Spontaneous fusion of phosphatidylcholine small unilamellar vesicles in the fluid phase

Using a high-sensitivity differential scanning microcalorimeter capable of performing cooling scans, we have examined the phase behavior of small unilamellar vesicles (SUV) as a function of time of storage above their order-disorder phase transition. Vesicles composed of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were examined. Cooling scans on fresh (5-7-h postsonication) samples revealed broad, relatively simple heat capacity peaks (peak temperatures: 19.9 degrees C for DMPC, 37.8 degrees C for DPPC) free of high-temperature spikes or shoulders. Subsequent heating scans displayed a sharp peak characteristic of previously described fusion products formed below the phase transition. SUV samples stored for 1 or more days above their phase transition displayed a moderately broad, high-temperature shoulder (23.8 degrees C for DMPC and 40.2 degrees C for DPPC) in the cooling profile. For DMPC, the enthalpy associated with this peak increased in a first-order fashion with time. Hydrolysis products were not detected until 12-20 days of storage. Both the rate and extent of shoulder appearance increased with temperature (k = 0.0017 h-1, fraction of total enthalpy = 0.1 at 36 degrees C; k = 0.0037 h-1, fraction = 0.2 at 42 degrees C). Freeze-fracture electron micrographs confirmed that an intermediate-sized vesicle population (diameters 400-500 A) appeared in SUV samples stored above their phase transition. Also, the trapped volume of DMPC SUV increased from 0.26 microL/mumol after 17 h of storage to 0.54 microL/mumol after storage for 16 days at 36 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

31P NMR saturation-transfer measurements in Saccharomyces cerevisiae: characterization of phosphate exchange reactions by iodoacetate and antimycin A inhibition

31P nuclear magnetic resonance (NMR) saturation-transfer (ST) techniques have been used to measure steady-state flows through phosphate-adenosine 5'-triphosphate (ATP) exchange reactions in glucose-grown derepressed yeast. Our results have revealed that the reactions catalyzed by glyceraldehyde-3-phosphate dehydrogenase/phosphoglycerate kinase (GAPDH/PGK) and by the mitochondrial ATPase contribute to the observed ST. Contributions from these reactions were evaluated by performing ST studies under various metabolic conditions in the presence and absence of either iodoacetate, a specific inhibitor of GAPDH, or the respiratory chain inhibitor antimycin A. Intracellular phosphate (Pi) longitudinal relaxation times were determined by performing inversion recovery experiments during steady-state ATP gamma saturation and were used in combination with ST data to determine Pi consumption rates. 13C NMR and O2 electrode measurements were also conducted to monitor changes in rates of glucose consumption and O2 consumption, respectively, under the various metabolic conditions examined. Our results suggest that GAPDH/PGK-catalyzed Pi-ATP exchange is responsible for antimycin-resistant saturation transfer observed in anaerobic and aerobic glucose-fed yeast. Kinetics through GAPDH/PGK were found to depend on metabolic conditions. The coupled system appears to operate in a unidirectional manner during anaerobic glucose metabolism and bidirectionally when the cells are respiring on exogenously supplied ethanol. Additionally, mitochondrial ATPase activity appears to be responsible for the transfer observed in iodoacetate-treated aerobic cells supplied with either glucose or ethanol, with synthesis of ATP occurring unidirectionally.     

Regulation of enterochelin synthetase in Escherichia coli K-12

Enterochelin synthetase activity is controlled by both repression and feed-back inhibition mechanisms. Inclusion of iron in growth media results in synthesis of all four (D, E, F and G) components of enterochelin synthetase being repressed. The specific inhibition of L-serine activation (partial reaction catalyzed by the F component) by the end products, ferric-enterochelin and 2,3-dihydroxybenzoylserine, is shown to inhibit overall enterochelin synthetase activity.     

Life Cycles of Ground Beetles (Coleoptera,Carabidae) from the Mountain Taiga and Mountain Forest-Steppe in the Eastern Sayan

Seasonal dynamics and demographic structure was studied in 15 dominant ground beetle species in the mountain taiga and mountain forest-steppe belts of the Eastern Sayan (Okinskoe Plateau). Life cycles of the dominant ground beetle species were classified by developmental time, seasonal dynamics, and intrapopulation groups with different reproduction timing. The strategies of carabid life cycles adapted to severe mountain conditions of the Eastern Sayan were revealed.     

Quantifying the ion selectivity of the Ca2+ site in photosystem II: evidence for direct involvement of Ca2+ in O2 formation.

Calcium is an essential cofactor in the oxygen-evolving complex (OEC) of photosystem II (PSII). The removal of Ca2+ or its substitution by any metal ion except Sr2+ inhibits oxygen evolution. We used steady-state enzyme kinetics to measure the rate of O2 evolution in PSII samples treated with an extensive series of mono-, di-, and trivalent metal ions in order to determine the basis for the affinity of metal ions for the Ca2+-binding site. Our results show that the Ca2+-binding site in PSII behaves very similarly to the Ca2+-binding sites in other proteins, and we discuss the implications this has for the structure of the site in PSII. Activity measurements as a function of time show that the binding site achieves equilibrium in 4 h for all of the PSII samples investigated. The binding affinities of the metal ions are modulated by the 17 and 23 kDa extrinsic polypeptides; their removal decreases the free energy of binding of the metal ions by 2.5 kcal/mol, but does not significantly change the time required to reach equilibrium. Monovalent ions are effectively excluded from the Ca2+-binding site, exhibiting no inhibition of O2 evolution. Di- and trivalent metal ions with ionic radii similar to that of Ca2+ (0.99 A) bind competitively with Ca2+ and have the highest binding affinity, while smaller metal ions bind more weakly and much larger ones do not bind competitively. This is consistent with a size-selective Ca2+-binding site that has a rigid array of coordinating ligands. Despite the large number of metal ions that competitively replace Ca2+ in the OEC, only Sr2+ is capable of partially restoring activity. Comparing the physical characteristics of the metal ions studied, we identify the pK(a) of the aqua ion as the factor that determines the functional competence of the metal ion. This suggests that Ca2+ is directly involved in the chemistry of water oxidation and is not only a structural cofactor in the OEC. We propose that the role of Ca2+ is to act as a Lewis acid, binding a substrate water molecule and tuning its reactivity.     

Two remarkable new species of scuttle-fly (Diptera: Phoridae) that parasitize termites (Isoptera) in Sulawesi

ABSTRACT. Diplonevra mortimeri sp.nov. and D.watsoni. sp.nov. are described from specimens caught at Nasutitermitinae termite colonies in Sulawesi. The female of both species has a remarkable pre-oviposition behaviour in which she lures a worker termite into following her away from the colony. She then oviposits in the termite's abdomen. Pupation takes place within the remains of the host's abdomen. Both species only mature a single egg at a time. The female probably guards the comatose host termite during the period of larval development. The morphology of the male hypopygia of the new species allows reinterpretation of the peculiar anal tube in typical male Diplonevra. Furthermore the suggested homologies reinforce the view that the epandrium (tergite 9) has not been replaced by a periandrium (fused gonocoxites) in the Cyclorrhapha and consequently the latter cannot be derived from the Empidoidea.     

Viscoelasticity of packed erythrocyte suspensions subjected to low amplitude oscillatory deformation.

Concentrated adult erythrocyte suspensions were subjected to low amplitude oscillatory shear in a Weissenberg rheogoniometer equipped with a cone-and-plate assembly. The dynamic viscoelastic properties of the suspension were measured over a broad range of frequency by a numerical solution that accounted for fluid inertia. Variation of shear amplitude and cell volume percent, and comparison of buffered saline, plasma, and dextran as suspending media showed that the cellular elements had undergone small bending and shearing deformations. Studies of normal adult erythrocytes, hypotonically swollen cells, temperature-altered cells, and erythrocyte ghosts suggested that the method was evaluating membrane material properties. The normal membrane was found to exhibit a shear rate dependent elastic modulus that increased by more than a factor of 20 over a frequency range from 0.0076 Hz to 60 Hz. The membrane viscosity showed a substantial drop with frequency indicative of a frequency thinning phenomenon. At high frequency of deformation the viscous response of normal erythrocytes was no longer indicative of a membrane property due to the dominant influence of the internal hemoglobin solution. The studies generally supported the ability of the method to quantify relative membrane material properties and detect changes in membrane structure.     

Effects of different sterols on the inhibition of cell culture growth caused by the growth retardant tetcyclacis

Cell division in cell suspension cultures can be completely blocked by the growth retardant tetcyclacis at a concentration of 10^-4 mol l^-1. In rice cells it has been demonstrated that the growth inhibition can be completely overcome by application of cholesterol independent of the duration of pretreatment with tetcyclacis. In suspension cultures of maize and soybean, too, the effect of tetcyclacis on cell division was neutralized by adding cholesterol. Other plant sterols, stigmasterol, campesterol and sitosterol were active in a decreasing order. Modifications in the cholesterol perhydro-cyclopentanophenanthrene-ring system indicate that the hydroxyl group at C-3 and the double bond between C-5 and C-6 in ring B are required for the activity. In contrast, gibberellic acid as well as ent-kaurenoic acid could not compensate retardant effects. Likewise, tetcyclasis did not change the level of gibberellins in rice cells as shown by radioimmunoassay. Thus, it is concluded that in cell suspension cultures sterols play a more important role in cell division than gibberellins.Abbreviation GA_x gibberelin A_x