On the mechanism of drug-induced acceleration of phospholipid translocation in the human erythrocyte membrane

Small amphiphilic compounds (M(r)<200 Da) such as anaesthetics and hexane derivatives with different polar groups produced a concentration-dependent acceleration of the slow passive transbilayer movement of NBD-labelled phosphatidylcholine in the human erythrocyte membrane. Above a threshold concentration characteristic for each compound, the flip rate gradually increased at increasing concentrations in the medium. For compound concentrations required to produce a defined flip acceleration, corresponding membrane concentrations were estimated using reported octanol/water partition coefficients. The effective threshold membrane concentrations (50-150 mmol l(-1)) varied in the order: hexylamine>isoflurane=hexanoic acid>hexanol=chloroform>hexanethiol=1,1,2,2-tetrachloroethane>chlorohexane. Apolar hexane, which mainly distributes in the apolar membrane core, was much less effective and supersaturating concentrations were required to enhance flip. Localization of the drug at the lipid-water interface seems to be required for flip acceleration. Such a localization may increase the lateral pressure in this region and the bilayer curvature stress with concomitant decrease of order and rigidity at the interface. This unspecific bilayer perturbation is proposed to enhance the probability of formation of hydrophobic defects in the bilayer, facilitating penetration of the polar head group of the phospholipid into the apolar membrane core.     

Factor h and properdin recognize different epitopes on renal tubular epithelial heparan sulfate

During proteinuria, renal tubular epithelial cells become exposed to ultrafiltrate-derived serum proteins, including complement factors. Recently, we showed that properdin binds to tubular heparan sulfates (HS). We now document that factor H also binds to tubular HS, although to a different epitope than properdin. Factor H was present on the urinary side of renal tubular cells in proteinuric, but not in normal renal tissues and colocalized with properdin in proteinuric kidneys. Factor H dose-dependently bound to proximal tubular epithelial cells (PTEC) in vitro. Preincubation of factor H with exogenous heparin and pretreatment of PTECs with heparitinase abolished the binding to PTECs. Surface plasmon resonance experiments showed high affinity of factor H for heparin and HS (K(D) values of 32 and 93 nm, respectively). Using a library of HS-like polysaccharides, we showed that chain length and high sulfation density are the most important determinants for glycosaminoglycan-factor H interaction and clearly differ from properdin-heparinoid interaction. Coincubation of properdin and factor H did not hamper HS/heparin binding of one another, indicating recognition of different nonoverlapping epitopes on HS/heparin by factor H and properdin. Finally we showed that certain low anticoagulant heparinoids can inhibit properdin binding to tubular HS, with a minor effect on factor H binding to tubular HS. As a result, these heparinoids can control the alternative complement pathway. In conclusion, factor H and properdin interact with different HS epitopes of PTECs. These interactions can be manipulated with some low anticoagulant heparinoids, which can be important for preventing complement-derived tubular injury in proteinuric renal diseases.     

Computer calculation-based quantitative structure-activity relationships for the oxidation of phenol derivatives horseradish peroxidase compound II

 The second-order rate constants for the oxidation of a series of phenol derivatives by horseradish peroxidase compound II were compared to computer-calculated chemical parameters characteristic for this reaction step. The phenol derivatives studied were phenol, 4-chlorophenol, 3-hydroxyphenol, 3-methylphenol, 4-methylphenol, 4-hydroxybenzoate, 4-methoxyphenol and 4-hydroxybenzaldehyde. Assuming a reaction of the phenolic substrates in their non-dissociated, uncharged forms, clear correlations (r = 0.977 and r = 0.905) were obtained between the natural logarithm of the second-order rate constants (ln k _app and ln k ₂ respectively) for their oxidation by compound II and their calculated ionisation potential, i.e. minus the energy of their highest occupied molecular orbital [E(HOMO)]. In addition to this first approach in which the quantitative structure-activity relationship (QSAR) was based on a calculated frontier orbital parameter of the substrate, in a second and third approach the relative heat of formation (ΔΔHF) calculated for the process of one-electron abstraction and H^• abstraction from the phenol derivatives was used as a parameter. Plots of the natural logarithms of the second-order rate constants (k _app and k ₂) for the reaction and the calculated ΔΔHF values for the process of one-electron abstraction also provide clear QSARs with correlation coefficients of –0.968 and –0.926 respectively. Plots of the natural logarithms of the second-order rate constants (k _app and k ₂) for the reaction and the calculated ΔΔHF values for the process of H^• abstraction provide QSARs with correlation coefficients of –0.989 and –0.922 respectively. Since both mechanisms considered, i.e. initial electron abstraction versus initial H^• abstraction, provided clear QSARs, the results could not be used to discriminate between these two possible mechanisms for phenol oxidation by horseradish peroxidase compound II. The computer calculation-based QSARs thus obtained for the oxidation of the various phenol derivatives by compound II from horseradish peroxidase indicate the validity of the approaches investigated, i.e. both the frontier orbital approach and the approach in which the process is described by calculated relative heats of formation. The results also indicate that outcomes from computer calculations on relatively unrelated phenol derivatives can be reliably compared to one another. Furthermore, as the actual oxidation of peroxidase substrates by compound II is known to be the rate-limiting step in the overall catalysis by horseradish peroxidase, the QSARs of the present study may have implications for the differences in the overall rate of substrate oxidation of the phenol derivatives by horseradish peroxidase. Received: 29 March 1996 / Accepted: 17 July 1996     

Spatial Structure Facilitates Cooperation in a Social Dilemma: Empirical Evidence from a Bacterial Community

Cooperative organisms are ubiquitous in nature, despite their vulnerability to exploitation by cheaters. Although numerous theoretical studies suggest that spatial structure is critical for cooperation to persist, the spatial ecology of microbial cooperation remains largely unexplored experimentally. By tracking the community dynamics of cooperating (rpoS wild-type) and cheating (rpoS mutant) Escherichia coli in well-mixed flasks and microfabricated habitats, we demonstrate that spatial structure stabilizes coexistence between wild-type and mutant and thus facilitates cooperator maintenance. We develop a method to interpret our experimental results in the context of game theory, and show that the game wild-type and mutant bacteria play in an unstructured environment changes markedly over time, and eventually obeys a prisoner’s dilemma leading to cheater dominance. In contrast, when wild-type and mutant E. coli co-inhabit a spatially-structured habitat, cooperators and cheaters coexist at intermediate frequencies. Our findings show that even in microhabitats lacking patchiness or spatial heterogeneities in resource availability, surface growth allows cells to form multi-cellular aggregates, yielding a self-structured community in which cooperators persist.     

Sour taste preferences of children relate to preference for novel and intense stimuli

Previous research has suggested that some children have a preference for sour tastes. The origin of this preference remains unclear. We investigated whether preference for sour tastes is related to a difference in rated sour intensity due to physiological properties of saliva, or to an overall preference for intense and new stimuli. Eighty-nine children 7-12 years old carried out a rank-order procedure for preference and category scale for perceived intensity for four gelatins (i.e. 0.0 M, 0.02 M, 0.08 M, 0.25 M added citric acid) and four yellow cards that differed in brightness. In addition, we measured their willingness to try a novel candy and their flow and buffering capacity of their saliva. Fifty-eight percent of the children tested preferred one of the two most sour gelatins. These children had a higher preference for the brightest color (P < 0.05) and were more likely to try the candy with the unknown flavor (P < 0.001) than children who did not prefer the most sour gelatins. Preference for sour taste was not related with differences in rated sour intensity, however those who preferred sour taste had a higher salivary flow (P < 0.05). These findings show that a substantial proportion of young children have a preference for extreme sour taste. This appears to be related to the willingness to try unknown foods and preference for intense visual stimuli. Further research is needed to investigate how these findings can be implemented in the promotion of sour-tasting food such as fruit.     

Single-molecule studies of nucleic acid motors

Nucleic acid motors comprise a variety of structurally, mechanistically and functionally very different enzymes. These motor proteins have in common the ability to directionally move DNA or RNA, or to move along DNA or RNA using a chemical energy source such as ATP. Recently, it became possible to study the action of a single motor on single DNA or RNA molecules in real time; this has provided unprecedented insight into the behavior and mechanism of these motors. As a result, the past few years have witnessed an enormous increase in such single-molecule studies of a variety of different motor systems. Particular highlights have included the investigation of the sequence-dependent behavior and helical tracking of motors, and the attainment of the ultimate (i.e. single base pair) resolution, which enables the detection of individual single base motor steps.     

Morphology and sedimentology of (clustered) cold-water coral mounds at the south Rockall Trough margins,NE Atlantic Ocean

Cold-water coral mounds on both margins of the Rockall Trough (NE Atlantic Ocean) have a strongly different morphology. Single, isolated mounds occur on the SE margin and are mainly found on the upper slope between 900 and 650 m water depth, while large mound clusters are found on the SW margin in water depths between 600 and 1,000 m, in a narrow zone almost parallel to the slope. Sedimentation rates on the mounds are higher than on the surrounding seabed as a result of baffling of biogenic carbonate debris and siliciclastic particles by the coral framework covering the mounds. This is confirmed by ²¹⁰Pb measurements. The individual coral growth rate can be three times higher then the vertical growth rate of the coral cover (±10 mm year⁻¹) which in turn is more than an order of magnitude higher then the present-day overall mound growth rate (±0.25 mm year⁻¹). The presence of extensive hardgrounds and firmgrounds and the three-dimensional coral framework are considered to be responsible for the stability of the relatively steep slopes of the mounds. High current velocities in the intramound areas result in local non-sedimentation and erosion, as is shown by the presence of IRD (ice-rafted debris) lag deposits on the seabed and moats around some of the mounds. The morphology and sedimentology of cold-water coral-covered (mainly Lophelia pertusa and Madrepora oculata) mounds on the southern Rockall Trough margins (NE Atlantic Ocean) is discussed and a model describing the development of these mounds is presented.