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PubMed2004

Preparation and physicochemical properties of niclosamide anhydrate and two monohydrates.

The intent of the study was to prepare and characterize three crystal forms of niclosamide namely the anhydrate and the two monohydrates and to investigate the moisture adsorption and desorption behavior of these crystal forms. The crystal forms were prepared by recrystallization and were characterized by differential scanning calorimetry, thermogravimetric analysis, isoperibol solution calorimetry, Karl Fischer titration, and X-ray powder diffractometry. Moisture adsorption by the anhydrate at increased relative humidities and two temperatures, 30 and 40 degrees C, was measured while the desorption from the monohydrates was determined at 45, 55, and 65 degrees C for monohydrate H(A) and 75, 90, and 100 degrees C for monohydrate H(B). Thermal analysis and solution calorimetry showed that monohydrate H(B) is more stable than monohydrate H(A) and solubility measurements showed the solubility of the crystal forms decreased in the order: anhydrate>>monohydrate H(A)>monohydrate H(B). With an increase in temperature and relative humidity niclosamide anhydrate adsorbed moisture to form monohydrate H(A) by a random nucleation process. Dehydration of monohydrate H(A) at increased temperatures followed zero order kinetics and resulted in a change to the anhydrate. Monohydrate H(B) was transformed to the anhydrate at higher temperatures by a three-dimensional diffusion mechanism.

Anthelmintics↗
PubMed1996

Energy expenditure in anorexia nervosa: can fat-free mass as measured by bioelectrical impedance predict energy expenditure in hospitalized patients?

Anorexia nervosa (AN) is associated with a reduced metabolically active fat-free mass (FFM) and basal metabolic rate (BMR). Excessive refeeding results in major fat deposition which is not well tolerated by patients. Prediction of BMR is, therefore, a clinical issue during refeeding, but measurement by indirect calorimetry is time-consuming and not widely available. The study aim was to determine if and when BMR could be estimated from prediction formulas based on FFM derived from bioelectrical impedance analysis (BIA) in AN patients during refeeding. Indirect calorimetry and BIA were prospectively measured bi-weekly in 9 AN patients (body mass index 13.7 +/- 0.5 kg/m2) for 10 weeks of refeeding. Initial BMR was 969 +/- 46.7 kcal/d and 27.7 +/- 1.4 kcal/kg FFM, and at week 10 increased to 1360 +/- 44.6 kcal/d and 35.8 +/- 0.8 kcal/kg FFM. While correlations improved with increasing weight, FFM and body mass index, prediction formulas are insufficient to permit prediction of BMR based on weight or FFM, even after 10 weeks of refeeding. To allow for optimal nutritional support, indirect calorimetry measurements may be useful in the early weeks of refeeding because of a large variability of basal metabolic rate between patients.

Journal Article↗
PubMed1999

Energy expenditure and substrate utilization in mechanically ventilated children.

The objective of the study was to determine the value of indirect calorimetry and nitrogen balance (N balance) in order to evaluate the current feeding protocols of mechanically ventilated children. The study was designed as a cross-sectional prospective study. In 36 mechanically ventilated children energy expenditure was measured by indirect calorimetry, and total urinary nitrogen excretion (TUN) was determined. Substrate utilization and respiratory quotient (RQ) were calculated from the measured values of oxygen consumption (VO2), carbon dioxide production (VCO2), and TUN. The RQ was compared with the RQ of the macronutrients administered (RQmacr) according to the modified criteria of Lusk. In results, the total measured energy expenditure (TMEE) showed a wide variation (range 155-272 kJ.kg-1.d-1). The N balance was positive in 20 and negative in 16 patients. The ratio of caloric intake/TMEE was significantly higher in patients with a positive N balance (1.50 +/- 0.06) as compared with those with a negative N balance (0.8 +/- 0.1, P < 0.001). There was a significant relationship between the difference of RQ-RQmacr versus the ratio caloric intake/TMEE (r = 0.72, P < 0.001). Carbohydrate and fat utilization were not significantly different in patients with a positive or negative N balance. Protein utilization was significantly higher in those patients with a negative N balance. We concluded that measurement of TMEE with indirect calorimetry results in accurate determination of energy needs in critically ill mechanically ventilated children. Feeding according to or in excess of the TMEE is correlated with a positive N balance. A combination of the RQ and the RQmacr can be helpful in differentiating under- or overfeeding.

Adolescent↗
PubMed2001

Surface activity at the planar interface in relation to the thermodynamics of intermolecular interactions in the ternary system: maltodextrin-small-molecule surfactant-legumin.

We report on the effect of potato maltodextrins with variable dextrose equivalent (Paselli SA-2, SA-6 and SA-10) on the surface behavior at the air-water interface of the mixture: legumin+small-molecule surfactant. Distinct in nature small-molecule surfactants (model: sodium salt of capric acid, Na-caprate; and commercially important: a citric acid ester of monoglyceride, CITREM) have been under our consideration. The role of the structure of both of the maltodextrins and the small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, value of weight average molecular weight of the maltodextrins and the thermodynamics of the pair maltodextrin-solvent and maltodextrin-protein interactions from laser static light scattering. The combined data of mixing calorimetry and light scattering suggest some complex formation between the small-molecule surfactants and the maltodextrins. Predominantly hydrophobic interactions along with hydrogen bonding form the basis of the complexes. The effect of the maltodextrins on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability in the presence of the small-molecule surfactants has been studied by differential scanning calorimetry. The interrelation between the thermodynamics of intermolecular interactions in a bulk and the surface behavior at the planar air-water interface of the ternary systems (maltodextrin+legumin+small-molecule surfactant) has been elucidated by tensiometry. The effect of the maltodextrins on the surface activity of mixtures of legumin with the small-molecule surfactants is governed by the competitive in relation to the protein interactions with the small-molecule surfactants and a subsequent change in the thermodynamic properties of the both biopolymers, which are favorable to the ternary complex formation.

Journal Article↗
PubMed2004

Structural, volumetric, and thermodynamic characterization of a micellar sphere-to-rod transition.

The thermotropic sphere-to-rod transition of nonionic surfactants was characterized in terms of a large set of parameters: the transition temperature and width, the partial volume, coefficient of thermal volume expansion, enthalpy, isobaric heat capacity, and structural parameters, such as radius of gyration and hydrodynamic radius. Data were recorded as a function of concentration of surfactants in H2O and in D2O. To this end, pressure perturbation calorimetry (PPC), small angle neutron scattering (SANS), dynamic light scattering (DLS), differential scanning calorimetry (DSC), and isothermal titration calorimetry (ITC) were applied in a study of aqueous solutions containing myristyl, tridecyl, and lauryl maltoside and heptaethyleneglycoltetradecyl ether (C14EO7). Small changes in the thermodynamic and volumetric parameters (e.g., the partial volume change is approximately +2 per thousand) are discussed in detail as the result of three effects governing the transition. (i) Reduction of the water accessible hydrophobic surface area (ASA(ap)) drives the transition. (ii) Shrinking in headgroup size by thermal dehydration triggers the transition. (iii) Hypothesized gradual ordering of the chains may control the effect of chain length on the transition.

Journal Article↗
PubMed2005

Pulsed-flow microcalorimetric study of the template-monolayer region of nonionic surfactants adsorbed at the graphite/water interface.

The formation of half-cylindrical surfactant aggregates at the graphite/aqueous solution interface is templated by an ordered monolayer of molecules disposed parallel to the graphite basal plane. Beyond a critical alkyl chain length, monolayer formation is effectively irreversible. Since enthalpic interactions in this template-monolayer region cannot be resolved with adequate accuracy by the traditional adsorption calorimetric methods, we applied a novel method, pulsed-flow calorimetry, for simultaneous measurement of the material balance and the enthalpy balance in this high-affinity region. For the three nonionic surfactants studied, n-octyl beta-D-glucoside (C(8)G(1)), dimethyl-n-decylamine oxide (C(10)DAO), and n-octyl tetraethylene glycol monoether (C(8)E(4)), the adsorption was found to be strongly exothermic and effectively irreversible at low adsorbate densities, and the differential heat of adsorption markedly decreased with increasing surface coverage in this region. This deviation from the ideal adsorption behavior was attributed to intermolecular interactions within the adsorption layer rather than to surface heterogeneity of the graphite basal planes. A thermodynamic consistency test clearly demonstrated that pulsed-flow calorimetry is a unique experimental method for the study of nonreversible adsorption phenomena at solid/solution interfaces, representing an excellent tool to complement traditional methods, e.g., frontal-flow and titration adsorption calorimetry. Studies by the frontal-flow method revealed that aggregation on top of the surfactant monolayer was endothermic and reversible.

Journal Article↗
PubMed2003

Observation and characterization of the interaction between a single immunoglobulin binding domain of protein L and two equivalents of human kappa light chains.

Detailed stopped-flow studies in combination with site-directed mutagenesis, isothermal titration calorimetry data and x-ray crystallographic knowledge have revealed that the biphasic pre-equilibrium fluorescence changes reported for a single Ig-binding domain of protein L from Peptostreptococcus magnus binding to kappa light chain are due to the binding of the kappa light chain at two separate sites on the protein L molecule. Elimination of binding site 2 through the mutation A66W has allowed the K(d) for kappa light chain binding at site 1 to be measured by stopped-flow fluorescence and isothermal titration calorimetry techniques, giving values of 48.0 +/- 8.0 nM and 37.5 +/- 7.3 nM respectively. Conversely, a double mutation Y53F/L57H eliminates binding at site 1 and has allowed the K(d) for binding at site 2 to be determined. Stopped-flow fluorimetry suggests this to be 3.4 +/- 0.8 microM in good agreement with the value of 4.6 +/- 0.8 microM determined by isothermal titration calorimetry. The mutation Y53F reduces the affinity of site 1 to approximately that of site 2.

Bacterial Proteins↗
PubMed1994

Extrapolation to water of kinetic and equilibrium data for the unfolding of barnase in urea solutions.

Assumptions about the dependence of protein unfolding on the concentration of urea have been examined by an extensive survey of the equilibrium unfolding of barnase and many of its mutants measured by urea denaturation and differential scanning calorimetry. The free energy of equilibrium unfolding and the activation energy for the kinetics of unfolding of proteins are generally assumed to change linearly with [urea]. A slight downward curvature is detected, however, in plots of highly precise measurements of logku versus [urea] (where ku is the observed rate constant for the unfolding of barnase). The data fit the equation logku = logkuH2O* + mku*.[urea] - 0.014[urea]2, where mku* is a variable which depends on the mutation. The constant 0.014 was measured directly on four destabilized mutants and wild-type, and was also determined from a global analysis of data from > 60 mutants of barnase. Any equivalent deviations from linearity in the equilibrium unfolding are small and in the same region, as determined from measurements on 166 mutants. The free energy of unfolding of barnase, delta GU-F, appears significantly larger by 1.6 kcal mol-1 when measured by calorimetry than when determined by urea denaturation. However, the changes in delta GU-F on mutation, delta delta GU-F, determined by calorimetry and by urea denaturation are identical. We show analytically how, in general, the curvature in plots of activation or equilibrium energies against [denaturant] should not affect the changes of these values on mutation provided measurements are made over the same concentration ranges of denaturant and the curvature is independent of mutation.

Bacillus↗
PubMed1997

Energy turnover of vascular endothelial cells.

Two noninvasive methods, calorimetry and 31P nuclear magnetic resonance (NMR), were used to further define energy-consuming and energy-providing reactions in endothelial cells. With 31P-NMR, cellular ATP content was measured; with calorimetry, heat flux as a result of ATP turnover was measured. For these measurements, pig aortic endothelial cells were cultured on microcarrier beads and perfused in a column at constant flow rate. Pig aortic endothelial cells synthesize ATP mainly through glycolysis and, as determined by NMR, contain no phosphocreatine. In such a system, calorimetry-measured heat flux reflects rate of cellular ATP turnover. By use of inhibitors of ATP-dependent processes, the following changes in basal heat flux (231 +/- 65.5 microW/mg protein) were obtained: 18% for 2,3-butanedione monoxime (inhibitor of actomyosin-ATPase), 17% for wortmannin (inhibitor of myosin light chain kinase), 10% for cytochalasin D (inhibitor of actin polymerization), 23% for cycloheximide (inhibitor of protein synthesis), 11% for thapsigargin (inhibitor of endoplasmic reticulum Ca(2+)-ATPase), and 6% for bafilomycin A1 (inhibitor of lysosomal H(+)-ATPase). Cytochalasin D, 2,3-butanedione monoxime, wortmannin, and thapsigargin caused changes in F-actin distribution, as revealed by rhodamine-phalloidin cytochemistry. In a separate experimental series, when cells were perfused with a medium containing no glucose, heat flux decreased by 40% while cellular ATP remained unchanged. Inhibition of glycolysis with 2-deoxy-D-glucose decreased heat flux by 73%, and ATP was no longer visible with 31P-NMR. Despite this massive ATP depletion, which was maintained for 3 h, cells fully recovered heat flux and ATP when 2-deoxy-D-glucose was removed. The results, together with previously published data for Na(+)-K(+)-ATPase [M. L. H. Gruwel, C. Alves, and J. Schrader. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H351-H358, 1995], demonstrate that > 70% of total ATP-consuming processes of endothelial cells can be attributed to specific cellular processes. Actomyosin-ATPase (18%) and protein synthesis (23%) comprise the largest fraction. At least three-fourths of ATP synthesized is provided by glycolysis. Endothelial cells exhibit the remarkable ability to coordinate downregulation of ATP synthesis and consumption when glycolysis is inhibited.

Actins↗
PubMed1991

How valid are isotopic measurements of fatty acid oxidation?

These studies were performed 1) to compare two isotopic methods (3H2O and 14CO2 production) of measuring free fatty acid (FFA) oxidation; 2) to determine whether isotopic estimates of fatty acid oxidation during hypoinsulinemia are plausible when compared with those obtained using indirect calorimetry; and 3) to examine whether the delay between the exit of [14C]FFA from plasma and the appearance of 14CO2 in breath is accounted for solely by bicarbonate kinetics. Studies in 11 healthy volunteers revealed that [14C]- and [3H]FFA tracers provide similar estimates of FFA turnover and oxidation. Isotopic estimates of fatty acid oxidation were less than those of indirect calorimetry under basal conditions but equaled or exceeded indirect calorimetry estimates after 3 h of acute hypoinsulinemia (somatostatin induced). After stopping tracer infusions, the half-life of plasma [14C]FFA was 3.7 +/- 0.1 min. The half-life of 14CO2 decay from [14C]bicarbonate was 37 +/- 1 min, and the half-life of 14CO2 decay after discontinuation of [14C]FFA infusion was 141 +/- 10 min. Intracellular preoxidative fatty acid pools (possibly triglycerides) most probably account for the marked delay between the exit of FFA tracers from plasma and the appearance of isotopic markers of oxidation in measurable spaces. This delay can result in erroneous estimates of "FFA oxidation." We conclude that tracer measurements of FFA oxidation are invalid under most circumstances.

Adult↗
PubMed1995

Use of [13C]bicarbonate infusion for measurement of CO2 production.

To determine whether infusion of 13C-labeled bicarbonate can be used to measure rates of CO2 production (VCO2), seven healthy adults received 6-h primed continuous intravenous infusions of NaH13CO3 and L-[1-14C]leucine in the post-absorptive state while VCO2 was measured by indirect calorimetry. Indirect calorimetry and the use of specific activity and rate of 14CO2 expired yielded identical values of VCO2: 8.97 +/- 0.82 and 8.80 +/- 0.83 mmol/min, respectively (P = NS). The concentration of NaH13CO3 in the infusates and the 13C enrichment in breath CO2 were determined using gas chromatography-isotope ratio mass spectrometry. The rate of appearance of CO2 measured using the NaH13CO3 infusion rate and the steady-state breath 13CO2 enrichments was 11.41 +/- 1.56 mmol/min, which was higher (P < 0.001) than that determined by either of the other two methods. When corrected for the recovery of labeled CO2 during the infusion of NaH13CO3 by use of published values, rate of appearance of CO2 was 9.24 +/- 0.78 mmol/min, which did not differ from VCO2 determined using the other two methods. We conclude that infusion of NaH13CO3 can be used to determine VCO2. This method should be useful to study the oxidation of substrates in populations such as ventilator-dependent neonates, in whom indirect calorimetry is laborious and inaccurate.

Adult↗
PubMed2004

The measurement of energy expenditure.

Proper nutrition support depends upon the clinician's ability to estimate the patient's energy expenditure. The accuracy of estimation is inversely proportional to the severity of the patient's illness. This fact has spurred academic and industry groups to pursue the measurement of energy expenditure. Harris and Benedict used indirect calorimetry to develop their now-famous equation nearly 100 years ago. The concept of indirect calorimetry is simple; if you know the concentration of inspired gases and expired gases, along with the flow, you can determine the amount of a gas consumed or produced. The complexity and expense of indirect calorimeters suggest that this simple concept is technically challenging. Because we desire to know the energy expenditure of the most critically ill patients, indirect calorimetry is further confounded by the presence of oxygen and mechanical ventilation. This paper will discuss the myriad of variables and obstacles that complicate the measurement of energy expenditure and will suggest methods to avoid or overcome them.

Journal Article↗
PubMed1992

Derived oxygen saturations are not clinically useful for the calculation of oxygen consumption.

In critically ill patients, oxygen consumption (VO2) and delivery (DO2) are used to determine optimal haemodynamic management and to grade severity of illness. VO2 may be measured by indirect calorimetry with metabolic gas monitoring systems or derived using the reverse Fick principle. Oxygen saturation (SaO2) may be measured directly by co-oximetry or derived by equations for incorporation into reverse Fick equations. A prospective study comparing VO2 measured by these methods was performed in 20 critically ill patients. The mean VO2 measured by the metabolic gas monitoring system (308 +/- 63.9 ml/min) was significantly greater than that measured by reverse Fick using measured SaO2 (284 +/- 72.0 ml/min) (P < 0.01). This difference may be due to intrapulmonary VO2. When SaO2 was calculated from three logarithmic equations and incorporated into the reverse Fick equations, calculated VO2's were significantly greater (P < 0.001) than those measured by indirect calorimetry. Correlation was poor and wide limits of agreement (-118 to +350 ml/min) were demonstrated. VO2 should ideally be measured by indirect calorimetry in the critically ill, or if reverse Fick is used, SaO2 should be measured by co-oximetry as the use of equations for clinical measurement of SaO2 is clinically suspect.

Adult↗
PubMed2005

Thermal Characterizations of silver-containing bioactive glass-coated Sutures.

This study utilized and compared a number of thermal analysis methods to characterize the thermal properties of commercial sutures with and without antimicrobial coatings of silver-doped bioactive glass (AgBG) interlocking particulates. The effect of a slurry dipping technique used to coat resorbable Vicryl (polyglactin 910) and non-resorbable Mersilk surgical sutures with AgBG was investigated using conventional differential scanning calorimetry (DSC), high speed calorimetry (or HYPERDSC), and modulated temperature DSC (MTDSC). These methods were compared in terms of their ability to resolve the thermal transitions of the types of suture materials. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were used to verify the thermal degradation temperatures of these materials and to quantify the AgBG coatings on the sutures. The use of complementary thermal analysis techniques enabled the understanding of the effect of the AgBG coating technique on the morphological properties of the sutures. The slurry dipping technique had no significant effect on the thermal transitions of both types of materials. The use of high speed calorimetry through DSC offered better resolution for the transitions that appeared to be weak through conventional heating regimes, and was able to separate broad double transitions. Furthermore, it was shown not to compromise either the melting temperature or the enthalpy of melting. Therefore this method allows for the accurate determination of thermal transitions through much shorter experimental times thus allowing for an increased sample throughput. The combined DTA and TGA indicated that a greater AgBG coating was obtained in the case of the Mersilk sutures.

Coated Materials, Biocompatible↗
PubMedDate not supplied

[Nutritional status assessment in a group of university students by means of dietary parameters and body composition].

OBJECTIVE: To assess the nutritional status in a young adults by means of dietary parameters and body composition. METHODOLOGY: The sample comprises 49 young men (35 women and 14 men) with a mean age of 21.9 +/- 2.9 ages, at Alfonso X el Sabio University of Madrid. Body composition was analyzed through anthropometrical parameters. By means of validated questionnaires, physical activity performed within a week was assessed, as well as energy and nutrients intake. The later was estimated through a registry of all foods and beverages consumed for the last 14 days performed by each one of the participants (previously instructed) and estimating the amounts by weight or home or standard servings. The resting energy waste was determined by indirect calorimetry and through the Harris-Benedict predictive formula. RESULTS: Body mass index was similar in both men and women (p = 0.10) (23.5 +/- 1.9 and 22.0 +/- 2.8 kg/m2). The percentage of fat obtained through anthropometrics was 16.4 +/- 3.5% and 27.1 +/- 3.8% (p < 0.05) in men and women, respectively. The physical activity was light-moderate. Daily energy intake was appropriate for the total needs when the later were determined by indirect calorimetry. The caloric intake for macronutrients was unbalanced: high in lipids, and very low in carbohydrates. With regards to micronutrients, the assessed diets were in general appropriate but for folic acid, calcium, magnesium for both genders and vitamin A in men, with levels below the recommended ones. CONCLUSIONS AND RECOMMENDATIONS: In some aspects, the diet from this group deviates from current dietary standards. It would be advisable to increase the physical activity that would allow a higher food intake in order to cover for the insufficient intake of some nutrients without energetically unbalancing the diet. The assessment of resting energetic waste by indirect calorimetry seems to better predict the energetic needs.

Adult↗
PubMed2003

Interrelationships between body composition and energy expenditure in cancer malnutrition. The role of bioimpedance assessment.

AIM: The aim of the experience was a case-control evaluation of the role of biological impedance assessment (BIA) for estimating resting energy expenditure (REE) instead of indirect calorimetry (IC) in cancer patients. METHODS: Thirteen patients with gastric cancer (GK) were studied vs 18 control (C) patients. GK patients were depleted in fat free mass (FFM) (33+/-3 vs 44+/-5 kg; p<0.0001) and in body cell mass (BCM), (19+/-1 vs 26+/-3 kg; p<0.0001). RESULTS: The REE, calculated with the Harris-Benedict formula (REE-HB) was higher in C than in GK (1,443+/-149 vs 1,196+/-109 Kcal/day; p<0.0001) despite that, when evaluated by means of Indirect Calorimetry (REE-IC), it showed similar values in both groups (1,456+/-157 vs 1,353+/-210 Kcal/day; p=NS; C vs GK), a remarkable decrease of BCM in GK notwithstanding. The REE-IC/actual weight ratio was 21+/-2 Kcal/kg/day in C and 25+/-3 Kcal/kg/day in GK (p<0.005) and the REE-IC/BCM ratio was 55+/-4 Kcal/kg/day in C, and 68+/-8 Kcal/kg/day, in GK (p<0.0001), with a significant correlation between BCM and REE-IC in controls vs GK. CONCLUSIONS: The group of gastric malnourished cancer patients showed an increase in REE-IC in comparison with REE-HB; the increase in REE-IC was not related to the size of the remaining BCM; it is impossible to get a satisfying linear regression function expressing REE in terms of body composition measures obtained by means of BIA instead of indirect calorimetry; the IC is more appropriate to evaluate REE in cancer malnourished patients because in the present experience the Harris-Benedict formula underestimates this parameter by more than 10% (11.06%).

Journal Article↗
PubMed1990

[Respiratory gas exchange during total body hyperthermia in man].

Respiratory gas exchange was evaluated using indirect calorimetry during total body hyperthermia (TBH) in 7 postoperative patients with disseminated gastric cancer to the peritoneal cavity. TBH was induced using veno-venous bypass and extracorporeal circuit incorporating a heat exchanger to keep pulmonary arterial temperature 42 degrees C. The high temperature was maintained for 3 hours under general anesthesia with droperidol (0.15 mg.kg-1), morphine (1 mg.kg-1) and enflurane (less than 0.5%). Oxygen consumption (VO2) was also calculated using the Fick equation and thermodilution cardiac output. Hyperthermia for three hours increased both VO2 and carbon dioxide output (VCO2) values to 1.5 times as compared to the control values before heating, respectively. VO2 values measured by indirect calorimetry correlated well with the values calculated from the Fick equation when respiratory and cardiovascular system were relatively stable during the procedure. Gradual but statistically insignificant increase in respiratory quotient was observed after recooling started. These results suggest that indirect calorimetry was valuable to measure respiratory gas exchange continuously during hyperthermic state as well as normothermic state. However, RQ value observed in the present study may be modified by several factors including lactate accumulation in the blood, and may not reflect substrate utilization during the hyperthermic state.

Adult↗
PubMed1999

The effect of inhibitor binding on the structural stability and cooperativity of the HIV-1 protease.

The effects of the peptide inhibitor acetyl pepstatin on the structural stability of the HIV-1 protease have been measured by high sensitivity calorimetric techniques. At 25 degrees C and pH 3.6, acetyl pepstatin binds to HIV-1 protease with an affinity of 1.6 x 10(7 )M-1 and an enthalpy of 7.3 +/- 0.5 kcal/mol, indicating that binding is not favored enthalpically and that the favorable Gibbs energy originates from a large positive entropy. Since the binding of acetyl pepstatin is associated with a negative change in heat capacity (-450 cal/K*mol) the association reaction becomes enthalpically favored at temperatures higher than 40 degrees C. The presence of the inhibitor stabilizes the dimeric structure of the protease in a fashion that can be quantitatively described by a set of thermodynamic linkage equations. The combination of titration and differential scanning calorimetry provides an accurate way of determining binding constants for high affinity inhibitors that cannot be determined by titration calorimetry alone. A structure-based thermodynamic analysis of the binding process indicates that the stabilization effect is not distributed uniformly throughout the protease molecule. The binding of the inhibitor selectively stabilizes those conformational states in which the binding site is formed, triggering a redistribution of the state probabilities in the ensemble of conformations populated under native conditions. As a result, the stability constants for individual residues do not exhibit the same change in magnitude upon inhibitor binding. Residues in certain areas of the protein are affected significantly whereas residues in other areas are not affected at all. In particular, inhibitor binding has a significant effect on those regions that define the binding site, especially the flap region which becomes structurally stable as a result of the additional binding free energy. The induced stabilization propagates to regions not in direct contact with the inhibitor, particularly to the strand between residues Pro9 and Ala22 and the helix between Arg87 and Gly94. On the other hand, the stability of the strand between Asp60 and Leu76 is not significantly affected by inhibitor binding. The structural distribution of binding effects define cooperative pathways within the protease molecule. Proteins 1999;36:147-156.

Algorithms↗