Thermally induced conformational changes in horseradish peroxidase.

Detailed differential scanning calorimetry (DSC), steady-state tryptophan fluorescence and far-UV and visible CD studies, together with enzymatic assays, were carried out to monitor the thermal denaturation of horseradish peroxidase isoenzyme c (HRPc) at pH 3.0. The spectral parameters were complementary to the highly sensitive but integral method of DSC. Thus, changes in far-UV CD corresponded to changes in the overall secondary structure of the enzyme, while that in the Soret region, as well as changes in intrinsic tryptophan fluorescence emission, corresponded to changes in the tertiary structure of the enzyme. The results, supported by data about changes in enzymatic activity with temperature, show that thermally induced transitions for peroxidase are irreversible and strongly dependent upon the scan rate, suggesting that denaturation is under kinetic control. It is shown that the process of HRPc denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme N -->k D where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated.

Compact residual structure in lentil lectin at pH 2.

Lentil lectin obtained from Lens culinaris collected in the La Armuña area (Salamanca, Spain) was examined by high-sensitivity differential scanning calorimetry, fluorimetry and measurements of circular dichroism at pH 2.0 and 7.4. At pH 2.0 the lentil lectin is not in the native state; however, at this pH it does show signs of a residual structure that breaks down upon heating. The lentil lectin at pH 2 shares some similarities with what has become known as the molten globule state. The thermal denaturation of intact (pH 7.4) and partially unfolded (pH 2.0) lentil lectin was irreversible and strongly dependent upon the scan rate, suggesting that its denaturation is under kinetic control. The process of lentil lectin denaturation is interpreted in terms of the simple kinetic model, Nk --> D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state.

Comparative calorimetric study of non-amyloidogenic and amyloidogenic variants of the homotetrameric protein transthyretin.

Familial amyloidotic polyneuropathy (FAP) is an autosomal dominant hereditary type of amyloidosis involving amino acid substitutions in transthyretin (TTR). V30M-TTR is the most frequent variant, and L55P-TTR is the variant associated with the most aggressive form of FAP. The thermal stability of the wild-type, V30M-TTR, L55P-TTR and a non-amyloidogenic variant, T119M-TTR, was studied by high-sensitivity differential scanning calorimetry (DSC). The thermal unfolding of TTR is a spontaneous reversible process involving a highly co-operative transition between folded tetramers and unfolded monomers. All variants of transthyretin are very stable to the thermal unfolding that occurs at very high temperatures, most probably because of their oligomeric structure. The data presented in this work indicated that for the homotetrameric form of the wild-type TTR and its variants, the order of stability is as follows: wild-type TTR approximately > T119M-TTR > L55P-TTR > V30M-TTR, which does not correlate with their known amyloidogenic potential.

pH-dependent thermal transitions of lentil lectin.

The thermal stability of lentil lectin in the 5.0-10.0 pH range was studied by high-sensitivity differential scanning calorimetry and infrared spectroscopy. The thermally induced transitions for protein were irreversible and strongly dependent upon the scan rate at all pH values, suggesting that the denaturation is under kinetic control. It is shown that process of lentil lectin denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme, N-->D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation, N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated.

Thermodynamic stability of two variants of xylanase (Xys1) from Streptomyces halstedii JM8.

In a continuation of our earlier study [Ruiz-Arribas, A., Santamaría, R.I., Zhadan, G. G., Villar, E. & Shnyrov, V. L. (1994) Differential scanning calorimetric study of the thermal stability of xylanase from Streptomyces halstedii JM8, Biochemistry 33, 13787-13791], we used high-sensitivity differential scanning microcalorimetry, intrinsic tryptophan fluorescence and far-ultraviolet circular dichroism to study the effect of regional sequence differences on the thermodynamic stability of xylanase (Xys1) from Streptomyces halstedii JM8 (1,4-beta-D-xylanohydrolase, EC 3.2.1.8). Thermal transitions were measured for original xylanase (Xys1S) and two variants. Thermal denaturation of all the xylanases studied revealed two structural domains, each of which, despite its partial irreversibility, follows a two-state thermal unfolding process under our experimental conditions. Both variants were found to exhibit slightly decreased stability, possessing the same activity as the original. The unfolding parameters for each domain of both variants, unlike the situation with wild-type xylanase (see our previous report), fit some correlations obtained for the most compact globular proteins. The values of enthalpy and entropy of unfolding/residue at 383 K were found to be inversely proportional to residual, well-regulated structures in unfolded states.

Protein involvement in thermally induced structural transitions of pig erythrocyte ghosts.

Thermal transitions in pig erythrocyte ghosts were studied by differential scanning calorimetry and thermal gel analysis (TGA). Heating of the suspension of pig erythrocyte ghosts induced at least six thermodynamically irreversible transitions. Each of these transitions is believed to be due to a localized structural transition induced by thermal stress. Using TGA and covalent attachment of the anionic transport inhibitor regions in the thermograms corresponding to the heat sorption of some proteins of the pig erythrocyte ghosts were identified.

A differential scanning calorimetric study of Newcastle disease virus: identification of proteins involved in thermal transitions.

The irreversible thermal denaturation of Newcastle disease virus was investigated using different techniques including high-sensitivity differential scanning calorimetry, thermal gel analysis intrinsic fluorescence, and neuraminidase activity assays. Application of a successive annealing procedure to the scanning calorimetric endotherm of Newcastle disease virus furnished four elementary thermal transitions below the overall endotherm; these were further identified as coming from the denaturation of each viral protein. The shape of these transitions, as well as their scanrate dependence, was explained by assuming that thermal denaturation takes place according to the kinetic scheme N-->(k)D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state; and D is the denatured state. On the basis of this model, activation energy values were calculated. The data obtained with the other methods used in this work support the proposed two-state kinetic model.

Differential scanning calorimetry of the irreversible thermal denaturation of cellulase from Streptomyces halstedii JM8.

High-sensitivity differential scanning calorimetry has been applied to characterize the irreversible thermal denaturation of a cellulase, assuming that thermal denaturation takes place according to the kinetic scheme N-k-->D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N the native state, and D the denatured one. On the basis of this model, the values of the rate constant as a function of temperature and the activation energy were calculated. The analytical data obtained with the fluorescence method as well by measurement of the enzymatic activity temperature dependence support this two-state kinetic model.

Differential scanning calorimetric study of the thermal stability of xylanase from Streptomyces halstedii JM8.

The thermal stability of two xylanases with molecular masses of 45 (Xys1L) and 35 (Xys1S) kDa has been characterized thermodynamically by high-sensitivity scanning microcalorimetry in the pH range 3.0-9.0. Thermal denaturation of Xys1L reveals three thermodynamically independent domains, and that of Xys1S, which is a proteolytic fragment of Xys1L (without a C-terminal part), reveals two thermodynamically independent domains, each of which follows a two-state thermal unfolding process under our experimental conditions. Nevertheless, the thermodynamic parameters of unfolding for each domain do not fit some of the correlations obtained for most compact globular proteins. It is known that if delta Hres(T) and delta Sres(T) are plotted against temperature for a number of water-soluble compact globular proteins, they all have a common value at approximately 110 degrees C (383 K). Calculation of the variations in the enthalpy and entropy of unfolding per residue for each domain of xylanase with temperature gave us delta Hres(383) and delta Sres(383) values of approximately 3 kcal/(mol of residue) and 9 cal/(K.mol of residue), respectively. This is practically 2-fold larger than those apparent for most medium-sized globular protein values. These discrepancies might be related to features of the folded and/or unfolded states of the protein.

Differential-scanning-calorimetric study of the irreversible thermal denaturation of 8 kDa cytotoxin from the sea anemone Radianthus macrodactylus.

A differential-scanning-calorimetric study of the thermal denaturation of a sea-anemone (Radianthus macrodactylus) 8 kDa cytolytic toxin was carried out. The calorimetric traces were found to be irreversible and scan-rate-dependent under the experimental conditions employed. Scan-rate-dependent thermograms were explained in terms of a two-state kinetic model N k -->D, where k is a first-order kinetic constant that changes with temperature as given by the Arrhenius equation, N is the native state of the toxin, and D the denatured one.

Multi-stage nature of the thermal denaturation process in collagen.

Thermal denaturation of acid-soluble collagen from polar cod (Eleginus gracialis) skin has been studied by scanning microcalorimetry and intrinsic spectrofluorimetry methods. The thermal denaturation process occurs in three independent stages reflecting the melting of 33 kDa, 97 kDa, and 230 kDa domains. Thermodynamical parameters of the collagen denaturation have been determined.

Calorimetric study of interactions of toxin from Radianthus macrodactylus with erythrocyte membrane.

Differential scanning microcalorimetry, intrinsic protein fluorescence and SDS-electrophoresis have been applied for the study of the hemolytic toxin from sea anemone, Radianthus macrodactylus, when it is incorporated into pure sphingomyelin/water systems and upon its effect on the human and dog erythrocyte membranes. The results obtained by using these techniques showed that one molecule of toxin withdraws six sphingomyelin molecules from the cooperative transition. Effect of hemolytic toxin on human erythrocyte ghosts causes an appearance of a new heat sorption peak in their differential scanning calorimetric curve with a maximum of 36 degrees C and, moreover, it leads to a loss of one of cytoskeleton proteins (actin). These effects are essentially weaker in the case of dog erythrocyte ghosts. This suggests differences in the structural organization of human and dog erythrocyte membranes.

Thermal inactivation of membrane proteins, volume-dependent Na+, K(+)-cotransport, and protein kinase C activator-induced changes of the shape of human and rat erythrocytes.

The heat sorption curves for human and rat erythrocyte membranes in the temperature region from 40 degrees C to 90 degrees C have been compared resulting from thermal gel analysis of these membranes. The main heat sorption peaks are located within the temperature regions from 49 degrees C to 52 degrees C (A transition), from 56 degrees C to 59 degrees C (B transition), from 62 degrees C to 65 degrees C (C transition) and from 74 degrees C to 82 degrees C (D transition). Thermoinactivation temperatures for most of the membrane proteins in human and rat erythrocytes are rather different, which suggests essential differences in the arrangement of their membrane frameworks. Addition of the protein kinase C activator TPA induces a fast increase in light scattering of human and rat erythrocyte suspension which is connected with some changes of cell shape. This reaction is completely blocked by a minimal thermal treatment of the membrane framework proteins (preincubation at the temperature of the A transition). Such treatment inhibits also the increase of Na+, K(+)-cotransport in rat erythrocytes induced by hyperosmotic shrinkage. It is assumed that the proteins of the membrane framework take part in the volume-dependent regulation of the ion-transport systems of plasma membranes.

Thermal transitions in rat erythrocyte ghosts.

Thermal transitions in rat erythrocyte ghosts have been studied by means of scanning microcalorimetry, intrinsic fluorescence, circular dichroism and infrared spectroscopy. Heating of the suspension of rat erythrocyte ghosts induced at least four thermodynamically irreversible transitions. Each of these transitions is believed to be due to a localized structural transition induced by thermal stress. The influence of pH, papain, phospholipase and other modifications on the thermal transitions in rat erythrocyte ghosts is demonstrated.

Calorimetric measurements of the effect of 330-MHz radiofrequency radiation on human erythrocyte ghosts.

Irreversible changes in the heat capacity of human erythrocyte ghost suspensions due to the effect of 330-MHz radiofrequency radiation (at a specific absorption rate of approximately 9 mW/g) were detected by the method of scanning differential microcalorimetry. Using the data obtained from the analysis of infrared spectra of air-dried films of erythrocyte membranes, it can be postulated that the observed microcalorimetric changes are connected with the local interaction of electromagnetic radiation with the channel-forming portion of band-3 protein.

[Microcalorimetric study of erythrocyte membrane suspensions in normal subjects and in hypertension]. / Mikrokalorimetricheskoe issledovanie suspenzii membran éritrotsitov v norme i pri gipertonicheskoi bolezni.

Microcalorimetric method was used to study the temperature dependence of thermal absorption of human red cell membrane suspension in health and essential hypertension. In disease, the temperature dependence of individual irreversible thermoinduced transitions characteristic for human red cell membranes was appreciably changed, which was comparable with disorders of protein-lipid interactions discovered before by fluorescent examination, while elevated enthalpy of one of the transitions pointed to the possibility of enriching these membranes with band 3 protein. Unlike normal, red cell membranes in essential hypertension exhibited a special thermoinduced irreversible transition with the thermoabsorption maximum about 41 degrees C.

[Effect of focused ultrasound on the electroreceptor system of skates and certain fish and amphibian tissues]. / Deistvie fokusirovannogo ul'trazvuka na élektroretseptornuiu sistemu skatov i na nekotorye tkani ryb i amfibii.

The effects of focused ultrasound have been investigated on the structures of electroreceptor system of the Black Sea skates (Raja clavata, Dasyatis pastinaca), as well as on the tissues of some other poikilotherms (fishes Belone belone and Solea lascaris, toad Bufo viridis, frog Rana temporaria). It was shown that focused ultrasound evokes excitation (onset of impulse activity) when applied to the pore of electroreceptors and inhibition of the activity when applied to the ampulla itself. The data obtained indicate that thermal conductivity of tissues in poikilotherms is lower than that in homoiotherms.

[Mechanism of temperature sensitivity of ampulla of Lorenzini electroreceptors]. / Issledovanie mekhanizma temperaturnoi chuvstvitel'nosti élektroretseptorov ampul Lorentsini.

In acute experiments on the Black sea skates (Raja clavata) the transepithelial potential of Lorenzinian ampullae and impulse activity of their single nerve fibres were studied during electrical and thermal stimulation. The potential inside the canal and input resistance of the ampulla were 0 divided by -2 mV and 250-400 k omega respectively. Heating of the capsule was accompanied by a negative wave of the potential, an increase in the ampulla resistance and a decrease in spike frequency. Cooling of the preparation caused the opposite changes in the parameters. Waves of the transepithelial potential are supposed to reflect changes in the potential difference across the basal membrane of a receptor cell, which is hyperpolarized by heating.