Calorimetry of tetraether lipids from Thermoplasma acidophilum: incorporation of alamethicin, melittin, valinomycin, and nonactin

The development and application of model membrane systems on the basis of tetraether lipids from Thermoplasma acidophilum has been proposed. In this respect incorporation of membrane proteins and ionophores is indispensable and is demonstrated in the case of alamethicin, melittin, nonactin, and valinomycin by calorimetry. Dipalmitoylphosphatidylcholine (DPPC) and dihexadecylmaltosylglycerol (DHMG) were chosen for comparison. Melittin and alamethicin prove to broaden the lipid phase transition and to reduce the melting temperature Tm and enthalpy change (delta H) of the main phospholipid from T. acidophilum (MPL) and DPPC. The decrease in Tm, however, is more pronounced in DPPC than in MPL. Valinomycin shows only a marginal effect on the temperature and width of the transition; delta H is reduced in MPL and remains constant in DPPC and DHMG. With nonactin the phase transition of DPPC is quenched, and delta H and the half-height width are increased. DHMG is affected to a lesser extent and MPL only marginally. The four ionophores exhibit different modulation of the phase transition behavior of the various lipids as expected from their varying molecular structures. Thus, the integral membrane protein alamethicin, the peripheral protein melittin, valinomycin, and nonactin interact primarily with lipid head groups and are readily incorporated into the tetraether lipid structures.

Fibroblasts from ataxia telangiectasia (AT) and AT heterozygotes show an enhanced level of residual DNA double-strand breaks after low dose-rate gamma-irradiation as assayed by pulsed field gel electrophoresis

Skin fibroblasts from ataxia telangiectasia (AT) patients, obligate AT heterozygotes (ATH) and normal individuals were studied for colony-forming ability and repair of DNA double-strand breaks (dsb) after gamma-irradiation. AT cells were three to four times more radiosensitive than normal cells at high and low dose-rate exposures; ATH cells, however, showed a marginally increased radiosensitivity after high dose-rate gamma-irradiation and an intermediate response after low dose-rate exposure. The repair of DNA dsb was studied by pulsed field gel electrophoresis. After high dose-rate gamma-irradiation the repair time constant (t1/2) was around 1 h for normal, ATH and AT cells. After low dose-rate gamma-irradiation the fraction of residual dsb was 1.4% for normal, 2.1% for ATH and 5.2% for AT cells, demonstrating a deficiency in the repair of a small fraction of dsb in AT. Thus the fraction of residual dsb after low dose-rate exposure was not only four times higher in AT than in normal cells, but was also significantly increased in ATH compared to normal cells.

Thermotropic properties of dispersions of cholesterol with tetraether lipids from Thermoplasma acidophilum

The main glycophospholipid from Thermoplasma acidophilum is composed of a diisopranol-2,3-glycerotetraether. The fraction of pentane cyclizations of its hydrocarbon chains increases with the growth temperature of the source organism (39-59 degrees C). Hydrated mixtures of these lipids together with cholesterol have been studied by calorimetry. With the reduction of the phase transition temperatures and enthalpy changes of the transitions, cholesterol is readily incorporated into lipid monolayers in the liquid-crystalline and the (metastable) solid-analogue phase. Lipid samples with a high number of acyclic hydrocarbon chains form a stable and a metastable solid-analogue phase. With the increasing concentration of cholesterol the metastable solid-analogue phase is stabilized and the time constant for the formation of the stable solid-analogue phase is prolonged.

Incorporation of synthetic peptide helices in membranes of tetraether lipids from Thermoplasma acidophilum. A calorimetric study

Four analogues of the membrane-modifying, alpha-helical polypeptide antibiotic alamethicin were synthesized. the alpha-helical deca-, undeca-, heptadeca-, and icosapeptides were mixed with the main tetraether lipid of the Archaebacterium Thermoplasma acidophilum (MPL), dipalmitoylphosphatidylcholine (DPPC) and dihexadecylmaltosylglycerol (DHMG) in various ratios and the modification of the lipid phase transition was determined by differential thermal analysis (DTA). The polypeptides form mixed phases with MPL and DPPC, however, not with DHMG. Heptadeca- and icosapeptide exert a much stronger reduction of enthalpy (delta H) than deca- and undecapeptide and bind about 0.5 molecule of MPL (or one molecule of DPPC) per peptide molecule. delta H of the DPPC pretransition is reduced by the deca- and the undecapeptides and completely disappears with heptadeca- and icosapeptides (at 0.2 mole of peptide/mole of lipid). The modulation of the melting point Tm by the incorporation of peptides is more pronounced with MPL than with DPPC, the heptadecapeptide exhibiting the strongest reduction (with MPL) and the strongest broadening of the transition peak (with DPPC). Helix length, amphiphilicity and charge of the polypeptides can be correlated with the observed modifications of the lipid phase transitions.

DNA double strand breaks in fibroblast cell lines, from non-Hodgkin's lymphoma patients, showing increased sensitivity to chronic gamma irradiation

Cultured skin fibroblast cell lines from two non-Hodgkin's lymphoma patients (NHL) and a normal subject were studied for cell killing, chromosomal aberrations (breaks, translocations, dicentrics and rings) and DNA double strand breaks (dsbs) following chronic gamma irradiation. Compared to the cell line from the normal donor, the NHL patients' fibroblasts showed enhanced radiosensitivity for both cell survival and chromosomal aberrations. While spontaneous breaks were observed in both normal and patients' cells, spontaneous translocations and radiation-induced dicentrics and rings were found only in the latter. Radiation-induced DNA double-strand breaks (dsb) were determined by CHEF electrophoresis. After chronic irradiation with gamma rays the fraction of residual dsb was significantly increased from 1.4% in controls to 1.9% in the NHL cell lines. These data, thus, suggest that the cellular and chromosomal sensitivity to chronic irradiation observed in NHL patients may be due to a deficiency in the repair of a small fraction of DNA double strand breaks.

Size distribution of DNA molecules recovered from non-denaturing filter elution

DNA fragments removed from the filter during non-denaturing filter elution were collected and loaded on top of neutral sucrose gradients. Their size distribution was determined by low-speed centrifugation in neutral sucrose gradients. The average size of eluted DNA was found to be approximately 110 S, yet the average size of DNA collected after short elution times was found to be slightly larger than that after long elution times. It is concluded that the size of eluted DNA fragments is not correlated with their elution rate, and it is proposed that shear forces generated at the pores of the filter cause degradation of the DNA. A comparison of the sedimentation profiles of carefully prepared cellular DNA before and after elution revealed that the generated shear forces during elution break down the DNA to an extent equivalent to around 20,000 DNA double-strand breaks (dsb) per G1 cell. The size of DNA fragments decreased with increasing radiation dose; however, five times more dsb were found than expected after exposure to radiation alone. It is proposed that this excess of dsb may derive from the transformation of other radiation-induced lesions to dsb under the action of the shear forces generated during elution.

Comparison of DNA double-strand break rejoining as measured by pulsed field gel electrophoresis, neutral sucrose gradient centrifugation and non-unwinding filter elution in irradiated plateau-phase CHO cells

The initial (up to 30 min) rate of DNA double-strand break (dsb) rejoining was measured in irradiated plateau-phase CHO cells, in a set of parallel experiments using the same cell suspension, by means of non-unwinding filter elution, neutral sucrose gradient centrifugation, and two pulsed-field gel electrophoresis assays: asymmetric field inversion gel electrophoresis (AFIGE) and clamped homogeneous electric field (CHEF) gel electrophoresis. The rate of DNA dsb rejoining was compared to the rate of rejoining of chromatin breaks measured, also in the same cell population, using the technique of premature chromosome condensation (PCC). Two radiation exposures, 25 Gy and/or 50 Gy, were used and applied to the individual parts of the experiments according to the sensitivity of the assay under investigation. Similar values for the initial rate of DNA dsb rejoining were obtained with all assays used, with t 1/2 ranging between 10 and 12 min after exposure to 25 Gy and between 15 and 20 min after exposure to 50 Gy. The initial rate of rejoining of chromatin breaks was slower than that of DNA dsb and occurred with t 1/2 of 87 min. The results suggest that all major techniques currently used for assaying rejoining of DNA dsb give similar results despite their widely different biophysical basis, and indicate that more information is required before a direct correlation between rejoining of DNA dsb and rejoining of chromatin breaks can be established.

Dose response in neutral filter elution

In neutral filter elution a nonlinear relationship between fraction of eluted DNA and dose is usually observed, which is often interpreted as a nonlinear induction of DNA double-strand breaks (DSBs) with dose. The conclusiveness of this hypothesis is questioned here on the basis of theoretical considerations regarding the size distribution of DNA fragments. A simple hydrodynamic model is proposed which generates the typical features of the dose response of neutral filter elution: (1) the shoulder at low doses, (2) a quasilinear correlation in an intermediate dose range, (3) a saturation at high doses, and (4) a linearization of the curve in the intermediate and higher dose range in a semilogarithmic plot. These features were derived even with the assumption of a linear induction of DSBs with dose. Thus it is demonstrated that the fraction of eluted DNA could conceivably be a nonlinear function of dose even if the induction of DSBs is directly proportional to the radiation dose.

DNA double-strand break analysis by CHEF (clamped homogeneous electrical field) electrophoresis

Pulsed-field gel electrophoresis (PFGE) has become a powerful tool for the study of high-molecular-weight DNA. In this technical note the application of clamped homogeneous electrical field (CHEF) electrophoresis for the determination of DNA double-strand breaks is described, and a number of parameters that influence the sensitivity of the assay are discussed. They include the concentration of the cell sample, agarose concentration in the plug and the electrophoresis gel, lysis conditions, pulse time, electric field gradient and the temperature of electrophoresis.

Physicochemical characterization of tetraether lipids from Thermoplasma acidophilum. V. Evidence for the existence of a metastable state in lipids with acyclic hydrocarbon chains

The main glycophospholipid of Thermoplasma acidophilum, grown at 39 degrees C, is composed of a di-isopranol-2,3-glycerotetraether. It has been characterized in hydrated systems by calorimetry. Unlike its equivalent grown at 59 degrees C, it shows complex phase properties, which include at least three different phases, (1) a liquid-analogue state (C), which is stable above 20 degrees C, (2) a metastable solid-analogue state (A) formed by supercooling of the liquid-analogue state (C) and (3) a stable solid-analogue state (B), which is slowly formed and may include a close chain packing of lipids and a network of hydrogen bonds between the headgroups. A high fraction of acyclic isopranol chains seems to be a prerequisite for the formation of state (B). A phase diagram, displaying the observed states and the transitions between them is proposed.

In CHEF electrophoresis a linear induction of dsb corresponds to a nonlinear fraction of extracted DNA with dose

A theory for DNA double-strand break (dsb) analysis by pulsed field electrophoresis is presented, based on the size distribution of DNA fragments, which converts an induction of dsb linear with dose into a nonlinear fraction of extractable DNA, in agreement with experimental observations.

CHEF electrophoresis, a sensitive technique for the determination of DNA double-strand breaks

It has been demonstrated that clamped homogeneous electrical field (CHEF) electrophoresis is a suitable method for the determination of DNA double-strand breaks in Chinese hamster ovary (CHO) cells. It allows the separation of DNA molecules up to 10 Mbp. The fraction of DNA fragments of this size is correlated with the number of radiation induced double-strand breaks. The resolution limit of the technique is equivalent to the effect of about 1 Gy (gamma-rays). Double-strand break repair was monitored after irradiation with Co-60 gamma rays and the repair time constant determined to t1/2 = 30-35 min. In combination with the detection of DNA by fluorescence, CHEF electrophoresis provides an easy and sensitive method for the determination of double-strand break repair which does not require the radioactive labelling of cells.

DNA double-strand break repair determines the RBE of alpha-particles

Radiation-induced DNA double-strand breaks (dsb) were studied in Ehrlich ascites tumour cells (EATC) by sedimentation in neutral sucrose gradients at low centrifuge speed. Dsb induction was found to be linear with dose with a frequency of: ndsbmr-1D-1 = (11.7 +/- 2) x 10(-12)Gy-1 for 140 kV X-rays and ndsbmr-1D-1 = (19.1 +/- 4) x 10(-12)Gy-1 for 3.4 MeV 241Am-alpha-particles. Postirradiation incubation of cells under non-growth conditions leads to repair of dsb, reaching a maximum after trep = 24 h. More than 97 per cent of dsb were repaired after an X-ray dose of 25 Gy. The number of residual dsb was found to be a linear-quadratic function of dose: nresmr-1 = (0.0161 +/- 0.0008) x 10(-12)Gy-2D2 for X-rays and nresmr-1 = (1.2 +/- 0.7) x 10(-12)Gy-1D + (0.105 +/- 0.017) x 10(-12)Gy-2D2 for alpha-particles. Thus, after cellular repair the RBE value of alpha-particles was increased from RBE = 1.6 +/- 0.4 (induction of dsb) to a dose-dependent value of RBE = 2.7 +/- 0.4 (at 100 Gy alpha-particles) to 3.8 +/- 1.2 (at 10 Gy alpha-particles) for residual dsb. From the data presented it is concluded that residual dsb are a major cause for loss of the reproductive capacity of EATC after irradiation with X-rays as well as alpha-particles.

The effects of 9-beta-D-arabinofuranosyladenine on the repair of DNA strand breaks in X-irradiated Ehrlich ascites tumour cells

The effects of the DNA synthesis inhibitor 9-beta-D-arabinofuranosyladenine (beta-ara A), a nucleoside analogue of desoxyadenine, on repair of DNA single and double strand breaks (ssb and dsb) were investigated in X-irradiated Ehrlich ascites tumour cells. Repair of ssb was followed using the unwinding method, and repair of dsb was measured with both the unwinding and the neutral sucrose centrifugation methods. Repair of ssb was inhibited in the presence of beta-ara A; however, even at high concentrations some repair took place. It is suggested that this proportion of the breaks (about 30 per cent) are joined by polynucleotide ligase, and do not require insertion of nucleotides. Dsb repair was strongly inhibited by beta-ara A, the inhibition being complete at high concentrations. It seems likely therefore that dsb repair has an absolute requirement for DNA polymerization. When cells were treated with beta-ara A (200 mumols/1, 2 hours) after irradiation dsb repair was inhibited; however, when the drug was washed away, repair of dsb returned. At 6 hours more breaks were found to have persisted in beta-ara A treated cells than in the untreated controls. Cells treated after X-irradiation with beta-ara A for 7 hours at 120 mumols/1 in conditioned medium and afterwards in fresh medium free of beta-ara A for 24 hours showed a higher number of residual dsb than control cells. It is suggested that these residual dsb may be relevant to the increased killing effect caused by adding beta-ara A to X-irradiated cultures.

DNA double strand breaks in Ehrlich ascites tumour cells at low doses of x-rays. II. Can cell death be attributed to double strand breaks?

The induction and repair of DNA double strand breaks (dsb) in early stationary Ehrlich ascites tumour cells by X-rays was determined using an improved sedimentation technique in neutral sucrose gradients. The disappearance of dsb was followed during post-irradiation incubation of the cells and was interpreted as dsb repair. Kinetics were approximated by exponential functions with time constants of t37 = 3.0 +/- 0.7 hours ('conditioned' medium) and t37 = 2.0 +/- 0.5 hours (growth medium). Maximal repair was reached after 24 hours and the relationship of the remaining breaks with dose was interpreted on the basis of a recombination repair model. Using these dsb data and on the assumption of one dsb being a lethal event, cell survival curves were calculated for different repair times and compared with experimental curves. It was shown that cell survival curves can be interpreted on the basis of one unrepaired dsb being a lethal event, when dsb repair continues for about 11 hours after plating the cells on nutrient agar.

DNA double strand breaks in Ehrlich ascites tumour cells at low doses of x-rays. I. Determination of induced breaks by centrifugation at reduced speed

DNA double strand breaks (dsb) were determined in Ehrlich ascites tumour cells at doses down to 5 Gy. The method is based on the separation of DNA from other components by heating in a solution of pronase and detergents held in wide-mouth syringes, which were also used to facilitate the application of the released high molecular weight DNA to sucrose gradients. Purified DNA was sedimented in neutral sucrose gradients at low speed to reduce speed artifacts. The sedimentation profiles were analysed using a computer program and the number of dsb was determined by simulation of random breaks in the mass distribution of the control sample and by comparison of this simulated profile with that of the irradiated one. The number of dsb formed was proportional to X-ray dose in the range of 5 to 2000 Gy. The induction per dose was found to be nmr-1 D-1 = (11.7 +/- 2) x 10(-12) Gy-1.

Measurement of the kinetics of DNA double strand break repair in Ehrlich ascites tumour cells using the unwinding method

Two main components of DNA strand break repair have been found using the unwinding method. The first has a time constant (t37) of some minutes and the second, much slower component, a time constant of several hours. The time constant for the slower component of repair was found to vary with the conditions of incubation and to depend on the quality of the radiation. The t37 values obtained for slow repair under various conditions after X-irradiation and after alpha-irradiation were found to be close to those for repair of double strand breaks as measured by velocity sedimentation. Values for initial breaks, obtained by extrapolation of slow repair data back to time zero, were also close to those obtained for double strand breaks. We therefore propose that the unwinding method can be a useful technique for monitoring the repair of double strand breaks.

The influence of oxygen on the survival and yield of DNA double-strand breaks in irradiated yeast cells

Survival and induction of DNA double-strand breaks were studied in cells of Saccharomyces cerevisiae irradiated under oxic or anoxic conditions with 30 MeV electrons. A linear relationship between DNA double-strand breakage and dose was found in both cases. The o.e.r.-value for colony forming ability was found to be 1.9 +/- 0.2, whereas the o.e.r.-value for DNA double-strand breakage was 3.0 +/- 0.1. These results are not inconsistent with the idea that DNA double-strand breaks are involved in killing of yeast cells. The frequency of induction of DNA double-strand breaks was found to be 0.74 x 10(-11) double-strand breaks per g/mol per Gy when cells were irradiated under oxygen and 0.24 x 10(-11) double-strand breaks per g/mol per Gy under nitrogen.