bluuebunny/crossref_metadata_2025_split huggingface dataset:
Adaptive Changes in Satellite Systems Related to Lipogenesis in Rat and Sheep Mammary Gland and in Adipose Tissue
Authors: Gumaa Khalid A. , Greenbaum A. Leslie , Mclean Patricia 
Published: March 2005

The activities of a range of enzymes from the glycolytic and pentose phosphate pathways, tricarboxylic acid cycle and lipogenic pathways have been measured in the mammary glands of rats at different stages of the lactation cycle and in the lactating sheep mammary gland.
Changes of the enzyme activities are considered in the light of constant and specific proportion enzymes; in particular, in relation to the alternative systems for the generation of NADPH in the cytosol.
Specifically considered are the metabolic transitions in the rat at parturition and weaning; comparison between lactating sheep and rat mammary gland (which manufacture a similar spectrum of fatty acids from different carbon sources) and a comparison between lactating rat mammary gland and rat adipose tissue.
Evidence is adduced from enzyme profiles and flux data that the major source and pathway of generation of NADPH differed in the three tissues, an adaptation dependent upon carbon source and over‐all energy requirement of the tissue. Specifically, rat mammary gland has a greater emphasis upon NADPH formation via the pentose phosphate pathway, sheep mammary gland from cytosolic isocitrate dehydrogenase (combined with glutamate dehydrogenase acting as a transhydrogenase system) and adipose tissue showing participation of both pentose phosphate pathway and ‘malic’ enzyme.
The adaptation of mammary tissue to the use of the pentose phosphate pathway or of isocitrate dehydrogenase confers considerable advantage in the ATP yields relative to adipose tissue, where partial generation of NADPH via the ‘malic’ enzyme necessitates an ATP‐consuming reaction.
The modification in other satellite systems of lipid synthesis, such as pyruvate carboxylase, citrate cleavage enzyme and glycerol‐1‐phosphate dehydrogenase, is linked to the relative emphasis on different hydrogen‐generating systems, to the source of carbon and to the function of the tissue as a secretory or storage and release tissue.
Genetic Variants of Human Erythrocyte Glucose‐6‐Phosphate Dehydrogenase
Authors: Cancedda Ranieri , Ogunmola Gabriel , Luzzatto Lucio 
Published: March 2005

Analysis of thermal inactivation profiles of human erythrocyte glucose‐6‐phosphate dehydrogenase, type B, reveals three distinct forms of the enzyme. The form with the lowest stability is observed at very low NADP+ concentration, and probably consists of the monomeric molecular species. As the NADP+ concentration is increased, the enzyme undergoes two successive transitions to forms of higher stability, which probably consist of two different conformational states of the dimer, in equilibrium with tetramer. Enzyme type A behaves similarly, but may have less tendency to monomer formation.
NADPH, like NADP+, is competent to stabilize the enzyme under suitable conditions and to induce dimer formation, but probably not to induce formation of the dimer with the highest thermostability. NADPH nay be integral part of the enzyme structure.
Stopped‐flow experiments suggest that the monomer of glucose‐6‐phosphate dehydrogenase is catalytically inactive, but that it recovers activity within 100 ms when exposed to a high concentration of NADPH+. By contrast, with the enzyme in dimeric form, the reaction starts immediately upon addition of glucose‐6‐phosphate.
Protonated Polynucleotide Structures
Authors: Sarocchi Marie‐Thérèse , Guschlbauer Wilhelm 
Published: March 2005

Circular dichroism and absorbance‐difference spectra of polydeoxynucleotides and DNAs of various base composition have been measured at different submelting temperatures. The following results were obtained.

The changes observed by both techniques correlate poorly with (G + C) content, but follow the normalized YpY, i.e. (fCpC+fTpT)/xG+C, and not the mixed RpY, i.e. (fApT+fTpA)/yA+T, nearest‐neighbour frequencies.
Small pH changes have considerable effect on the difference spectra.
The metastable protonated form of poly(dG) · poly(dC) Correlates better with the circular dichroism spectra than the non‐protonated form.
It is concluded that dC clusters may be at the origin of DNA premelting, especially if they are in the metastable protonated form.
The Problem of Cell Heterogeneity of Liver Tissue in the Study of Fructose Metabolism
Authors: Zehner Jürgen , Loy Eberhard , Müllhofer Gerd , Bücher Theodor 
Published: March 2005

Livers of rats fasted for 48 h were perfused with [2‐14C]fructose, [6‐14C]fructose or [U‐14C]‐fructose in a non‐recirculating perfusion system.From the 14C‐labeling patterns of the metabolites glucose and lactate, which were isolated from the effluent perfusate, as well as from the labeling patterns of lactate, phosphoglycerate and phosphoenolpyruvate, which were separated from the tissue extract, the following conclusions were drawn:

The 14C activity of lactate in the perfusate leaving the liver is almost independent of the position of 14C label in the fructose, which was infused. This indicates that this lactate originates from a metabolic pathway in which both triose units of fructose are converted to lactate. On the other hand the 14C activity of cellular lactate depends on the position of the 14C label in the infused fructose. This lactate seems to be formed only from the C4‐C5‐C6 unit of the fructose. These two pathways of fructose metabolism are postulated to be located in different cell compartments of liver tissue and are initiated by phosphorylation of fructose at carbon position 6 and 1, respectively. We refer to them as the “glycolytic” and “gluconeogenic” compartments.
In experiments with [2‐14C]fructose there is no correlation between the specific 14C activities of phosphoglycerate, phosphoenolpyruvate and intracellular lactate indicating compartmentation of these metabolites.
By comparing the specific 14C activities of the C4‐C5‐C6 triose unit of newly formed glucose (taken as representative of the specific 14C activity of glyceraldehyde‐3‐phosphate in the “gluconeogenic” compartment) with the specific 14C activity of cellular lactate, the following can be postulated. Glyceraldehyde is transformed into lactate either by a pathway not involving glyceraldehyde‐3‐phosphate as an intermediate or a glyceraldehyde‐3‐phosphate which is spatially separated from the intermediate glyceraldehyde‐3‐phosphate pool of gluconeogenesis. The measured high tissue levels of 2‐phosphoglycerate in our experiments support the first concept.
In experiments with [2‐14C]fructose, the specific 14C activity of newly formed glucose corresponds to the 14C activity of fructose infused, while in experiments with [6‐14C]fructose a high dilution of the 14C activity of glucose was observed. This dilution must originate from a crossing over of some intermediate metabolite in the conversion of fructose to glucose with the inactive intermediate metabolite of some other metabolic pathway. An interaction with the citric acid cycle as possible source of this dilution was excluded.
Studies on the Conformation of Yeast Pyruvate Decarboxylase in Solution by Small‐Angle X‐Ray Measurements
Authors: Pilz Ingrid , Ullrich Johannes 
Published: March 2005

Pyruvate decarboxylase, holo‐ and apo‐enzyme, was studied by X‐ray small‐angle scattering.A concentration series of the holo‐enzyme in 50 mM sodium phosphate buffer pH 6.8 was measured. The solutions were homodisperse and the following data could be determined: radius of gyration = 4.13 nm, molecular weight = 185000 (partial specific volume 0.751), volume = 400 nm3, hydration = 0.55 g H2O/g protein. The overall shape of the holo‐enzyme can be described by an elongated body (ellipsoid of revolution or double sphere) with an axial ratio of about 1:1:2.2.From the apo‐enzyme no homodisperse solutions could be obtained. There was always a mixture of particles, which had nearly the same radius of gyration as the holo‐enzyme, and larger aggregates. Therefore no detailed evaluation was possible. The partial specific volume was found to be identical with that of the holo‐enzyme within the limits of error.
The Solution Conformation and Some Spectroscopic Properties of 1,N6‐Ethenoadenosine Monophosphate, a Fluorescent Analogue of AMP
Authors: Penzer Geoffrey R. 
Published: March 2005

The conformation of ethenoadenosine monophosphate in aqueous solution at pH 2.0 has been shown to be very similar to that of AMP through use of the lanthanide‐shift proton‐magnetic‐resonance technique.
The pKa, for protonation of the 9‐position of etheno‐AMP is 4.3 at 298 K, but at all pH values in the range 1.0–9.6, fluorescence is emitted from the protonated form of the heterocyclic ring system. This conclusion is based on the pH, temperature and KI dependence of the emission spectrum.
When dissolved in ethanol the quantum yield of etheno‐AMP fluorescence falls to about 40% of its value in water.
There are unlikely to be large changes in the energies of the emissions of ethenoadenosine derivatives when bound to biological structures, but their quantum yields may well be reduced, especially if binding affects the ease with which the rings can be protonated.
Functional Behaviour of Transferrin
Authors: Hahn Dietrich 
Published: March 2005

Radioactive iron was preferentially bound in vitro to the two functionally different binding sites of transferrin, referred to as hemoglobin‐orientated and nonhemoglobin‐orientated binding site, respectively. Its availability from these binding sites for different organs was tested in vivo in rats.To distinguish different iron uptake characteristics by different organs from the two binding sites of the injected transferrin, the normally high percentage of the erythropoiesis‐related iron clearance was cut down to a level similar to the degree of radioactive iron clearance to the storing organs liver and spleen by a hypertransfusion‐induced polycythemia.It was observed that the iron from the hemoglobin‐orientated binding site was taken up predominantly by bone marrow and intestine, while liver and spleen had a higher affinity to iron bound to the nonhemoglobin‐orientated binding site, thus indeed pointing out a different function of the two binding sites.
The Kinetics of the Redox Reactions of Ubiquinone Related to the Electron‐Transport Activity in the Respiratory Chain
Authors: KrÖGER Achim , Klingenberg Martin 
Published: March 2005

The role of ubiquinone (Q) in the respiratory chain is quantitatively analyzed by correlating both the steady‐state redox level of Q and the kinetics of oxidation and reduction with the flux of the overall electron transport in uncoupled submitochondrial particles. This is achieved by experimentally defining donor and acceptor activities for Q.

The degree of reduction of Q in the steady state is proportional to the respiratory activity with NADH and succinate, if the respiratory activity is varied by titrating the donor side for Q with rotenone and malonate, respectively. The proportionality constant (acceptor activity, Vox) is independent of the substrate used.
The degree of oxidation of Q in the steady state is proportional to the respiratory activity as varied by titration of the acceptor side for Q with antimycin. The proportionality constant (donor activity, Vred) is independent of the acceptor activity and depends on the dehydrogenase activity for NADH and succinate.
From these experimental relations, the redox state of Q in the steady state and the respiratory activity can be described as functions of the donor and the acceptor activity only. The equations are valid with both NADH and succinate as the substrates.
The kinetics of oxidation of Q on the addition of oxygen as measured by the quench‐flow method are in agreement with that measured by direct absorption recording in a mixing chamber. The reaction is first order with a rate constant equal to the acceptor activity divided by the amount of redox‐active Q (Vox/Qa). The final steady‐state level (in the present case 15% reduction) is a result also of the reduction reaction with the first‐order rate constant, VredQa.
The acceptor activity can also be measured as maximum respiratory activity with duro‐hydroquinone. This activity is independent of the presence of Q but sensitive to antimycin. Thus, the acceptor activity for ubiquinone can be measured by three independent methods.
It is concluded, that also in the steady state the reduction and oxidation of the active Q‐pool follow pseudo‐first‐order reactions, the rates of which are equal to the respiratory rate. The total amount of redox‐active Q is kinetically and functionally homogeneous and is not divided into substrate‐specific compartments.
The Steric Course with Respect to the Reduced Nicotinamide‐Adenine Dinucleotide of the Reduction of 3‐Oxo Steroids Catalysed by Cortisone Reductase
Authors: Gibb William , Jeffery Jonathan 
Published: March 2005

The stereochemistry of hydrogen transfer from NADH during the reduction of the 3‐oxo groups of various steroids by cortisone reductase has been investigated for a series of substrates showing interesting structural features or reaction characteristics. It was found that the 4B‐hydrogen of the NADH was transferred to the substrate in all cases whether the resulting hydroxyl group was 3α or 3β. The 4B‐hydrogen was also transferred when the same enzyme catalysed the reduction of the 20‐oxo group of pregn‐4‐ene‐3,20‐dione, so, although the enzyme can bind steroids allowing hydrogen transfer in various ways, it binds NADH in such a way that only the 4B‐hydrogen is available for catalytic transfer to any considerable extent. No reaction was detected with NADPH.The reaction rate with NADH was lowered by the presence of NAD+ in the two cases tested and, in the one case in which the reaction was studied in the reverse direction, NADH lowered the rate of reaction with NAD+. In all three cases, the apparent V values did not change significantly, whereas the apparent Km values for the reacting nucleotide changed considerably (“competitive inhibition”). This was consistent with an ordered reaction sequence in which the nucleotide bound before and left after the steroid, but these findings did not eliminate other possibilities.
Rat‐Liver‐Sialidase Activity Utilizing a Tritium‐Labeled Sialic‐Acid Derivative of Glycoprotein Substrates
Authors: Bernacki Ralph J. , Bosmann H. Bruce 
Published: March 2005

Plasma glycoproteins were oxidized with periodic acid and reduced with tritiated KBH4, resulting in macromolecules specifically labeled with tritium on their sialic acid residues. The resulting tritiated seven‐carbon analog of N‐acetylneuraminic acid, abbreviated [3H]AcNeu, covalently attached as part of the fetuin molecule, [3H‐AcNeu]fetuin, had a specific activity of 3.67 counts × min−1× ng−1. It served as a very suitable substrate for the measurement of bacterial and mammalian neuraminidase. Clostridium perfringens neuraminidase had a Km of 9 μM and a V of 50 counts × min−1× h−1× ng−1. [3H‐AcNeu]Fetuin was homogeneous on polyacrylamide gel electrophoresis and migrated exactly as native fetuin following treatment by sodium dodecyl‐sulfate, mercaptoethanol, and boiling. Mild acid hydrolysis removed 70% of the radioactivity, which chromatographed as a homogeneous product utilizing butylacetate, acetic acid and water (3:2:1, v/v/v) as the solvent system.Mammalian neuraminidase was located in a particulate fraction of rat liver. The enzyme had an acid pH optimum and may be lysosomal in origin. The use of [3H‐AcNeu7]glycoprotein substrates increased the sensitivity of measurement of the released sialic acid and provided a more reliable estimate of neuraminidase levels in mammalian tissue sources.Livers from rats fed a vitamin‐K‐deficient diet were also assayed for activity. The results indicated that vitamin‐K‐deficient animals had a slightly elevated amount of neuraminidase activity with [3H‐AcNeu7]prothrombin substrate. Prothrombin, unlabeled or labeled, was not extensively degraded by Cl. perfringens neuraminidase. Therefore, either differences exist between bacterial enzyme and mammalian enzyme in the substrate specificities, or the mammalian enzyme source may contain more than one neuraminidase, i.e. one capable of the degradation of the sialic acid residues of fetuin and another capable of the degradation of the sialic acid residues of prothombin.
Internal pH, ΔpH, and the Kinetics of Electron Transport in Chloroplasts
Authors: Bamberger Elchanan S. , Rottenberg Hagai , Avron Mordhay 
Published: March 2005

The relation between the rate of electron transport, ΔpH, and internal pH was studied in lettuce chloroplasts, by following, in parallel, the fluorescence quenching of 9‐aminoacridine and the rate of electron transport. At low external pH (< 8.0) there was an initial burst in the rate of electron transport (R1) followed by a slower steady‐state rate (R2). However, at high external pH (> 8.5) R1 was slower than R2. Nevertheless, when analyzed as a function of internal pH, both R1 and R2 showed the same dependence with a maximal rate observed around internal pH 5.0. Thus, at low external pH, the initial pH is initially around 5.0 and, therefore, R1 is high, but as ΔpH is increased, the internal pH falls below 5.0 and R2 is lower. At high external pH, the internal pH is initially above 5.0 and, therefore, R1 is relatively low. As ΔpH increases the internal pH is lowered toward 5.0 and R2 becomes faster. A similar conversion of the ratio R1/R2 from R1/R2 > 1 to R1/R2 < 1 was also observed at constant external pH by varying the light intensity, or concentration of the uncoupler nigericin.In the presence of nigericin or at low light intensities, the maximal rate was shifted to higher internal pH values. At internal pH values above 5.0 in the presence of nigericin, the rate of electron transport was inversely proportional to ΔpH. These results indicate that the internal pH is not the only parameter that controls the rate of electron transport. It is shown that when all these diverse results are plotted as a function of an average pH (between the internal and external pH) they show a single optimum. Thus, it is suggested that the rate controlling site is embedded in the thylakoid membrane, and is a function of both the internal and external pH. This membrane pH and the size of ΔpH seem to be the major factors controlling the rate of electron transport in chloroplasts.
The Problem of Cation‐Binding Sites in the Energized Membrane of Intact Mitochondria
Authors: Colonna Raffaele , Massari Stefano , Azzone Giovanni Felice , Ziche Bruno , Veronese Paolo 
Published: March 2005

Weak bases, such as the acridine and the aminophenazine dyes, which undergo protonation in the chromophoric group, are actively bound in sonicated fragments. On the other hand strong bases such as the divalent cations, the safranine and the cyanine dyes, which carry a permanent charge, are actively bound in intact mitochondria. In the latter case the process results in stimulation of the respiration, H+ ejection, and a large change in absorbance, which is attributed to a stacking of the dyes. The effects are inhibited or reversed by uncouplers, and insensitive to nigericin.Four alternatives are considered: accumulation in the inner aqueous space; binding to matrix proteins; binding to the outer membrane surface; and binding to sites in the membrane fabric. The view that the energized membrane contains high affinity sites for cationic dyes is supported by: comparison of the spectra of the membrane‐bound and polyanion‐bound dye; competition with divalent cations; osmotic activity; and dependence of the spectral shift on the protein/dye ratio. The data are in accord with the concept that the mitochondrial membrane acts as a fixed negative‐charge system where electrostatic interactions occur in microenvironments of low polarity.
Interprétation du rôle antilipolytique de la tétracycline
Authors: Counis Raymond , Raulin Jeanine , Koumanov Kamen , Infante Recaredo 
Published: March 2005

In order to observe whether or not the lipolytic inhibitory effect due to the injection of tetracycline to rats could be related to the decrease of enzymic reactivity in the membrane, adenylate cyclase activities were determined in adipose cell ghosts.Basal, noradrenaline and glucagon‐stimulated activities were significantly depressed by added tetracycline (in vitro experiments). It appeared that tetracycline could influence both V (inhibition of catalytic subunit) and the apparent affinity for hormones, as indicated on dose‐response curves. Fluoride‐stimulated adenylate cyclase activity was also affected, with no change in Km. Phosphodiesterase activity was identical whatever the concentration of added tetracycline.The adenylate cyclase inhibition by added tetracycline was not related to any chelation effect on Mg2+ of this anionic‐antibiotic compound.
Yeast 3‐Phosphoglycerate Kinase
Authors: Roustan Claude , Brevet Annie , Pradel Louise‐Anne , van Thoai Nguyen 
Published: March 2005

The mechanism of the transphosphorylation reaction catalyzed by yeast 3‐phosphoglycerate kinase has been studied by means of spectrophotometric investigations.The binding sites, one for the nucleotide substrates and one for the acceptor, have been characterized. The interaction of nucleotide substrates with the enzyme, which is not metal‐dependent, resulted only in a perturbation of the spectrum of the nucleotide chromophore characterized by hypochromic and red shift effects. The hydrophobicity of this site was estimated by using a reporter group reagent, 2‐(dansylamino)ethyl monophosphate. The interaction of 3‐phosphoglycerate with its corresponding site produced perturbation of a tyrosyl chromophore. Furthermore, spectrophotometric studies evidenced the formation of binary and ternary complexes in accord with a random reaction mechanism proceeding via interconversion of ternary complexes.We also observed partial exchange reaction between ATP and ADP. Properties of this exchange reaction have been investigated. The partial reaction points out the existence of a covalent phosphoryl‐enzyme intermediate.
Reversible Photolysis of the Purple Complex in the Purple Membrane of Halobacterium halobium
Authors: Oesterhelt Dieter , Hess Benno 
Published: March 2005

A suspension of the purple membrane from Halobacterium halobium in concentrated salt solutions saturated with diethylether is bleached by light. During this process, the purple complex is converted in to a compound absorbing maximally at 412 nm (412‐nm complex). The purple complex reappears in the dark with a rate constant of 0.04 s−1 at 20°C.
The initial rate of formation of the 412‐nm complex is proportional to the intensity of the bleaching light and a quantum yield of 0.79 is measured. The action spectrum of the photolytic reaction is identical to the absorption spectrum of the purple complex.
The light reaction is temperature independent. The regeneration of the purple complex requires an activation energy of 11.4 kcal × mol−1.
Illumination causes a decrease in the tryptophan fluorescence of the membrane. Regeneration of the purple complex restores the original fluorescence level. The half times of both processes correlate with the half times observed by absorption spectroscopy.
The photochemical reaction is accompanied by release of protons during bleaching and uptake of the same amount of protons during regeneration of the purple complex. The half times of the measured pH changes again correspond to the one observed by absorption spectroscopy. The rapid release of protons upon illumination is temperature independent. For the dark uptake of protons an activation energy of 14 kcal × mol−1 has been determined. The significance of this isolated photochemical system is discussed with respect to a possible function of the purple membrane in vivo.