Synchrotron X-ray investigations into the lamellar gel phase formed in pharmaceutical creams prepared with cetrimide and fatty alcohols.

Semisolid liquid paraffin-in-water emulsions (aqueous creams) prepared from cetrimide/fatty alcohol mixed emulsifiers, and ternary systems formed by dispersing the mixed emulsifier in controlled percentages of water were examined as they aged using a combination of low and high angle X-ray diffraction measurements (Daresbury Laboratory Synchrotron Radiation Source). The results were correlated with the rheological properties measured in earlier studies. The cationic emulsifying wax showed phenomenal swelling in water. The reflection that incorporates interlamellar water increased continuously from 74 A at 28% water to over 500 A at 93% water. The trend was not influenced by the method of incorporation of the components and swollen lamellar phase was also identified in the corresponding emulsion. The swelling, which was due to electrostatic repulsion, was suppressed by salt and was reduced when the surfactant counterion was changed from Br(-) to Cl(-). Changes in rheological properties on storage and in the presence of salt were correlated with changes in water layer thickness. High angle diffraction confirmed that the hydrocarbon bilayers were in the hexagonal alpha-crystalline mode of packing. Ternary systems and creams prepared from pure alcohols, although initially semisolid, were rheologically unstable and broke down. Low angle X-ray study into the kinetics of structure breakdown showed that the swollen lamellar gel phase formed initially swells even further on storage before separating.

A near perfect temperature adaptation of bilayer order in vertebrate brain membranes.

The bilayer order of a brain synaptic membrane fraction from a number of fish, mammalian and avian species have been compared in relation to their respective body temperatures using steady-state and time-resolved fluorescence anisotropy techniques. Fluorescence anisotropy for both 1,6-diphenyl-1,3,5-hexatriene and trans-parinaric acid increased in the order: antarctic Notothenia, trout, perch, cichlid, rat and starling, this also being the order of increasing body temperature. This suggests that cold-adapted fish species possess more disordered brain membranes than warm-adapted fish species, and mammals and birds membranes were more ordered than fish membranes. Comparison of temperature profiles for both fluorescence probes showed that fish species display similar anisotropies, and by inference bilayer order, to mammals and birds when measured at their respective body temperatures. Time-resolved analysis showed that the interspecific differences in (P2) order parameter was consistently related to body temperature whilst the rotational diffusion coefficient was not. These results suggest that brain membrane order is highly conserved within the vertebrates despite large differences in thermal habits and phylogenetic position. Polar fish species have by far the lowest bilayer order indicating that invasion of extreme cold habitats involved an adaptive decrease in bilayer order and conversely adoption of a high body temperature by mammals involved an adaptive increase in bilayer order. The conservation of membrane static order for these species at their respective body temperatures indicates a regulatory control of this aspect of membrane hydrocarbon structure and the functional importance of this structure.

Construction of a stable dimer of Bacillus stearothermophilus lactate dehydrogenase.

A molecular graphics analysis of the features which prevent cytosolic malate dehydrogenase dimers from forming tetramers was evaluated by its success in predicting the synthesis of a version of the LDH framework which is a stable dimer. Surface residues responsible for malate dehydrogenases being dimers were revealed by superimposing the structures of two dimers of pig cytosolic malate dehydrogenase on one homologous tetramer of L-lactate dehydrogenase from Bacillus stearothermophilus. Four regions were identified as composing the P-axis dimer-dimer interface. Two regions of the dimer were surface loops that collided when built as a tetramer: a large loop (residues 203-207, KNOBI) and a small loop (residues 264-269, KNOBII), and these were candidates to explain the dimeric character of malate dehydrogenase. The analysis was tested by constructing a synthetic B. stearothermophilus lactate dehydrogenase (KNOBI) containing the large malate dehydrogenase loop (residues 203-207 being AYIKLQAKE, and extra four amino acids). The new construct was thermotolerant (90 degrees C) and enzymically active with kcat and KM (pyruvate) values similar to those of the wild-type enzyme. However, whereas the allosteric activator fructose 1,6-bisphosphate decreased KM 100 times for wild type, it had no influence on KNOBI. The molecular volumes of 1-120 microM concentrations of the construct were measured by time-resolved decay of tryptophan fluorescence anisotropy and by gel filtration. Both methods showed the molecular weight of wild type increased from dimer to tetramer with Kd about 20 microM dimer. KNOBI remained a dimer under these conditions.(ABSTRACT TRUNCATED AT 250 WORDS)