Response of carbonic anhydrase to polyethylene glycol-mediated water stress in wheat

Carbonic anhydrase (CA) activity in wheat leaves changed upon leaf dehydration: it decreased at mild stress (relative water content, RWC, 81 %), but increased at severe water stress (RWC 74 %). Phosphoenopyruvate carboxylase activity was not significantly affected by these stresses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Discovery of PPi-type Phosphoenolpyruvate Carboxykinase Genes in Eukaryotes and Bacteria

Phosphoenolpyruvate carboxykinase (PEPCK) is one of the pivotal enzymes that regulates the carbon flow of the central metabolism by fixing CO₂ to phosphoenolpyruvate (PEP) to produce oxaloacetate or vice versa. Whereas ATP- and GTP-type PEPCKs have been well studied, and their protein identities are established, inorganic pyrophosphate (PP_i)-type PEPCK (PP_i-PEPCK) is poorly characterized. Despite extensive enzymological studies, its protein identity and encoding gene remain unknown. In this study, PP_i-PEPCK has been identified for the first time from a eukaryotic human parasite, Entamoeba histolytica, by conventional purification and mass spectrometric identification of the native enzyme, followed by demonstration of its enzymatic activity. A homolog of the amebic PP_i-PEPCK from an anaerobic bacterium Propionibacterium freudenreichii subsp. shermanii also exhibited PP_i-PEPCK activity. The primary structure of PP_i-PEPCK has no similarity to the functional homologs ATP/GTP-PEPCKs and PEP carboxylase, strongly suggesting that PP_i-PEPCK arose independently from the other functional homologues and very likely has unique catalytic sites. PP_i-PEPCK homologs were found in a variety of bacteria and some eukaryotes but not in archaea. The molecular identification of this long forgotten enzyme shows us the diversity and functional redundancy of enzymes involved in the central metabolism and can help us to understand the central metabolism more deeply.     

Transient impairment of the adaptive response to fasting in FXR-deficient mice

The farnesoid X receptor (FXR) has been suggested to play a role in gluconeogenesis. To determine whether FXR modulates the response to fasting in vivo, FXR-deficient (FXR^−/−) and wild-type mice were submitted to fasting for 48 h. Our results demonstrate that FXR modulates the kinetics of alterations of glucose homeostasis during fasting, with FXR^−/− mice displaying an early, accelerated hypoglycaemia response. Basal hepatic glucose production rate was lower in FXR^−/− mice, together with a decrease in hepatic glycogen content. Moreover, hepatic PEPCK gene expression was transiently lower in FXR^−/−mice after 6 h of fasting and was decreased in FXR^−/−hepatocytes. FXR therefore plays an unexpected role in the control of fuel availability upon fasting.     

Studies on γ-Radiolysis of Sulfur Containing Amino Acids

In connection with flavor deterioration accompanied by food irradiation, the effect of γ-irradiation on sulfoxide amino acids in air free aqueous system was investigated. The major radiolysis products from S-n-propyl-l-cysteine sulfoxide (PCSO) were alanine, cysteic acid, dipropyl disulfide, etc., and from S-allyl-l-cysteine sulfoxide (ACSO) were S-allyl-l-cysteine, cystine, cysteic acid, etc., which were isolated chromatographycally and identified by using IR and mass spectrometry. The sulfoxide in ACSO was more easily reduced to sulfide than that of in PCSO, and the bond of S-C (β-carbon in alanine moiety) in ACSO was difficult to cleave. These differences observed between PCSO and ACSO in the radiolysis products and their yields indicate that the radiolysis degradation is considerably influenced by the structure of alkyl group. From the experiments with N₂O or KBr addition during irradiation, principal roles of the active species in irradiated water in the degradation processes were partly elucidated.     

马齿苋叶片PEP羧化酶的分子特性

马齿苋叶片PEPCase由四个相同的亚基组成,亚基分子量为83kD。远紫外CD光谱分析表明,此酶含有36.6％α—螺旋结构。马齿苋叶片PEPCase可被G6P激活,但不能被Gly、Ser激活。G6P可防止酶的尿素变性和枯草杆菌蛋白酶的作用。这种保护效应与G6P诱导的酶构象变化有关。 从酶对低温、高温及尿素的反应来看,马齿苋叶片PEPCase的稳定性高于高粱叶片PEP—Case,两者的免疫特性和电泳特性亦不同。     

Carbon: terrestrial C4 plants

The carbon isotope composition of terrestrial C₄ plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO₂ to the carboxylation sites, but is also influenced by the final carboxylation of ribulose-1,5-bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C₄ photosynthesis. As a new feature, the model considers four types of limitation of the overall CO₂ assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO₂, irradiance and water stress on short-term discrimination by leaves of several C₄ species measured simultaneously with CO₂ gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C₄ plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C₄ plants, evolutionary and genetical aspects, and some ecological implications are reviewed.     

The photosynthetic characteristics of differently shaped leaves in Populus euphratica Olivier

There is a distinct leaf shape polymorphism within a single plant of P. euphratica Olivier. The anatomical structure, carbon isotope discrimination (Δ¹³C), and stomatal and photosynthetic behaviour were investigated in broad-ovate (BOL) and lanceolate (LL) leaves, located at the top and bottom in crown, respectively, of a mature Euphrates poplar growing in its native habitat. Both types of leaves had a non-Kranz anatomy and low Δ¹³C values. However, Δ¹³C of a LL was in average 3.2‰ larger than that of a BOL. In comparison with the LL, the BOL had a smaller stomatal conductance, causing subsequent decreases in transpiration rate and ratio of CO₂ concentrations in intercellular spaces to air. Carbon assimilation rate and water use efficiency were higher in the BOLs than in the LLs. The BOL exhibited C₄-like enzymological features, the activity of glycollate oxidase, and the ratio of activities of ribulose-1,5-bisphosphate carboxylase (RuBPC) to phosphoenolpyruvate carboxylase (PEPC) was lower in BOL than in LL throughout the whole growing season. The lowered ratio of RuBPC/PEPC in BOL was mainly associated with a marked decline in the activity of RuBPC, and only a slight increase in the activity of PEPC. These differences might contribute to microclimate adaptation in both types of leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Affinity labeling of phosphoenolpyruvate carboxykinase with 1, 5-I-AEDANS.

The concentrations of zinc thionein and cytosolic zinc in rat liver were examined in male rats five days after bilateral adrenalectomy. Zinc in metallothionein increased 10 fold, as compared with control animals. Cytosolic zinc increased 79% as compared with controls. 65% of this increase could be accounted for bound to metallothionein. Sham operated animals after five days showed a 4 fold increase in hepatic zinc thionein and a 23% increase in cytosolic zinc, 71% of this increase being bound to metallothionein. Adrenalectomized rats, maintained on daily injections of corticosterone (4mg/100g b.w.), exhibited the same levels of zinc thionein and cytosolic zinc as adrenalectomized rats receiving no treatment. Adrenalectomized rats, maintained on daily injections of aldosterone (5μg/100g b.w.), exhibited the same levels of zinc thionein as the sham operated rats, but the cytosolic zinc remained elevated at the level found in adrenalectomized rats receiving no treatment. These results indicate that there is adrenal involvement in the control of hepatic zinc and zinc thionein levels in the rat.