PubMed Health⌕ Search

Biomedical subjects

E Valero

Publications and source records attributed to E Valero.

At least 19 recordsLinked to original sources
PubMed2006

Expressions for the fractional modification in different monocyclic enzyme cascade systems: analysis of their validity tested by numerical integration.

This paper presents the derivation, under a minimal set of assumptions, of a general expression for the steady-state fractional modification of an interconvertible protein involved in four different schemes of monocyclic enzyme cascade systems. From this general expression we derive, as particular cases, other, simpler expressions by applying additional assumptions and which have, therefore, a smaller range of validity. Some of these particular expressions coincide with those already obtained in previous contributions on individualised analyses. We discuss the relationships between the kinetic parameters and the concentrations needed for the fulfilment of the additional assumptions. The goodness of the analysis was tested by reference to the shape in the steady-state of the simulated time progress curves obtained by numerical integration.

Algorithms↗
PubMed2003

Activity of oral antibiotics against respiratory tract pathogens in Spain.

The aim of this study was to carry out a nationwide survey to assess the susceptibility of clinical isolates of four respiratory pathogens against nine antibiotics. Eight Spanish centers participated in the study, collecting a total of 977 isolates of Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae and Moraxella catarrhalis. The susceptibility of S. pneumoniae to penicillin was 37.46% susceptible, 30.43% intermediate and 32.11% resistant. MIC(90) of all antibiotics against this microorganism were 48 mg/l, except cefaclor, cefixime and azithromycin. For S. pyogenes, all the strains were susceptible to penicillins and cephalosporins, and azithromycin was the least active with a rate of resistance of 11.43%. A total of 95 isolates of H. influenzae were betalactamase positive (26.32%). With regard to M. catarrhalis, only penicillin and amoxicillin showed MICs(90) >=8 mg/l.

Administration, Oral↗
PubMed2002

Kinetic analysis of the general modifier mechanism of Botts and Morales involving a suicide substrate.

Suicide substrates are widely used in enzymology for studying enzyme mechanisms and designing potential drugs. The presence of a reversible modifier decreases or increases the rate of substrate-induced inactivation, with evident physiological and experimental consequences. To date, only the action of a competitive or uncompetitive inhibitor of an enzyme system involving suicide substrate has been reported. In this paper, we analyse the kinetics of enzyme-catalysed reactions which evolve in accordance with the general modifier mechanisms of Botts and Morales in which enzyme inactivation is induced by suicide substrate. Rapid equilibrium of all of the reversible reaction steps involved is assumed and the time course equations for the residual enzyme activity, the inactive enzyme forms and the reaction product are derived. Partition ratios giving the relative weight of the product and inactive enzyme concentrations, and the relative contribution to the product formation of each of the unmodified and modified catalytic routes, are studied. New indices pointing to the conditions under which the modifier acts as inhibitor or as activator are suggested. The goodness of the analytical solutions is tested by comparison with the simulated curves obtained by numerical integration. An experimental design and kinetic data analysis to evaluate the kinetic parameters from the time progress curves of the product are proposed. From these results, those corresponding to several reaction mechanisms involving both a suicide substrate and a modifier, and which can be regarded as particular cases of the general case analysed here, can be directly and easily derived.

Animals↗
PubMed2002

Linear mixed irreversible inhibition of the autocatalytic activation of zymogens. Kinetic analysis checked by simulated progress curves.

Autocatalytic zymogen activation is a phenomenon of great importance for understanding some fundamental physiological processes involved in the enzyme regulation of gastrointestinal-tract enzymes, blood coagulation, fibrinolysis and the complement system. Examples of such processes are the activation of prekallikrein, trypsinogen and pepsinogen, all of which are controlled by natural proteinase inhibitors. This work studies the kinetics of a general autocatalytic zymogen activation process overlapped by two two-step irreversible inhibitions, i.e. a linear mixed irreversible inhibition. The kinetic equations for the whole course of the reaction are derived for this mechanism. In addition, we determine the corresponding kinetics for a number of particular cases of the general model analyzed, i.e. for reversible and irreversible non-competitive, competitive and uncompetitive inhibition systems which are considered particular cases of the general mechanism studied. The kinetic behavior of the system is related to a parameter, a dimensionless quantity, which shows whether the inhibition or the activation route prevails, in a similar way to that which we have previously carried out for other mechanisms. Finally, based on the kinetic equations obtained, a procedure for discriminating between the different mechanisms considered is suggested. The results of this contribution can be directly applied to most physiological autocatalytic zymogen activations in the presence of an inhibitor, allowing their complete kinetic characterization and suggesting procedures for varying the relative weight of the catalytic and inhibition routes or for changing the predominant route.

Algorithms↗
PubMed2001

Changes in nitrogen compounds in must and wine during fermentation and biological aging by flor yeasts.

Urea, ammonium, and free amino acid contents were quantified in a must from Vitis vinifera cv. Pedro Ximenez grapes and in fermented wine and after a short aging of this wine by Saccharomyces cerevisiae race capensis yeast under variable oxygen availability conditions. The previous compounds were also determined in a wine in which the nitrogen source was depleted by the same race of flor yeast (old wine) and also following the addition of ammonium ion, L-glutamic acid, and L-proline. Under specific conditions such as low oxygen level and the absence of some nutrients, the yeasts release some amino acids including L-threonine, L-tryptophan, L-cysteine, and L-methionine to the medium. These amino acids must originate primarily in a de novo synthesis from ethanol that regenerates NAD(P)+. On the basis of these results, the yeasts may be able to use amino acids not only as nitrogen sources but also as redox agents to balance the oxidation-reduction potential under conditions of restricted oxygen, when electron transport along the respiratory chain may be hindered or limited.

Aging↗
PubMed2001

Direct thermal desorption in the analysis of cheese volatiles by gas chromatography and gas chromatography-mass spectrometry: comparison with simultaneous distillation-extraction and dynamic headspace.

Direct thermal desorption (DTD) has been used as a technique for extracting volatile components of cheese as a preliminary step to their gas chromatographic (GC) analysis. In this study, it is applied to different cheese varieties: Camembert, blue, Chaumes, and La Serena. Volatiles are also extracted using other techniques such as simultaneous distillation-extraction and dynamic headspace. Separation and identification of the cheese components are carried out by GC-mass spectrometry. Approximately 100 compounds are detected in the examined cheeses. The described results show that DTD is fast, simple, and easy to automate; requires only a small amount of sample (approximately 50 mg); and affords quantitative information about the main groups of compounds present in cheeses.

Cheese↗
PubMed2001

Influence of pre-fermentative treatment on the fatty acid content of Saccharomyces cerevisiae (M(3)30-9) during alcoholic fermentation of grape must.

The effect of skin maceration and must filtration on the growth and fermentative capacity of one strain of Saccharomyces cerevisiae was studied. Skin maceration increased the fatty acid concentration of the must, thus affecting the lipid composition of the yeast cell membranes. Sterile filtration resulted in a reduction of the fatty acid content of the must extracted during maceration, but increased the fatty acid content of the yeast membranes, thus increasing the fermentative capacity without affecting either growth or cell viability.

Journal Article↗
PubMed2001

Influence of oxygen addition during growth phase on the biosynthesis of lipids in Saccharomyces cerevisiae (M(3)30-9) in enological fermentations.

The fatty acid, phospholipid and sterol composition of one strain of Saccharomyces cerevisiae under different oxygen levels during grape must fermentation was studied. Anaerobiosis partially inhibited yeast growth and resulted in a drop in the unsaturation index of the fatty acids throughout the fermentation, as well as an abnormally low ergosterol/phospholipid ratio. Aeration after 48 h of fermentation in anaerobiosis stimulated growth and increased cellular viability, as well as raised the unsaturation index of fatty acids. Although the ergosterol/phospholipid ratio remained low in the days following the aeration, by the end of the fermentation the value was that considered optimum for the maintenance of membrane functions.

Journal Article↗
PubMed2001

Influence of skin maceration and oxygen on anaerobic fermentation of grape musts with high sugar content.

Saccharomyces cerevisiae race cerevisiae was used to ferment grape musts in strictly anaerobic conditions, subjected to a prefermentative treatment of skin maceration and following a short aeration after 48 h of fermentation. Skin maceration caused an increase in the cellular phospholipid content which affected neither viability nor the fermentative capacity of the yeasts. The short aeration had no significant effect on the unsaturation index of the cellular fatty acids, although it did increase the ergosterol/phospholipid ratio. This was reflected by an increase in the growth rate, viability and fermentative capacity of the yeasts. Maceration did not increase the effect of aeration.

Anaerobiosis↗
PubMed2001

[Activity of different antimicrobial agents against penicillin-resistant Streptococcus pneumoniae].

Since the first strains of penicillin-resistant Streptococcus pneumoniae were isolated in the 1960s, the rate of resistance has increased world- wide, though with geographic variations. Currently, the prevalence and patterns of antibiotic resistance in this microorganism vary widely from one country to another, as well as within in the same country. In our study we evaluated the in vitro susceptibility of 299 isolates of S. pneumoniae from patients with community-acquired respiratory tract infections from 1998-2000 to different antimicrobial agents. The following resistance results were obtained: 32.11% to penicillin, 4.35% to amoxicillin, 3.68% to amoxicillin-clavulanic acid, 69.9% to cefaclor, 32.44% to cefpodoxime, 34.11% to cefuroxime, and 24.41% to azithromycin. For cefixime and ceftibuten there are no NCCLS breakpoint criteria.

Amoxicillin↗
PubMed2001

[Activity of new fluoroquinolones against clinical isolates of Acinetobacter baumannii].

Acinetobacter baumannii is an opportunistic pathogen associated with numerous nosocomial infections. In recent years it has shown extraordinary ease in developing resistance to most antimicrobial agents, which is a serious problem as it makes these infections difficult to treat. We determined the in vitro activity of eight quinolones, five betalactam agents and colistin in 160 clinical isolates of A. baumannii. In general, we observed a high rate of resistance to the quinolones (90%), excluding clinafloxacin (25%), and to ampicillin-sulbactam (61.25%) and imipenem (50%). Colistin is the agent with least resistance (13.125%), although its toxicity limits its therapeutic use. Clinafloxacin may be a good option to treat A. baumannii infections, especially in cases of therapeutic failure with other antimicrobial agents.

Acinetobacter↗
PubMed2000

Kinetics of a self-amplifying substrate cycle: ADP-ATP cycling assay.

A kinetic study of an ATP-ADP amplification cyclic system involving the enzymes adenylate kinase, pyruvate kinase and L-lactate dehydrogenase has been made. The stoichiometry of the cycle is 2:1, because two molecules of ADP are synthesized from one each of ATP and AMP, and one molecule of ADP is converted back into one of ATP at each turn of the cycle. This results in a continuous exponential increase in the concentrations of ATP and ADP in the reaction medium, according to the equations obtained. This is therefore a substrate cycle that amplifies itself, the cycling rate increasing continuously with time. The background signal of the reagent was reduced by using apyrase to degrade ATP and ADP in the reagent, permitting detection limits as low as 16 pmol of ATP and/or ADP in a continuous spectrophotometric assay.

Adenosine Diphosphate↗
PubMed2000

Stochastic independence of color-vision mechanisms confirmed by a subthreshold summation paradigm.

We have used a subthreshold summation protocol to analyze spatial color-color interaction. By means of a CRT color monitor, we measured the threshold contours for a spatial frequency of 0.5 cycles/degree. Heterochromatic flicker photometry was used to obtain isoluminance. The results suggest that the blue-yellow (b-y) and red-green (r-g) contrast thresholds remained unchanged by the addition of fixed r-g and b-y subthreshold pedestals. Our subthreshold summation data then support the stochastic independence of colorvision mechanisms derived from Mullen and Sankeralli's work [Vision Res. 39, 733 (1999)] despite the differences that exist between the two experimental methods.

Color Perception↗
PubMed2000

[Pharmacodynamic basis for the use of amoxicillin-clavulanic acid in respiratory infections due to Streptococcus pneumoniae: In vitro studies in an experimental model].

Amoxicillin-clavulanic acid is a first choice treatment for respiratory tract infections caused by Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis. In a previous study we observed its high efficacy against penicillin-susceptible and intermediate-resistant strains of S. pneumoniae. We aimed to study the efficacy of this antibiotic against three strains of S. pneumoniae (susceptible, intermediate and resistant to penicillin) in a mouse model of pneumonia, and to determine the influence of the time of starting treatment and the in vitro postantibiotic effect. We also determined the serum levels of the antimicrobial agent in the mice, and correlated the pharmacodynamic parameters (Cmax/MIC, AUC/MIC and T>MIC) with the survival rate to establish the best predictor of efficacy. MICs with amoxicillin-clavulanic acid were 0. 03 mg/l, 0.25 mg/l and 2 mg/l for the penicillin-susceptible, -intermediate and -resistant strains, respectively. The ED90 were approximately 5 mg/kg for susceptible strains, 25 mg/kg for the intermediate and 50 mg/kg for the resistant strains. We observed a lower survival rate (approximately 55%) when the treatment began 31 h after infection than when it began 5 h (100%) and 19 h (approximately 90-100%) afterwards. Serum levels were dose dependent and the correlation with the pharmacodynamic parameters showed a significant association between survival and the T>MIC (r = 0.946). In vitro postantibiotic effects with 1, 4 and 10 times the MIC were 0.96 to 1.69 h for susceptible strains, 0.38 to 1.23 h for intermediate, and 1.52 to 2. 20 h for resistant strains. These results show the high efficacy of this antibiotic combination against strains with variable susceptibility to penicillin, with this activity being related mainly to the T>MIC of the microorganism. The postantibiotic effect would prolong the effect of the antibiotic in the dosing interval. These parameters and antimicrobial effects are important in terms of the clinical application of this antimicrobial agent.

Amoxicillin-Potassium Clavulanate Combination↗
PubMed1999

[Atrioventricular node catheter ablation with condenser discharge and radiofrequency].

The AV junction ablation was useful to treat patients with drug-refractory supraventricular arrhythmias. The purpose of this study was to determine short and long-term success and complications of the atrioventricular nodal catheter ablation and to compare direct current and radiofrequency energy. Forty patients underwent AV nodal ablation with direct current energy (Group I) and forty patients with radiofrequency (Group II). They were followed up for a mean of 76 +/- 49 and 28 +/- 20 months, respectively. Persistent complete AV block was successfully induced during the first ablation session in 45% of 40 patients who underwent DC energy, while in 50% it was modulated. All patients in the radiofrequency group had complete AV block. The rate of recurrence of AV conduction was 7.5% and 2.5% respectively. Immediate complications did not occur after either procedure. One patient died suddenly in each group during follow-up. AV nodal ablation with radiofrequency energy appears to be as efficacious and safe as direct current energy.

Adult↗
PubMed1998

A continuous spectrophotometric method based on enzymatic cycling for determining L-glutamate.

A continuous spectrophotometric assay for determining low levels of L-glutamate is described. The assay, which involves the enzymes L-glutamate oxidase and glutamic-pyruvic transaminase, is based on the recycling of L-glutamate into alpha-ketoglutarate, with the concomitant appearance of one molecule of hydrogen peroxide in each turn of the cycle. This is subsequently reduced by means of a peroxidase-coupled reaction, using 2, 2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) as substrate. In this way the interference observed in the cyclic assay using glutamic-oxalacetic transaminase, which is due to the fact that L-aspartate is also a substrate of L-glutamate oxidase, is eliminated. A kinetic study of the system is presented, with the accumulation of chromophore being seen to undergo a transient phase, which is dependent both on the cycling rate and on the auxiliary enzyme concentration. The kinetic parameters characterizing the system have been determined, making it possible to optimize costs with respect to the enzymes involved in the cycle, since the minimum amount needed for a given rate constant of the cycle can be calculated.

Amino Acid Oxidoreductases↗
PubMed1997

Modulation of the protein kinase CK2 activity by a synthetic peptide corresponding to the N-terminus of its beta regulatory subunit.

Using a synthetic peptide corresponding to the sixteen amino-acid N-terminus of the beta subunit, the structure-activity relationship study of casein kinase 2 (CK2) was performed with regard to its previously reported property to polymerize and oligomerize in vitro. Velocity sedimentation experiments show that the peptide beta 1-16 prevents the thick filament formation and stabilizes the ring-like structure of the kinase. Furthermore, the peptide beta 1-16 stimulates the kinase activity by 3-fold toward exogenous substrates as well as the intrinsic autophosphorylation of the kinase. Such observations are in agreement with the proposed model of an activated state of CK2 when the ring-like structure is adopted. Comparison of the effects of spermine and peptide beta 1-16 on CK2 structure and activity suggests that these two activating molecules may function in a different way. Our study suggests that the N-terminal region of the beta subunit of CK2 could regulate the kinase activity by controlling the quaternary structure of the enzyme.

Amino Acid Sequence↗