Influence of partial defoliation of green pepper on the senescence,growth, and nitrate reductase of the remaining leaf

Summary Removal of all but one leaf from pepper plants prevented the senescence of the remaining leaf and caused increases of approximately 140, 200, and 200%, respectively in leaf area, weight, and nitrate reductase activity. Development of the fruit (fresh and dry weight increases) was only approximately 65% of that of fruit on control plants.     

Effects of seasonal changes of soil salinity and soil nitrogen on the N-metabolism of the halophyteArthrocnemum fruticosum (L.) Moq.

Summary The N-metabolism ofArthrocnemum fruticosum (L.) Moq., growing in a saline area north-east of the Dead Sea in Jordan, was studied over its vegetative growth period from March to September 1981. Plant and soil samples were taken at monthly intervals. Water content, Na⁺, K⁺, Cl⁻, NH ₄ ⁺ , NO ₂ ⁻ and NO ₃ ⁻ concentrations were determined in the soil extracts, and the same determinations plus ash weight, soluble carbohydrates, proline, proteins andin vivo nitrate reductase in the plant roots and shoots. Soil humidity decreased and salinity increased from March to August, with re-wetting occurring in late July. K⁺ and Cl⁻ were much lower in the soils than Na⁺. Plant relative dry weight increased during summer due to the absorption of Na⁺ in addition to increased organic dry weight. The uptake of Na⁺ was not balanced by a similar uptake of Cl⁻. Ammonium and nitrate decreased in soil and plants in parallel with increasing salinity. Nitrite was only found in the roots and always in very low quantities. Proline was found only in March. The total soluble carbohydrates in the roots showed a short increase in June when the sodium in the plants also increased. It was concluded that carbohydrates may be used to balance osmotic shocks, but that another compatible compounds is necessary to maintatin long-term osmotic equilibrium. The nitrate reductase activity, measuredin vivo, and the soluble protein changed roughly in parallel with the internal nitrate from May to August, suggesting that nitrogen uptake and reduction in the plant is inhibited during summer when the soil is dry and very saline. This could be a direct effect of drought and/or salinity on the plants, or an indirect onevia an inhibition of nitrifying bacteria.     

Amino acid sequence of COOH-terminal 20K Da fragment from pig liver microsomal NADPH-cytochrome P-450 reductase

We have determined the complete amino acid sequence of a 20K Da COOH-terminal fragment of porcine NADPH-cytochrome P-450 reductase. The 20K Da fragment is probably produced by a proteolytic cleavage of the intact protein in porcine liver microsomes, and since the cleavage does not affect enzymatic activity, the fragment has been studied as a distinct domain. The sequence comprises 175 amino acids including three cysteine residues, one of which has been previously identified as protected by NADPH from S-carboxymethylation. The NADPH-protected cysteine lies in a stretch of 12 residues with partial homology to glutathione reductase, and is adjacent to a hydrophobic region containing a glycine-rich stretch homologous to other FAD-containing proteins. The predicted secondary structure over this entire region is beta-sheet/beta-turn/beta-sheet/alpha-helix/beta-sheet/beta-turn/alpha-h elix corresponding to hydrophobic residues 21-28/glycine-rich residues 29-33/residues 34-38/residues 39-54/residues 56-61/NADPH-protected cysteine residues 62-78/residues 71-82. It is possible that the 20K Da domain provided a significant portion of the sequence responsible for binding FAD and NADPH in the intact enzyme. This data provides a basis for further active site studies.     

The effect of Cd2+ on the biosynthesis of chlorophyll in leaves of barley

The effect of cadmium on the biosynthesis of chlorophyll has been investigated in the leaves of dark-grown seedlings of barley ( Hordeum vulture L. cv. Proctor). Cd2+ inhibited the production of chlorophyll by affecting 1) the synthesis of 5-aminolacvulinic acid and 2) the protoehlorophyllide reductase ternary complex with its substrates. Cd2+ had no effect on the constituent enzymes that catalyse the synthesis of free protoehlorophyllide from 5-aminolaevulinic acid. The results obtained are consistent with Cd2+ inhibiting the formation of chlorophyll by reacting with essential thiol groups in both the protochlorophyllide reductase protein and the enzyme(s) involved in the light dependent synthesis of 5-aminolaevulinic acid.     

Detection of antimycin-binding subunits of Complex III by photoaffinity-labeling with an azido derivative of antimycin

Deformamidoazidoantimycin A (DAA), a photoactive derivative of antimycin A containing an azido group substituting for the formamido group attached to the phenyl ring, was synthesized. The ultraviolet spectrum of DAA was almost identical to that of antimycin A, indicating little alteration of the electronic structure of the substituted phenyl ring by the azido substitution. However, the inhibitory effectiveness of DAA toward ubiquinol-cytochromec reductase (Complex III) purified from bovine heart (K _i =ca. 0.5 µM) was considerably less than that of antimycin (K _i 3 pM), indicating a direct rather than a supporting role of the formamido group in the inhibitory activity of antimycin. Exposure of purified Complex III to [³H]DAA plus ultraviolet light caused a major labeling by tritium of SDS-PAGE band 7 (m=13 kDa by SDS-PAGE) and lesser but significant labeling of bands 3, 6, 8, and 9. Pretreatment of Complex III with antimycin greatly suppressed the labeling of bands 5, 6, and 7 but caused an apparent increased labeling of bands 8 and 9 by [³H]DAA, respectively. The labeling of band 7 by [³H]DAA also was strongly suppressed by reduction of Complex III by either sodium borohybride or ascorbate. Based on magnitude of labeling by [³H]DAA and the degree of suppression of labeling by antimycin, the protein of band 7 qualified as the principal component for specific binding of antimycin with the protein of band 6 (m=16 kDa) showing a lesser but significant amount of specific binding.     

The presence and photoregulation of protochlorophyllide reductase in green tissues

Summary The enzyme protochlorophyllide (pchlide) reductase has been identified amongst the peptides, resolved by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), of chloroplast membranes from oat and barley plants. In support of this identification the enzymic activity associated with the enzyme has also been measured in the same preparations. A higher level of enzyme was found in plants which had been darkened prior to extraction. Based on this data, mechanisms for the light regulated diurnal variation of the reductase are discussed.     

Redox interconversion of Escherichia coli glutathione reductase. A study with permeabilized and intact cells

Summary The redox interconversion of Escherichia coli glutathione reductase has been studied both in situ, with permeabilized cells treated with different reductants, and in vivo, with intact cells incubated with compounds known to alter their intracellular redox state.The enzyme from toulene-permeabilized cells was inactivated in situ by NADPH, NADH, dithionite, dithiothreitol, or GSH. The enzyme remained, however, fully active upon incubation with the oxidized forms of such compounds. The inactivation was time-, temperature-, and concentration-dependent; a 50% inactivation was promoted by just 2 M NADPH, while 700 M NADH was required for a similar effect. The enzyme from permeabilized cells was completely protected against redox inactivation by GSSG, and to a lesser extent by dithiothreitol, GSH, and NAD(P)⁺. The inactive enzyme was efficiently reactivated in situ by physiological GSSG concentrations. A significant reactivation was promoted also by GSH, although at concentrations two orders of magnitude below its physiological concentrations. The glutathione reductase from intact E. coli cells was inactivated in vivo by incubation with DL-malate, DL-isocitrate, or higher L-lactate concentrations. The enzyme was protected against redox inactivation and fully reactivated by diamide in a concentration-dependent fashion. Diamide reactivation was not dependent on the synthesis of new protein, thus suggesting that the effect was really a true reactivation and not due to de novo synthesis of active enzyme. The glutathione reductase activity increased significantly after incubation of intact cells with tert-butyl or cumene hydroperoxides, suggesting that the enzyme was partially inactive within such cells. In conclusion, the above results show that both in situ and in vivo the glutathione reductase of Escherichia coli is subjected to a redox interconversion mechanism probably controlled by the intracellular NADPH and GSSG concentrations.     

高等植物叶片中的硝酸还原酶活性稳定因子

从小麦、油菜、浮萍、番茄、烟草的叶片中分离得到NR-SF。不同植物材料中NR及NR-SF能起交叉反应;不同NR-SF影响NR酶动力学性质相同;不同NR-SF的凝胶电泳谱带显示蛋白和糖蛋白性质。NR-SF广泛存在于植物细胞中。     

Comparative studies of diurnal variations of nitrate reductase activity in wheat, oat and barley

Diurnal variations of in vitro and in vivo (intact tissue assay) nitrate reductase (EC 1.6.6.1) activity and stability were examined in leaves of wheat ( Triticum aestivum L. cv. Runar), oat ( Avcna saliva L. cv. Mustang) and barley ( Hordeum vulgure L. cv. Agneta and cv. Gunillu). Nitrate reductase activity was generally higher for wheat than for oat and barley. However, the diurnal variations of nitrate reductase activity and stability were principally the same for all species, e.g. the high activity during the photoperiod was associated with low stability. All species showed a rapid (30-60 min) increase in the in vitro and in vivo activity when the light was switched on. When light was switched off the in vitro activity decreased rapidly whereas decrease in in vivo activity was slower. These experiments support the hypothesis that an activation/ deactivation mechanism is involved in the regulation of diurnal variations in nitrate reductase activity. Red light enhanced nitrate reductase activity in etiolated wheat and barley leaves. In green leaves, however, the daily increase in nitrate reductase activity was not induced by a brief red light treatment. Indications of different regulation mechanisms for the diurnal variations of nitrate reductase activity among the cereals were not found.