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At least 721 records · Page 40Linked to original sources
PubMed1991

Differential scanning calorimetry of bovine rhodopsin in rod-outer-segment disk membranes.

Rhodopsin-containing retinal rod disk membranes from cattle have been examined by differential scanning calorimetry. Under conditions of 67 mM phosphate pH 7.0, unbleached rod outer segment disk membranes gave a single major endotherm with a temperature of denaturation (Tm) of 71.9 +/- 0.4 degrees C and a thermal unfolding calorimetric enthalpy change (delta Hcal) of 700 +/- 17 kJ/mol rhodopsin. Bleached rod outer segment disk membranes (membranes that had lost their absorbance at 498 nm after exposure to orange light) gave a single major endotherm with a Tm of 55.9 +/- 0.3 degrees C and a delta Hcal of 520 +/- 17 kJ/mol opsin. Neither bleached nor unbleached rod outer segment disk membranes gave endotherms upon thermal rescans. When thermal stability is examined over the pH range of 4-9, the major endotherms of both bleached and unbleached rod outer segment disk membranes were found to show maximum stability at pH 6.1. The observed delta Hcal values for bleached and unbleached rod outer segment disk membranes exhibit membrane concentration dependences which plateau at protein concentrations beyond 1.5 mg/mL. For partially bleached samples of rod outer segment disk membranes, the calorimetric enthalpy change for opsin appears to be somewhat dependent on the degree of bleaching, indicating intramembrane nearest neighbor interactions which affect the unfolding of opsin. Delta Hcal and Tm are particularly useful for assessing stability and testing for completeness of regeneration of rhodopsin from opsin. Other factors such as sample preparation and the presence of low concentrations of ethanol also affect the delta Hcal values while the Tm values remain fairly constant. This shows that the delta Hcal is a sensitive parameter for monitoring environmental changes of rhodopsin and opsin.

Animals↗
PubMed1992

Differential scanning calorimetry of chicken erythrocyte nuclei.

Investigation of structural features of native chromatin requires the use of intact nuclei, a turbid material which cannot be analyzed by optical methods. Differential scanning calorimetry does not require optically clear samples and has been proved by a number of authors to be a powerful tool in this field of study. By this technique, chicken erythrocyte nuclei were found to undergo at least four thermal transitions, centered at 59, 74, 88 and 98 degrees C. The highest temperature transition is strongly dependent on age and storage conditions of the nuclei. Adequate storage conditions overcame this problem and reproducible scans were obtained over a period of several months. This technical improvement has permitted the reconsideration of the occurrence of the fourth calorimetric transition, previously believed to be displayed only in replicating nuclei. Evidence gathered in the presence of perturbants and possible ligands allows the assignment of the four transitions to a nuclear protein scaffold, histones, nucleosomal DNA and a superstructured form of DNA. Moreover, it suggests that the higher-order structure is stabilized by fibronectin-like proteins.

Animals↗
PubMed1995

Assessment of glucose metabolism in humans with the simultaneous use of indirect calorimetry and tracer techniques.

Concomitant measurements of sytemic glucose delivery and carbohydrate oxidation are frequently performed in human investigations. Systemic glucose delivery (SGD) is usually determined using dilution of infused glucose tracers; net carbohydrate oxidation rate (net CHOOX) can be calculated from respiratory gas exchanges and urinary nitrogen excretion (indirect calorimetry); alternatively, glucose oxidation can be measured from labelled CO2 production during infusion of carbon-labelled glucose tracers. In this paper, the theory underlying the use of each of these techniques is briefly reviewed and qualitative differences are outlined. SGD represents the sum of hepatic glucogenolysis, gluconeogenesis from amino acids or glycerol, and, according to the glucose tracer used, glucose cycles (glucose-phosphate cycle, fructose-phosphate cycle, Cori and glucose-alanine cycles); systemic delivery of exogenous glucose after oral or i.v. glucose administration is also measured. Net CHOOX represents oxidation of glucose arising from hepatic or muscle glycogen or from exogenous glucose; it does not take into account oxidation of glucose formed from amino acids or glycerol, which is included in net protein or lipid oxidation. In contrast, isotopic determination of glucose oxidation corresponds to oxidation of glucose originating from hepatic glycogen breakdown, of exogenously administered glucose, and of glucose formed from amino acids and glycerol. Non-oxidative glucose disposal, calculated as SGD-net CHOOX, corresponds to the sum of gluconeogenesis from amino acids or glycerol (which are included in net protein and lipid oxidation), glucose cycles, and glycogen synthesis.

Blood Glucose↗
PubMed2006

Whole-body fat oxidation determined by graded exercise and indirect calorimetry: a role for muscle oxidative capacity?

During whole-body exercise, peak fat oxidation occurs at a moderate intensity. This study investigated whole-body peak fat oxidation in untrained and trained subjects, and the presence of a relation between skeletal muscle oxidative enzyme activity and whole-body peak fat oxidation. Healthy male subjects were recruited and categorized into an untrained (N=8, VO(2max) 3.5+/-0.1 L/min) and a trained (N=8, VO(2max) 4.6+/-0.2 L/min) group. Subjects performed a graded exercise test commencing at 60 W for 8 min followed by 35 W increments every 3 min. On a separate day, muscle biopsies were obtained from vastus lateralis and a 3 h bicycle exercise test was performed at 58% of VO(2max). Whole-body fat oxidation was calculated during prolonged and graded exercise from the respiratory exchange ratio using standard indirect calorimetry equations. Based on the graded exercise test, whole-body peak fat oxidation was determined. The body composition was determined by DEXA. Whole-body peak fat oxidation (250+/-25 and 462+/-33 mg/min) was higher (P<0.05) and occurred at a higher (P<0.05) relative workload (43.5+/-1.8% and 49.9+/-1.2% VO(2max)) in trained compared with untrained subjects, respectively. Muscle citrate synthase activity and beta-hydroxy-acyl-CoA-dehydrogenase activity were higher (49% and 35%, respectively, P<0.05) in trained compared with untrained subjects. Both lean body mass and maximal oxygen uptake were significantly correlated to whole-body peak fat oxidation (r(2)=0.57, P<0.001), but leg muscle oxidative capacity was not correlated to whole-body peak fat oxidation. In conclusion, whole-body peak fat oxidation occurred at a higher relative exercise load in trained compared with untrained subjects. Whole-body peak fat oxidation was not significantly related to leg muscle oxidative capacity, but was related to lean body mass and maximal oxygen uptake. This may suggest that leg muscle oxidative activity is not the main determinant of whole-body peak fat oxidation.

Absorptiometry, Photon↗
PubMed1993

Non-invasive techniques for assessing carbohydrate flux: I. Measurement of depletion by indirect calorimetry.

Glycogen forms the smallest yet most labile energy substrate store. Therefore studying carbohydrate flux may be crucial to understanding the regulation of energy balance. Indirect calorimetry has been used to measure carbohydrate oxidation overnight and during exercise in nine fasted subjects. Overnight carbohydrate oxidation (averaging 2.85 +/- 0.8 g h-1) was assumed to be derived primarily from hepatic glycogen since subjects were inactive or asleep, and since glucose oxidized after gluconeogenesis from protein is measured as protein oxidation. Lower-limb muscle glycogen stores were depleted by repeated 30-min periods of cycle ergometry at 45% VO2max until exhaustion (8 +/- 1 periods). The carbohydrate oxidation rate decreased as exercise progressed. Quadratic curves yielded a close fit to each individual's exercise carbohydrate depletion data (mean multiple correlation r = 0.9996) and provided excellent inter-subject discrimination. Total (muscle plus liver) glycogen stores prior to exercise were estimated by extrapolation of the depletion curves to zero oxidation rate. This produced an extrapolation of the depletion curves to zero oxidation rate. This produced an estimate (174 +/- 61 g) which compared well with predictions (208 +/- 43 g) based on reference values for muscle mass and initial glycogen content. The results demonstrate that non-invasive estimates of glycogen status can be obtained from accurate respiratory exchange data.

Adult↗
PubMed1990

Determination of triplet excitation energies of cyclic enones by time-resolved photoacoustic calorimetry.

Energies and lifetimes of triplet states of a series of 2-cyclohexenones have been measured in solution under ambient conditions by time-resolved photoacoustic calorimetry (PAC). The PAC triplet lifetimes are in excellent agreement with lifetimes measured by kinetic absorption spectrophotometry (KAS) using nanosecond flash techniques, indicating that the present data indeed pertain to the enone triplets previously studied using KAS. The data demonstrate that, as previously proposed, the pi pi triplet energies in these systems are indeed correlated with triplet lifetimes, and vary sensibly with the anticipated extent of conformational flexibility of the C = C bond. No evidence supporting formation of metastable trans-cyclohexenones was obtained. The data also demonstrate greater precision and, for lifetimes, greater accuracy than has hitherto been claimed for the PAC technique for study of transients with lifetimes in the 25 ns range.

Calorimetry↗
PubMed1990

Efficiency of the photoprocesses leading to singlet oxygen (1 delta g) generation by alpha-terthienyl: optical absorption, optoacoustic calorimetry and infrared luminescence studies.

The triplet energy of alpha-terthienyl has been determined by heavy atom-induced optical absorption: the value of 39.7 +/- 1.5 kcal/mol is consistent with earlier energy transfer work. Combining this result with calorimetric data from optoacoustic calorimetry indicates that intersystem crossing occurs with at least 90% efficiency in polar and non-polar solvents. The quantum yields for singlet oxygen formation via energy transfer from triplet alpha-terthienyl have been obtained from time-resolved measurements of its IR phosphorescence: these yields are in the 0.6-0.8 range in non-polar and polar (hydroxylic and non-hydroxylic) solvents.

Calorimetry↗
PubMed1996

Model studies of DNA photorepair: enthalpy of cleavage of a pyrimidine dimer measured by photothermal beam deflection calorimetry.

The enzyme DNA photolyase mediates the repair of pyrimidine dimers. This repair step, a net retro [2 + 2] reaction, proceeds through either the cation or anion radical of the pyrimidine dimer. In order to understand how electron transfer makes the repair process possible, its energetics have been examined by photothermal beam deflection calorimetry, fluorescence quenching and quantum yield studies. The enthalpy for the cleavage reaction of cis-syn 1,3-dimethylthymine dimer itself was found to be -19 kcal/mol. In addition, from the redox potentials, the enthalpies for the cleavage reactions of the dimer cation radical and the anion radical were determined to be -19 kcal/mol and -28 kcal/mol, respectively.

Calorimetry↗
PubMed1997

Photothermal beam deflection calorimetry in solution photochemistry: recent progress and future prospects.

Photothermal beam deflection (PBD) calorimetry is a technique that measures changes in the solvent's refractive index that accompanies photothermal heating. This method is capable of extracting both kinetic and thermodynamic information from photochemical reactions. A qualitative description of physical basis of time-resolved PBD is presented. Several recent examples of its application to photochemical and photobiological problems are discussed. Finally, the advantages and limitations of PBD are described.

Calorimetry↗
PubMed1978

Interaction of cortisol-21-palmitate with liposomes examined by differential scanning calorimetry.

Liposomes have been suggested as carriers for corticosteroids in the local treatment of arthritis by intra-articular injection. The long chain 21-esters of cortisol such as the palmitate or octanoate are taken up and retained by liposomes in higher concentration than cortisol itself. Differential scanning calorimetry has been used to show that the cortisol ester is anchored in the liposome phospholipid bilayer by the acyl side chain. In addition, the limiting concentration of cortisol-21-palmitate which can be incorporated into dipalmitolyl phosphatidylcholine liposomes has been measured by observing changes in the DSC spectrum at different steroid concentrations. Steroid in excess of this concentration limit forms a separate phase which can be identified by nuclear magnetic resonance. For optimum effect, the treatment of arthritis with liposomes must be carried out with liposomes containing steroid below the limiting concentration.

Arthritis, Rheumatoid↗
PubMed1987

A quantitative assessment of the reactivity of the fatty alcohols with cetrimide using immersion calorimetry.

The reactivity of the fatty alcohols with cetrimide has been quantitatively assessed using immersion calorimetry. In all cases the reaction was endothermic i.e. it had a positive enthalpy. For the n-alcohols the reactivity, as evaluated by the rate of enthalpy change, decreased with increasing chain length although branching on the tetradecanol and hexadecanol resulted in a higher reactivity. Adding 1-octadecanol to 1-hexadecanol resulted in an increased reactivity rising to a maximum for mixtures containing 20-40% w/w 1-octadecanol. The results have been interpreted in terms of the polymorphic form of the alcohol.

Calorimetry↗
PubMed1990

Differential scanning calorimetry characterization of process-induced variations in an ointment base.

Preparation of an experimental emollient wax-gelled ointment base by two processes differing only in cooling rate produced material with markedly different physical properties. Differential scanning calorimetry showed that a major endotherm, possibly related to a phase change in a major triglyceride wax component, Synchrowax HGLC, was different in the two products. Mean enthalpies for this major endotherm for the two products were 7.36 J g-1 (s.d. = 0.49, n = 5) in slow cooled samples and 4.35 J g-1 (s.d. = 0.21, n = 5) in fast cooled samples. The degree of order of the Synchrowax HGLC in the ointment is suggested as being different in the two preparations and it is this that controls the physical properties of the ointment.

Calorimetry, Differential Scanning↗
PubMed1995

An attempt to clarify the mechanism of the penetration enhancing effects of lipophilic vehicles with differential scanning calorimetry (DSC).

In a previous in-vivo skin penetration study, it was observed that certain lipophilic liquid vehicles enhanced drug penetration, whilst others did not. To clarify the mechanism of skin penetration enhancement, isolated sheets of human stratum corneum were measured by differential scanning calorimetry (DSC), either untreated or after pretreatment with various lipophilic liquids (highly purified light mineral oil, isopropyl myristate, caprylic/capric acid triglycerides containing 5% phospholipids, dibutyl adipate, dimethicone 100, cetearyl iso-octanoate, caprylic/capric acid triglycerides), commonly used in ointment bases. All samples were analysed over a heating range of at least--10-130 degrees C. All DSC curves were evaluated with regard to the phase-transition enthalpies (peak areas) and peak maximum temperatures of the lipid-phase transitions at ca 75 and 85 degrees C. With the exception of dimethicone 100, cetearyl iso-octanoate and caprylic/capric acid triglycerides, all vehicles showed characteristic alterations of the phase-transition temperatures and enthalpies of the stratum corneum lipids. Mineral oil and isopropyl myristate caused a reduction of the enthalpy and a decrease of the phase-transition temperatures. These two vehicles are thought to fluidize the lamellar-gel phase of the stratum corneum lipids, and possibly partially dissolve the lipids. Dibutyl adipate and caprylic/capric acid triglycerides containing 5% phospholipids decreased the phase-transition enthalpy only, probably due to dissolution or extraction of the stratum corneum lipids. These DSC results provide an explanation for the in-vivo penetration-enhancing effects observed previously.

Calorimetry, Differential Scanning↗
PubMedDate not supplied

Absorbed dose calorimetry: bridge nulling and voltage analysis techniques.

A theoretical analysis is made of the conventional calorimeter approach utilizing a two-thermistor Wheatstone bridge--an approach which requires balancing the bridge by adjusting variable resistances. However, with the current availability of computers and digital precision measuring instruments, the conventional calorimeter approach can now be readily replaced by an automated, nonmechanical, electrical systems approach called differential calorimetry. The analytical theory outlining such an approach is also presented.

Calorimetry↗
PubMed1988

Time-resolved photoacoustic calorimetry: probing the energetics and dynamics of fast chemical and biochemical reactions.

Time-resolved photoacoustic calorimetry is a new experimental technique that measures the dynamics of enthalpy changes on the time scale of nanoseconds to microseconds for reactions initiated by absorption of light. When the reaction is carried out in water, it is also possible to obtain the dynamics of the corresponding volume changes. This method has been applied to a variety of biochemical, organic, and organometallic reactions.

Calorimetry↗
PubMed1986

Lipid domains in fluid membranes: a quick-freeze differential scanning calorimetry study.

The application of rapid-freezing techniques to differential scanning calorimetry (DSC) provides a new approach for understanding the organization of lipids in biomembranes. Use of quick-freeze DSC on membranes of mixed lipid composition supports the existence of nonrandom distributions of lipids (domains) in fluid bilayers. In addition to allowing investigations on the organization of lipids in fluid bilayers, the quick-freeze technique now allows calorimetric studies to be carried out on mammalian membranes which, because of their high cholesterol content, have not been previously amenable to study by DSC.

Calorimetry, Differential Scanning↗
PubMed1983

Quaternary and quinternary structures of native chromatin DNA in liver nuclei: differential scanning calorimetry.

Differential scanning calorimetry of chromatin isolated from rat liver cells revealed three discrete thermal transitions whose temperatures and melting enthalpies depend on ionic strength in the range 0 to 600 millimolar NaCl. Intact nuclei showed a fourth thermal transition at a lower temperature and different melting enthalpies for the other three transitions still present at temperatures similar to those obtained in isolated chromatin. The data are discussed in terms of the tertiary, quaternary, and quinternary structures of chromatin DNA.

Animals↗
PubMed2002

Evaluation of the heat inactivation of Escherichia coli and Lactobacillus plantarum by differential scanning calorimetry.

Differential scanning calorimetry (DSC) is used to evaluate the thermal stability and reversibility after heat treatment of transitions associated with various cellular components of Escherichia coli and Lactobacillus plantarum. The reversibility and the change in the thermal stability of individual transitions are evaluated by a second temperature scan after preheating in the DSC to various temperatures between 40 and 130 degrees C. The viability of bacteria after a heat treatment between 55 and 70 degrees C in the DSC is determined by both plate count and calorimetric data. The fractional viability values based on calorimetric and plate count data show a linear relationship. Viability loss and the irreversible change in DSC thermograms of pretreated whole cells are highly correlated between 55 and 70 degrees C. Comparison of DSC scans for isolated ribosomes shows that the thermal stability of E. coli ribosomes is greater than that of L. plantarum ribosomes, consistent with the greater thermal tolerance of E. coli observed from viability loss and DSC scans of whole cells.

Calorimetry, Differential Scanning↗