Synchrotron radiation-based experimental determination of the optimal energy for cell radiotoxicity enhancement following photoelectric effect on stable iodinated compounds

This study was designed to experimentally evaluate the optimal X-ray energy for increasing the radiation energy absorbed in tumours loaded with iodinated compounds, using the photoelectric effect. SQ20B human cells were irradiated with synchrotron monochromatic beam tuned at 32.8, 33.5, 50 and 70 keV. Two cell treatments were compared to the control: cells suspended in 10 mg ml1 of iodine radiological contrast agent or cells pre-exposed with 10 mM of iodo-desoxyuridine (IUdR) for 48 h. Our radiobiological end point was clonogenic cell survival. Cells irradiated with both iodine compounds exhibited a radiation sensitisation enhancement. Moreover, it was energy dependent, with a maximum at 50 keV. At this energy, the sensitisation calculated at 10% survival was equal to 2.03 for cells suspended in iodinated contrast agent and 2.60 for IUdR. Cells pretreated with IUdR had higher sensitisation factors over the energy range than for those suspended in iodine contrast agent. Also, their survival curves presented no shoulder, suggesting complex lethal damages from Auger electrons. Our results confirm the existence of the 50 keV energy optimum for a binary therapeutic irradiation based on the presence of stable iodine in tumours and an external irradiation. Monochromatic synchrotron radiotherapy concept is hence proposed for increasing the differential effect between healthy and cancerous tissue irradiation

Lateral Protein-Protein Interactions at Hydrophobic and Charged Surfaces as a Function of pH and Salt Concentration

Surface adsorption of Thermomyces lanuginosus lipase (TLL) - a widely used industrial biocatalyst - is studied experimentally and theoretically at different pH and salt concentrations. The maximum achievable surface coverage on a hydrophobic surface occurs around the protein isoelectric point and adsorption is reduced when either increasing or decreasing pH, indicating that electrostatic protein-protein interactions in the adsorbed layer play an important role. Using Metropolis Monte Carlo (MC) simulations, where proteins are coarse grained to the amino acid level, we estimate the protein isoelectric point in the vicinity of charged surfaces as well as the lateral osmotic pressure in the adsorbed monolayer. Good agreement with available experimental data is achieved and we further make predictions of the protein orientation at hydrophobic and charged surfaces. Finally, we present a perturbation theory for predicting shifts in the protein isoelectric point due to close proximity to charged surfaces. Although this approximate model requires only single protein properties (mean charge and its variance), excellent agreement is found with MC simulations

Influence of product phase separation on phospholipase A(2) hydrolysis of supported phospholipid bilayers studied by force microscopy.

In situ atomic force microscopy studies reveal a marked influence of the initial presence of hydrolysis products on the hydrolysis of supported phospholipid bilayers by phospholipase A(2). By analysis of the nano-scale topography of a number of supported bilayers with different initial product concentrations, made by Langmuir-Blodgett deposition, we show that small depressions enriched in products are efficiently promoting enzyme degradation of the bilayer. These small depressions, which are indicative of phase separation, are initially present in samples with 75% products. The kinetics of phospholipase A(2) exhibit under certain conditions an initial phase of slow hydrolysis, termed the latency phase, followed by a marked increase in the hydrolysis rate. The appearance of the phase-separated bilayer is strikingly similar to that of bilayers at the end of the latency phase. By analysis of individual nano-scale defects we illustrate a quantitative difference in the growth rates of defects caused by product aggregation and other structural defects. This difference shows for the first time how the enzyme prefers one type of defect to another

Increase in phospholipase A2 activity towards lipopolymercontaining liposomes

AbstractPhospholipase A2 (PLA2)-catalyzed hydrolysis of dipalmitoylphosphatidylcholine (DPPC) liposomes incorporated with submicellar concentrations of polyethyleneoxide covalently attached to dipalmitoylphosphatidylethanolamine (DPPE-PEG2000) has been studied in the gel-to-fluid transition region of the host DPPC lipid bilayer matrix. By means of fluorescence and light-scattering measurements, the characteristic PLA2 lag time has been determined as a function of lipopolymer concentration and temperature. The degree of lipid hydrolysis was followed using radioactive labeled lipids. Differential scanning calorimetry has been applied to characterize the thermodynamic phase behavior of the lipopolymer-containing liposomes. A remarkable lipopolymer concentration-dependent decrease in the lag time was observed over broad temperature ranges. The radioactive measurements demonstrate an increase in catalytic activity for increasing amounts of lipopolymers in the bilayer. Hence, the lipopolymers act as a promoter of PLA2 lipid hydrolysis resulting in a degradation of the bilayer structure and a concomitant destabilization of the liposomes. This behavior is in contrast to the generally observed protective and stabilization effect in biological fluids exerted by lipopolymers in polymer-grafted liposomes. It is proposed that the enhanced activity of the small water soluble and interfacially active enzyme may involve a non-uniform distribution of the lipopolymers in the lipid matrix due to a coupling between local lipid bilayer curvature and composition of the non-bilayer-preferring lipopolymers