bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermodynamics and Kinetics of Base‐Stacking Interactions
Authors: Pörschke Dietmar , Eggers Frieder 
Published: March 2005

The thermodynamics and kinetics of the self‐association of N6, N9‐dimethyladenine has been studied as a model system for base stacking. Molal osmotic coefficients have been determined by vapor pressure osmometry in aqueous solutions at four different temperatures. These data, indicating a very high degree of association much beyond the dimer stage, are consistent with a series of consecutive equivalent association processes. The analysis is performed with an isodesmic model, which allows for cooperativity of the first binding step. Cooperativity can almost be neglected in the present case; the thermodynamic parameters for the formation of stacks are ΔH°=– 8.7 (° 1.5) kcal/mol and ΔS°=– 21.6 cal/K resulting in a stability constant K= 60 at 20°C.The kinetics of base stacking has been investigated by means of sound absorption techniques in the 10–100 MHz frequency range. The excess absorption profiles are only slightly broader than theoretically expected for a single relaxation process. The data can be explained by an isodesmic kinetic model. Formation of stacks proceeds with a rate constant of about 109 M−1 sec−1 and an activation enthalpy of + 6 kcal/mol. Evaluation of the amplitudes leads to a volume change of 6.8 ml (at 25°C) per mol of stacking sites. The volume change decreases with increasing temperature. Optical measurements in a pressure cell show that the volume decreases with increasing number of stacks.These results are discussed with respect to current theories of base stacking. Both ΔH and ΔV found for the stacking equilibrium are in contrast to the thermodynamic parameters of hydro‐phobic interactions according to the standard view. The present data demonstrate a characteristic difference of hydrophobic interactions between unpolar, unpolarisable particles like simple hydrocarbons and between compounds like nucleic acid bases with high polarisability and a dipole structure.
Studies on the Active Site of IRC‐50 Arvin, the Purified Coagulant Enzyme from Agkistrodon rhodostoma Venom
Authors: Collins J. Peter , Jones J. Gareth 
Published: March 2005

A number of the properties of the active site of IRC‐50 arvin, a coagulant enzyme from Agkistrodon rhodostoma, have been studied. The results indicate that the enzyme is in many ways similar to the mammalian serine proteinases.

IRC‐50 arvin will hydrolyse α‐N‐benzoyl‐L‐arginine esters and a study of the rates of hydrolysis of a number of these esters indicates that the hydolysis may proceed via a common acylenzyme intermediate.
IRC‐50 arvin is inactivated by di‐isopropylfluorophosphate and phenylmethanesulphonyl fluoride but at much lower rates than the mammalian serine proteinases.
Both the catalytic activity and rate of inactivation of the enzyme by di‐isopropylfluorophosphate and phenylmethanesulphonyl fluoride are dependent on the ionisation of a group with pK(app)∼ 7.0; this group is probably a histidine residue.
Evidence is presented which suggests that the esterase and coagulant activity of the enzyme are associated with the same active site.
Titration of the active site of the enzyme with p′‐nitrophenol p′‐guanidinobenzoate demonstrates that IRC‐50 arvin has one active site per mole, based on a molecular weight of 55000.
Enzyme Activities Related to Fatty‐Acid Synthesis in Liver and Adipose Tissue of Rats Treated with Triiodothyronine
Authors: Diamant Sophia , Gorin Erela , Shafrir Eleazar 
Published: March 2005

The effect of triiodothyronine treatment on fatty acid synthesis was investigated in liver and adipose tissue of fed and 48‐h fasted rats. In the liver, the thyrotoxic state was accompanied by an increased incorporation of [1‐14C]acetyl‐CoA to fatty acids and a rise in the activity of acetyl‐CoA carboxylase and fatty acid synthetase, both in the fed and fasted rats. The activity of glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase, NADP‐malate dehydro‐genase and ATP citrate lyase was also enhanced by triiodothyronine treatment. Liver malate and citrate content was not changed in fed thyrotoxic rats, but was increased in fasted triiodo‐thyronine‐treated animals. Palmitate oxidation was increased in the liver of the same animals, either fed or fasted.The rise in acetyl‐CoA carboxylase activity due to triiodothyronine administration was apparent when assayed both before and after citrate activation in vitro. Maximal activity of this enzyme in the presence of citrate was reached earlier in the liver of triiodothyronine‐treated rats.In adipose tissue, triiodothyronine treatment induced an increase in the activity of acetyl‐CoA carboxylase, fatty acid synthetase and in glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase. The activity of NADP‐malate dehydrogenase and ATP citrate lyase was slightly but insignificantly increased. Malate and citrate levels in adipose tissue were unchanged by triiodothyronine treatment.It is suggested that the triiodothyronine‐induced simultaneous stimulation of opposing pathways (the synthesis of fatty acids and their oxidation) creates a futile metabolic cycle which contributes to the general energy waste and thermogenesis characteristic of thyrotoxicosis.
The DNA‐RNA Hybridisation of [3H]Methylated Mouse‐Globin mRNA in Conditions of RNA Excess
Authors: Morrison Marcelle R. , Paul John , Williamson Robert 
Published: March 2005

Milligram quantities of mouse reticulocyte 9‐S RNA, which has been demonstrated to contain the mRNA for globin, have been isolated using the zonal ultracentrifuge. In vitro methylation of the 9‐S RNA with dimethylsulphate has allowed investigation of its hybridisation to mouse DNA. The RNA saturates between 0.084% and 0.15% of the DNA.The hybridisation is rapid, being complete in under 3 min. Repeated hybridisations of the RNA eliminate the possibility of the high saturation levels being due to the presence of contaminating traces of other RNA species. Similar saturation levels are found for mouse sperm DNA and total mouse embryo DNA. Slightly higher values are found for 14‐day mouse embryo liver DNA. There was no hybridisation with Escherichia coli DNA.The 9‐S RNA formed hybrids which had a Tm 28 °C lower than that predicted from the 50.7% (G + C) content of the RNA. The size of the hybridised RNA was small after elution from RNAase treated filters, the largest fragments being approximately 60–120 nucleotides in length.Control experiments with [3H]methylated 32P‐labelled 5‐S ribosomal RNA showed that, although the saturation values obtained for hybridisation of methylated and non‐methylated RNA are identical, the methylated RNA‐DNA hybrid melts at a lower temperature. This difference in itself cannot, however, explain the low Tm of the mRNA‐DNA hybrids.
Steroid Metabolism in Rats Given [1‐2H2]Ethanol
Authors: Cronholm Tomas 
Published: March 2005

The rate of excretion of different corticosterone metabolites in rat bile after administration of [1‐2H2]ethanol, and the transfer of deuterium to these metabolites have been studied by gas chromatography‐mass spectrometry. The excretion of corticosterone metabolites increased after injection of ethanol, but the relative amounts of different steroids remained essentially constant. The deuterium excess in 3α,11β, 15α,21‐tetrahydroxy‐5α‐pregnan‐20‐one and 3β(and 3α),11β,21‐trihydroxy‐5α‐pregnan‐20‐one in female rats and of 3β,11β,21‐trihydroxy‐5α‐pregnan‐20‐one, 5α‐pregnane‐3β‐(and 3α),11β,20β,21‐tetrol and 5β‐pregnane‐3α,11β,20β,21‐tetrol in male rats varied between 7 and 20 atoms per cent. Only monodeuterated molecules were found. All the deuterium in 3β,11β,21‐trihydroxy‐5α‐pregnan‐20‐one was present in the 3α‐position, and little or no deuterium was found in the 20α‐position of the corresponding 20β‐hydroxysteroid.When 4‐androstene‐3,17‐dione was given to male bile‐fistula rats, 3α‐hydroxy‐5α‐androstan‐17‐one, 3β‐hydroxy‐5α‐androstan‐17‐one, and 3α‐hydroxy‐5β‐androstan‐17‐one, and various hydroxylated metabolites were excreted in bile. Of the former compounds, the 3α,5β‐epimer was predominant in the glucuronide fraction, whereas the 3β,5α‐epimer was the major epimer in the sulphate fraction. During metabolism of [1‐2H2]ethanol the 3β‐ and 3α‐positions of these compounds contained 5–20 atoms per cent of deuterium. No deuterium was found in the 5α‐ or 5β‐positions.The results indicate that the coenzyme pool(s) used in different reductions at C‐3 is metabolically related to NADH formed in the alcohol dehydrogenase reaction. Since little or no deuterium was found at C‐5 and C‐20 other coenzyme pools are probably used for the reductions of these positions.
The Oxidation of Fatty Acids by Rabbit Reticulocytes and Their Isolated Mitochondria
Authors: Schultze Maria , Rost Johann , Augustin Wolfgang , Gellerich Frank , Rapoport Samuel 
Published: March 2005

The oxidation of fatty acids by rabbit reticulocytes and their isolated mitochondria has been studied with [1‐14C]octanoate, [U‐14C]palmitate and [U‐14C]oleate.The fatty acids, particularly octanoate, represent good respiratory substrates.They are activated both by carnitine‐dependent and carnitine‐independent systems, the former predominating for palmitate, the latter for octanoate.The oxidation of palmitate presents a nearly hyperbolic substrate‐saturation curve. The limitation lies presumably in the capacity of the activating enzymes for the long‐chain fatty acids on in their translocation through the mitochondrial membrane. With octanoate no indication of saturation was observed. It may contribute as much as 65% of the respiratory substrate.
Oxaloacetate exerts a double effect on the oxidation of fatty acids. In low concentrations it activates their oxidation as an acceptor for acetyl‐CoA; in high concentrations it serves itself as a source of acetyl‐CoA and produces a competitive inhibition.
It is estimated that fatty acids furnish about 10% of the respiratory substrate of reticulocytes under physiological conditions.
The Interaction of the Bowman‐Birk Inhibitor with Trypsin and Chymotrypsin
Authors: Steiner Robert F. 
Published: March 2005

The combination of trypsin and chymotrypsin with the Bowman‐Birk inhibitor results in characteristic changes in absorption and fluorescence emission spectra, indicating an alteration of the microenvironments of the enzyme chromophores as a consequence of the interaction. One of the two tyrosine groups of this inhibitor is accessible to nitration with tetranitromethane. The pK of the resultant nitrotyrosine group is shifted to lower values upon combination of the inhibitor with trypsin or chymotrypsin, suggesting a proximity to the zone of contact.The reaction of trypsin with Bowman‐Birk inhibitor is biphasic, an initial second‐order combination being followed by a slower first‐order change. The rate constant for dissociation of the complex increases at acid pH. The dissociation has a high activation energy. Bowman‐Birk inhibitor which has been converted to the modified form by prolonged incubation with catalytic amounts of trypsin forms a complex with trypsin which has the same rate constant and activation energy for dissociation as the complex formed by the native inhibitor.
Protection of Escherichia coli Ribosomes against Streptomycin by Purified Initiation Factors
Authors: Lelong Jean‐Claude , Cousin Marie‐Anne , Gros François , Miskin Ruth , Vogel Zvi , Groner Yoram , Revel Michel 
Published: March 2005

The effect of the purified initiation factors F1, F2 and F3 on the binding of radioactive dihydrostreptomycin to 30‐S subunits and 70‐S sensitive ribosomes of Escherichia coli was studied in the absence of other substrates.Neither of the three factors alone inhibits dihydrostreptomycin binding on 30‐S subunits; the same applies to 70‐S ribosomes except that F2 alone exerts a partial inhibitory effect when present in large excess.In contrast, certain associations of factors appear very efficient in preventing dihydrostreptomycin from binding on ribosomes: thus a mixture of F1 with either F2 or F3 strongly inhibits radioactive dihydrostreptomycin binding to 70‐S ribosomes; the same is observable with 30‐S subunits although the F1– F3 association exerts the most potent effect, this effect being even more pronounced by the further addition of F2.The competing effect exerted by factors toward streptomycin binding does not manifest itself at low temperatures. Elongation factors have no effect.In line with these results, it was also found that combination of the three initiation factors can protect ribosomes against the streptomycin‐induced release of initiator tRNA which is bound in the presence of natural messenger‐RNA and GTP.The observed competition between initiation factors and streptomycin for the ribosomes indicates that the factors either bind or induce conformational changes in the P10 region of the ribosome.
Regulation of Pyruvate Metabolism in Rat‐Liver Mitochondria by Adenine Nucleotides and Fatty Acids
Authors: Stucki Jörg W. , Brawand Fritz , Walter Paul 
Published: March 2005

Fluxes of pyruvate through the pyruvate dehydrogenase and the pyruvate carboxylase reactions in intact rat liver mitochondria were measured under various experimental conditions. When amounts of pyruvate not limiting for pyruvate dehydrogenase were present in the incubation medium, pyruvate carboxylase was found to be regulated essentially by the ATP:ADP ratio within the mitochondrial matrix. No direct correlation between pyruvate carboxylase activity and the level of intramitochondrial acetyl‐CoA could be observed under these conditions.When the ATP:ADP ratio within the matrix was decreased by elevation of the ADP concentration in the medium, a strong inhibition of the pyruvate carboxylase was observed. After addition of oleate to the medium this low ATP:ADP ratio within the matrix was increased and the inhibition of pyruvate carboxylase was partially reversed. It is demonstrated that this effect of oleate is probably caused by an inhibition of the adenine nucleotide translocase.The effects of oleate on pyruvate carboxylase were compared to those of octanoate. In contrast to oleate, octanoate diminished the pool size of the exchangeable adenine nucleotides, ATP and ADP, within the matrix and oxidative phosphorylation was stimulated. The reduced level of intramitochondrial ADP and the increased rate of oxidative phosphorylation elevated the ATP:ADP ratio within the matrix and partially reversed the inhibition of the pyruvate carboxylase by ADP. Thus, both fatty acids reversed the inhibition of pyruvate carboxylase by ADP via changes in the intramitochondrial ATP:ADP ratio but they did this by differing mechanisms.Octanoate was found to be much more rapidly activated and oxidized within the matrix than oleate. Furthermore, octanoate strongly inhibited pyruvate oxidation whereas with oleate this effect was much smaller.The physiological implications of these findings are discussed in relation to the stimulation of gluconeogenesis by fatty acids in the liver.
3′:5′‐Cyclic Nucleotide Phosphodiesterases in Rat Tissues
Authors: Campbell Mary T. , Oliver Ivan T. 
Published: March 2005

The rates of enzymatic hydrolysis of cyclic AMP, cyclic GMP, cyclic IMP, cyclic UMP, cyclic CMP and cyclic dTMP (all 3′:5′‐phosphates) have been assayed in several rat tissues. Subcellular distribution studies indicated that, in all tissues except brain and heart, most of the enzymatic activity is confined to the soluble cell fraction.The purine cyclic nucleotides are the preferred substrates for soluble 3′:5′‐cyclic nucleotide phosphodiesterase in all tissues. Cyclic AMP is a slightly better substrate than cyclic GMP and cyclic IMP in most tissues. However, spleen, lung and erythrocytes hydrolyse cyclic AMP 2–3 times faster than the other purine cyclic nucleotides. Hydrolysis of the cyclic pyrimidine nucleotides was very low or not detected in most tissues. The exceptions were cyclic UMP and cyclic dTMP hydrolysis by brain, and cyclic dTMP hydrolysis by liver.Polyacrylamide gel electrophoresis resolved the soluble cyclic nucleotide phosphodiesterase activity into a number of components. Two components, designated B and C, arc present in all tissues except brain although there are quantitative differences between tisues. These two components appear to be specific for purine cyclic nucleotides. A third form, A, is present in spleen, lung and erythrocytes. It hydrolyses all the cyclic nucleotides except cyclic CMP, although cyclic AMP is the best substrate. However, this component also has 5′‐nucleotidase activity which may indicate that A is not specific for 3′:5′‐cyclic nucleotides. A fourth component D, detected only in liver, hydrolyses cyclic AMP and cyclic dTMP at comparable rates.The electrophoretic patterns obtained for brain phosphodiesterase differ from those of all other tissues investigated and suggest that the enzyme(s) probably has markedly different properties from the cyclic nucleotide phosphodiesterases of other tissues.
Étude d'un glycolipide polyinsaturé synthétisé par Mycobacterium phlei
Authors: Asselineau Cécile P. , Montrozier Henri L. , PromÉ Jean‐Claude , Savagnac Arlette M. , Welby Michèle 
Published: March 2005

Mycobacterium phlei synthesizes an homologous series of polyunsaturated fatty acids, the main member of which is hexatriaconta‐4,8,12,16,20‐pentaenoic acid. These acids will be refered to as “phleic acids”. Natural derivatives of these acids and their subcellular localization are studied in this paper.A series of esters of trehalose and phleic acids, called “lipids P”, have been isolated from Mycobacterium phlei. They differ by their number of esterified hydroxyl groups. Because of their low stability, it is difficult to make a distinction between natural product and artefact. The main member of this series, lipid P1, has been isolated in an homogeneous form by chromatographic procedures.Saponification of lipid P1 gives only phleic acids and trehalose. Quantitative estimation by gas chromatography, as methyl phleate and trehalose trimethylsilyl ether, and examination of infrared and nuclear magnetic resonance spectra of the whole lipid P1 suggest the structure of octaphleate of trehalose for this lipid. Then lipid P1 is a glycolipid having a strong lipidic character, that differs from most other natural lipid compounds by its high molecular weight (4400) and its high unsaturation (40 double bonds).Lipids P appear to be the main form of phleic acid derivatives. They are located in the outer part of the cell, and can be extracted by a simple washing of the bacteria with hexane; bacterial cells are still living after this treatment.Phleic acids are not obligatory components of mycobacteria, as they have been found in two saprophytic strains (Mycobacterium phlei and Mycobacterium smegmatis), but not in two strains of Mycobacterium tuberculosis and one strain of Mycobacterium bovis (BOG). On the basis of chemical structure, subcellular localization and species distribution, lipids P exhibit analogies with other kinds of esters of trehalose found in some strains of mycobacteria devoid of phleic acids.Although the content of phleic acids in a culture of Mycobacterium phlei reaches a constant value long before the end of the logarithmic phase of growth, phleic acids do not appear to be simple excretion products. They are not storage compounds: unlike common fatty acids, they are neither used when no carbon source is supplied to the medium, nor accumulated when the main nitrogen source of the medium (asparagin) is omitted. An important decrease of the content of phleic acids is observed when the medium is supplemented with a surfactant, which prevents bacterial aggregation.
Purification and Subunit Interactions of Yeast Hexokinase
Authors: Easterby John S. , Rosemeyer Michael A. 
Published: March 2005

A method is described for the purification of two native isoenzymes of yeast hexokinase (A and B) in the presence of phenylmethyl sulphonylfluoride, an inhibitor of yeast proteases. In the absence of this inhibitor two proteolytically modified forms (A′ and B′) were obtained.Investigation of the effects of solvent ionic strength and pH on the dissociation of the native form A and the proteolytically modified form A′ suggests that these enzymes have a common molecular weight and quaternary structure. Both forms exist as dimers of molecular weight 108000 ± 3000 in reversible dissociation equilibrium with monomers of molecular weight 54000 ± 1500.Hexokinase A dissociates to monomer both with increasing ionic strength and in the presence of hexose substrates. Dissociation to constituent polypeptide chains of 54000 molecular weight is effected by maleylation or succinylation of the native enzyme, confirming its dimeric structure. Hexokinase A′ exhibits a pH‐dependent dissociation from dimer to monomer. This dissociation is attributable to two ionising groups on the monomer which are predominantly unionised in the dimer.
A Glycopeptide from the Posterior Lobe of Pig Pituitaries
Authors: Holwerda Dirk A. 
Published: March 2005

The primary structure of the glycopeptide from the posterior lobe of the pig pituitary is: Ala‐Ser‐Asp‐ Arg‐Ser‐Asn‐Ala‐Thr‐Leu‐Leu‐Asp‐Gly‐Pro‐Ser‐Gly‐Ala‐Leu.The, carbohydrate is linked to the peptide chain via the asparagine residue.
The Amino‐Acid Sequence of Neurotoxin II of Androctonus australis Hector
Authors: Rochat Hervé , Rochat Catherine , Sampieri François , Miranda François , Lissitzky Serge 
Published: March 2005

The complete amino acid sequence of neurotoxin II of the North African scorpion Androctonus australis Hector has been determined.From the N‐terminal sequence (43 residues) obtained by automatic phenylisothiocyanate degradation in the protein sequenator, and from the sequence of tryptic peptides obtained by digestion of the reduced and methylated toxin of which amino groups had been previously masked by reaction with ethylthioltrifluoroacetate the complete amino acid sequence of the protein has been inferred. It is composed of a single polypeptide chain of 64 ammo acid residues, including 8 half‐cystines, ended by an histidinamide residue at the C‐terminus.