in insects. The enzymatic recycling of ascorbate, first noted in green plants, may also exist in insects. A greater understanding of these antioxidant systems may provide greater understanding about the ecological relationships of insects with their hosts. © 1995 Wiley-Liss, Inc.     

The expression of redox proteins of denitrification inThiosphaera pantotropha grown with oxygen,nitrate, and nitrous oxide as electron acceptors

The redox proteins and enzymes involved in denitrification inThiosphaera pantotropha exhibited a differential expression in response to oxygen. Pseudoazurin was completely repressed during batch or continuous culture under oxic conditions. Cytochromecd ₁ nitrite reductase was also heavily repressed after aerobic growth. Nitrite, nitric oxide, and nitrous oxide reductase activities were detected in intact cells under some conditions of aerobic growth, indicating that aerobic denitrification might occur in some circumstances. However, the rates of denitrification were much lower after aerobic growth than after anaerobic growth. Growth with nitrous oxide as sole electron acceptor mimicked aerobic growth in some respects, implying that expression of parts of the denitrification apparatus might be controlled by the redox state of a component of the electron transport chain rather than by oxygen itself. Nevertheless, the regulation of expression of nitrous oxide reductase was linked to the oxygen concentration.     

Uptake of atmospheric 15NO2 and its incorporation into free amino acids in wheat (Triticum aestivum)

Five-week-old wheat plants were exposed, under controlled environmental conditions, to 60 nl 1^?115NO₂ or to purified air. After 48 and 96 h of exposure, leaves, stalks and roots were analysed for ¹⁵N-enrichment in α-amino nitrogen of soluble, free amino acids. In addition, the in vitro nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NIR, EC 1.7.7.1) activities were determined in the leaves. NR activity in the leaves decreased continously during the 96-h exposure to purified air. In the leaves exposed to ¹⁵NO₂, NR activity increased within the first 24 h, then decreased, and reached the level of controls after 96 h. NiR activity in leaves exposed to purified air was almost constant during the 96-h exposure. In leaves exposed to ¹⁵NO₂, NiR activity increased within the first 48 h, then decreased, and reached the level of controls after 72 h, Exposure to ¹⁵NO₂ enhanced the total content of soluble, free amino acids in all tissues analysed. Most of this increase was attributed to Glu in the leaves and to Asn plus Gln the α-amino group of soluble, free amino acids was observed in the leaves, the lowest enrichment in the roots. The main labelled amino compounds were Glu (with 8.0%¹⁵N enrichment compared to the control), γ-aminobutyric acid (GABA; 7.9%), Ala (7.2%). Ser (6.8%), Asp (5.5%) and Gln (4.6%). Appreciable incorporation of ¹⁵ into Asn was not found. After 96 h exposure to ¹⁵NO₂ the ¹⁵N enrichment in the α-amino group of soluble, free amino acids in the leaves declined as compared to the values obtained after 48 h fumigation. The possible pathway and the time course of ¹⁵N incorporation into soluble, free amino acids from the ¹⁵NO₂ absorbed are discussed.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Nitrate reductase activity of plasma membranes from cultured carrot cells

Summary Cultured carrot cells (Daucus carota L.) reduced nitrate to nitrite at a slow rate (0.4 moles/g dry wt · h) without any additions to the reaction medium. This rate was doubled or tripled in presence of 100 M NADH. Ethanol and other alcohols stimulated the basal rate 8–10-fold. Isolated carrot plasma membranes also reduced nitrate to nitrite at a rate of 80 nmoles/mg protein · h. This plasma membrane-bound nitrate reductase activity was estimated to be 1.7% of the total activity. Nitrate reduction by carrot cells was inhibited 56% by sodium tungstate, 57% by potassium cyanide, and 87% by gold chloride. It was stimulated by plasma membrane electron transport inhibitors (retinoic acid and chloroquine) and ATPase inhibitors (diethylstilbestrol). From differential effects of some stimulators or inhibitors in the presence or absence of NADH, it can be implied that the nitrate reductase activity of cultured carrot cells was due to a transmembrane enzyme exhibiting an exogenous nitrate reductase activity when NADH was added.Abbreviation DMSO dimethyl sulfoxide - SHAM salicyl hydroxamic acid