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161.
Most terrestrial animals face the challenge of having to conserve water in a desiccating environment. Not surprisingly, the ability to produce concentrated urine has been relatively well studied in birds. Nectar-feeding birds are unusual among terrestrial animals in that they often ingest and excrete prodigious water volumes to obtain adequate energy. Thus, they confront the unusual challenge of having to conserve electrolytes. The diluting abilities of birds and the renal mechanisms that may correlate with them have been relatively neglected. To elucidate diluting and concentrating abilities in nectar-feeding birds, we fed rufous hummingbirds Selasphorus rufus an electrolyte-free nectar and a nectar containing a range of NaCl concentrations. Hummingbirds had a spectacular (and possibly unique) diluting ability: when fed on electrolyte-free food they produced excreta containing less than 0.5 mM l−1 each of sodium and potassium. Hummingbirds also had a poor concentrating ability, retaining sodium and chloride when their food (0.632 M l−1 sucrose) contained more than 35 mM l−1 of NaCl. The kidneys of hummingbirds do not appear to be suited for concentrating urine, and possibly contain structural features that give them a unique diluting ability compared with those of birds that do not feed on nectar. 相似文献
162.
Carlos García-Echeverría 《Letters in Peptide Science》1996,2(6):369-373
Summary In order to minimise the formation of the pyrophosphate derivative of the target peptide when side-chain-unprotected phopshotyrosine is used in solid-phase peptide synthesis, this building block can be incorporated using benzotriazolyloxy-tris-(dimethylamino)phosphonium hexafluorophosphate/1-hydroxybenzotriazole/N-methylmorpholine (1:1:2.3) in the presence of a chaotropic salt (0.4 M LiCl in N-methyl-2-pyrrolidinone).Abbreviations BOP
benzotriazolyloxy-tris-(dimethylamino)phosphonium hexafluorophosphate
- DIEA
diisopropylethylamine
- Fmoc
9-fluorenylmethoxycarbony
- HATU
N-[(dimethylamino)1H-1,2,3-triazolo[4,5-b]pyridin-1-ylmethylene]-N-methylmethan-aminium hexafluorophosphate N-oxide
- HOBt
1-hydroxybenzotriazole
- HPLC
high-performance liquid chromatography
- MALDI-TOF
matrix-assisted laser-desorption ionization time-of-flight mass spectrometry
- NMM
N-methylmorpholine
- NMP
N-methyl-2-pyrrolidinone
- Pmc
2,2,5,7,8-pentamethyl-chroman-6-sulfonyl
- ®
solid support
- TFA
trifluoroacetic acid
- TPTU
2-(2-pyridon-1-yl)-1,1,3,3-tetramethyluroniumfluoroborate. Abbreviations used for amino acids follow the recommendations of the IUPAC-IUB Commission of Biochemical Nomenclature [Eur. J. Biochem., 138 (1984) 9] 相似文献
163.
Summary Triple-resonance experiments can be designed to provide useful information on spin-system topologies. In this paper we demonstrate optimized proton and carbon versions of PFG-CT-HACANH and PFG-CT-HACA(CO)NH straight-through triple-resonance experiments that allow rapid and almost complete assignments of backbone H, 13C, 15N and HN resonances in small proteins. This work provides a practical guide to using these experiments for determining resonance assignments in proteins, and for identifying both intraresidue and sequential connections involving glycine residues. Two types of delay tunings within these pulse sequences provide phase discrimination of backbone Gly C and H resonances: (i) C–H phase discrimination by tuning of the refocusing period a_f; (ii) C–C phase discrimination by tuning of the 13C constant-time evolution period 2Tc. For small proteins, C–C phase tuning provides better S/N ratios in PFG-CT-HACANH experiments while C–H phase tuning provides better S/N ratios in PFG-CT-HACA(CO)NH. These same principles can also be applied to triple-resonance experiments utilizing 13C-13C COSY and TOCSY transfer from peripheral side-chain atoms with detection of backbone amide protons for classification of side-chain spin-system topologies. Such data are valuable in algorithms for automated analysis of resonance assignments in proteins. 相似文献
164.
Alfonso Pompella Caterina Cambiaggi Silvia Dominici Aldo Paolicchi Roberto Tongiani Mario Comporti 《Histochemistry and cell biology》1996,105(3):173-178
Confocal laser scanning fluorescence microscopy coupled to image analysis was employed in order to develop and evaluate procedures for the appraisal at the single-cell level of: (1) protein-bound 4-hydroxynonenal, the specific product of membrane peroxidation (by means of immunocytochemistry with biotin-avidin revelation); (2) protein oxidation (by reaction of protein carbonyls with 2,4-dinitrophenyl-hydrazine followed by immunocytochemistry of dinitrophenyl moieties); and (3) cellular protein thiols (by direct alkylation of sulfhydryl groups with thiol-specific fluorescent reagents possessing different cell permeabilities). The procedures proved able to reveal the subcellular distribution of cytochemical parameters useful as indices of oxidative stress conditions, and may allow redox phenotyping of isolated cells, which would provide an efficient tool in selected experimental models. 相似文献
165.
Alfonso Pompella Aldo Paolicchi Silvia Dominici Mario Comporti Roberto Tongiani 《Histochemistry and cell biology》1996,106(3):275-282
A number of studies indicate that cell proliferation can be modulated by changes in the redox balance of (soluble and protein)
cellular thiols. Free radical processes, including lipid peroxidation (LPO), can affect such a balance, and a role for LPO
in multistage carcinogenesis has been envisaged. The present study was aimed to assess the relationships between the protein
thiol redox status and the LPO process in chemically induced preneoplastic tissue. The Solt-Farber's initiation-promotion
model of chemical carcinogenesis in the rat liver was used. In fresh cryostat sections, preneoplastic lesions were identified
by the reexpression of γ-glutamyltranspeptidase (GGT) activity. In serial sections, different classes of protein thiols were
stained; in additional sections, LPO was elicited by various prooxidant mixtures and determined thereafter by the hydroxynaphthoic
hydrazide-Fast Blue B procedure. The incubation of sections in the presence of chelated iron plus substrates for GGT activity
leads to the development of LPO in selected section areas closely corresponding to GGT-positive lesions, indicating the ability
of GGT activity to initiate LPO. Protein-reactive thiols, as well as total protein sulfur, were decreased by 20–25% in cells
belonging to GGT-positive preneoplastic nodules, suggesting the occurrence of oxidative conditions in vivo. The incubation
of additional adjacent sections with the prooxidant mixture H2O2 plus iron(II), in order to induce the complete oxidation of lipid present in the section, showed a decreased basal concentration
of oxidizable lipid substrate in GGT-rich areas. The decreased levels of both protein thiols and lipid-oxidizable substrate
in GGT-positive nodules suggest that the observed GGT-dependent path-way of LPO initiation can be chronically operative in
vivo during early stages of chemical carcinogenesis, in cells expressing GGT as part of their transformed phenotype. 相似文献
166.
Ricardo O. Louro Teresa Catarino Carlos A. Salgueiro Jean LeGall António V. Xavier 《Journal of biological inorganic chemistry》1996,1(1):34-38
Using potentiometric titrations, two protons were found to participate in the redox-Bohr effect observed for cytochrome c
3 from Desulfovibrio vulgaris (Hildenborough). Within the framework of the thermodynamic model previously presented, this finding supports the occurrence
of a concerted proton-assisted 2e– step, ideally suited for the coupling role of cytochrome c
3 to hydrogenase. Furthermore, at physiological pH, it is shown that when sulfate-reducing bacteria use H2 as energy source, cytochrome c
3 can be used as a charge separation device, achieving energy transduction by energising protons which can be left in the acidic
periplasmic side and transferring deenergised electrons to sulfate respiration. This mechanism for energy transduction, using
a full thermodynamic data set, is compared to that put forward to explain the proton-pumping function of cytochrome c oxidase. 相似文献
167.
Paula Veríssimo Cristina Esteves Carlos Faro Euclides Pires 《Biotechnology letters》1995,17(6):621-626
The flowers of cardoon (genus Cynara) are traditionally used in Portugal for cheese making. In this work the vegetable rennet of the species Cynara cardunculus L. was characterized in terms of enzymic composition and proteolytic specificity of its proteinases (cardosin A and cardosin B). Cardosin A was found to cleave insulin B chain at the bonds Leu15-Tyr16, Leu17-Val18 and Phe25-Tyr26. In addition to the bonds mentioned cardosin B cleaves also Glu13-Ala14, Ala14-Leu15 and Phe24-Phe25 indicating that it has a broader specificity. The kinetic parameters for the hydrolysis of the synthetic peptide Leu-Ser-Phe(NO2)-Nle-Ala-Leu-oMe were also determined and compared to those of chymosin and pepsin. The results obtained indicate that in terms of specificity and kinetic parameters cardosin A is similar to chymosin whereas cardosin B is similar to pepsin. It appears therefore that the enzyme composition of cardoon rennet closely resembles that of calf rennet. 相似文献
168.
We examined the effects of the conversion of tropical forest to pasture on soil organic matter (SOM) origin and quality along
a chronosequence of sites, including a primary forest and six pastures. Bulk soil samples received a physical size-fractionation
treatment to assess the contribution of each compartment to total SOM pool. Besides a general increase in total C and N stocks
along the chronosequence, we observed a reduction of the relative contribution of the coarser fractions to total soil C content,
and an increased concentration in the finer fractions. The origin of the C in each size fraction was established from measurements
of13C abundance. After 80 years about 93% of the C in the least humified fraction of the top 10 cm of soil was of pasture origin,
while in the most humified it was 82%. Chemical analyses indicated that the fine silt and coarse clay fractions contained
the most refractory carbon. 相似文献
169.