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Biomedical subjects

G W Brady

Publications and source records attributed to G W Brady.

18 recordsLinked to original sources
PubMed1987

Environmental influences on DNA superhelicity. The effect of ionic strength on superhelix conformation in solution.

The techniques of small-angle X-ray scattering and analysis that have been developed by the authors are used to investigate the influence of ionic strength on the superhelical conformation of native COP608 plasmid DNA in solution. For salt concentrations below 0.1 M, the superhelicity is partitioned between twisting (Tw) and writhing (Wr) in the ratio delta Tw/Wr = 2. Near the physiological salt concentration, [Na+] = 0.2 M, a co-operative transition is observed in which the pitch angle of the toroidal superhelix is drastically decreased. This results in an almost complete relaxation of writhe. At salt concentrations in excess of the threshold for this transition, the superhelical partitioning occurs in the ratio delta Tw/Wr greater than 25. Energetic considerations support the suggestion that this transition results from co-operative, superhelical B to Z transconformation reactions at susceptible sites. A method is discussed that will enable the direct measurement of this secondary structural transition by means of X-ray scattering.

DNA, Superhelical↗
PubMed1986

The role of charge microheterogeneity of basic protein in the formation and maintenance of the multilayered structure of myelin: a possible role in multiple sclerosis.

Isolation of several of the charge isomers from both normal and multiple sclerosis (MS) myelin basic protein (MBP) was achieved on CM-52 columns at pH 10.6. In liquid x-ray diffraction and aggregation experiments, corresponding charge isomers were equally effective in the formation of multilayers, demonstrating the dominant role of overall net positive charge. These studies demonstrated that the change in overall charge of MBP of one net positive charge was sufficient to produce large changes in aggregation and in multilayer formation. The x-ray diffraction experiments showed that component 1 was twice as effective as component 2 although they differed in charge by a single positive charge. Component 3 was less effective than component 2 and component "8" was not effective at all. Vesicle aggregation also showed a dependence on net positive charge. In order of decreasing effectiveness, component 1 greater than component 2 greater than component 3 greater than component "8". Since overall charge on MBP is determined by contributions from the various charge isomers, the relative proportions of these charge isomers favoring the less cationic components could explain the observation that MBP from MS victims was less effective than MBP from normal brain in vesicle aggregation and multilayer formation. The isolation of myelin-containing white matter fractions from both normal and MS tissue in which the loss of some of the most cationic charge isomers was correlated with presence of less compact myelin supports this hypothesis.

Cell Fractionation↗
PubMed1985

The role of charge microheterogeneity of human myelin basic protein in the formation of phosphatidylglycerol multilayers.

Human myelin basic protein (HBP) was fractionated into its various charge isomers by chromatography on CM-52 columns at pH 10.6. Components 1,2,3 and "8" (C-1, C-2, C-3, and C-"8") were cleanly separated. Each component was combined with phosphatidylglycerol (PG) vesicles, at neutral pH at a concentration of 30% (w/w), protein/lipid. C-1, the most cationic of the components was the most effective at inducing the formation of multilayers when studied by liquid X-ray diffraction. C-3, which differs from C-1 by 2 positive charges was less effective than C-2. C-"8" was totally ineffective since the scattering pattern with this component was no different from that of the pure lipid. Thus a seemingly small change in net charge of the protein had a dramatic effect on the ability of the protein to organize the lipid into a crystalline, multilayer arrangement characteristic of compact myelin.

Chromatography↗
PubMed1983

X-ray scattering from the superhelix in circular DNA.

This communication presents measurements, made with a newly constructed position-sensitive detector, of the small-angle x-ray scattering from the first-order superhelix of native COP608 plasmid DNA. This instrument measures intensities free of slit effects and provides good resolution in the region of interest. The reported observations, made both in the presence and in the absence of intercalator, closely fit the scattering patterns calculated for noninterwound helical first-order superhelices. These results are consistent with a toroidal helical structure but not with interwound conformations. The pitch angle alpha and contour length per turn c are reported for the native molecule at several concentrations of the platinum intercalating compound. From these parameters, the best-fitting toroidal helix is constructed and its geometry is investigated. The specific linking difference of the native molecule is estimated to be delta Lk/Lk0 approximately equal to -0.055. If the best-fitting toroidal helix is taken to be the actual structure, the partitioning of superhelicity between twist and writhe occurs in the approximate ratio of 2:1.

DNA, Superhelical↗
PubMed1982

Liquid diffraction analysis of sarcoplasmic reticulum. II. Solvent electron contrast variation.

Intensities of x-ray scattering from rabbit muscle sarcoplasmic reticulum membrane have been measured over the range of s = 0.05-0.25, at solvent densities varying between p0 = 0.335 and 0.389 electrons/A3. Analysis of the results shows agreement with the elements of structure deduced earlier by a different technique, based on the variation of the lipid-protein concentration ratio. In addition, the present work extends the analysis to allow isolation of the lipid contribution to the total scattering, from which a profile of the lipid electron density normal to the membrane face is evaluated. The scattering arising from electron correlations within the plane of the bilayer has also been identified.

Animals↗
PubMed1981

A liquid diffraction analysis of sarcoplasmic reticulum. I. Compositional variation.

Intensities of x-ray scattering from a series of fragmented rabbit muscle sarcoplasmic reticulum (SR) samples have been measured over the range x = 0.05 to s = 0.25. By varying the relative concentrations of lipid and protein (chiefly the Mg++-dependent, Ca++- stimulated ATPase) in the membranes of this series, and by employing methods of analysis appropriate to the scattering from binary liquid mixtures, we have identified the separable contributions of protein and lipid, and the protein-lipid interaction contributions to the total scattering profiles. The shape of the protein term is consistent with scattering from a cylindrical ATPase particle 142 A in length and 35 A in diameter. These data imply that the dominant ATPase species is monomeric. The protein-lipid interaction term has been analyzed by a novel treatment based on a determination of the pair correlation function between the electrons of the protein molecule with the electrons of the lipid bilayer in terms of the asymmetry of the transbilayer disposition of the protein. Applied to our results, the analysis indicates a fully asymmetric disposition of ATPase, in which one end of the molecule is contiguous with either the lumenal or cytoplasmic surface of the bilayer.

Animals↗
PubMed1980

X-ray scattering from randomly oriented superhelices. Circular superhelical DNA.

The scattering functions of randomly oriented filaments of finite length exhibiting two orders of helicity have been calculated. It is shown to a good approximation that each order scatters as if present alone as a first order helix of the same contour length and pitch angle. These results show that the measured scattering pattern from dissolved superhelical DNA molecules is consistent with the scattering pattern calculated for a coiled coil geometry.

DNA, Bacterial↗
PubMed1979

Liquid diffraction analysis of the model membrane system. Egg lecithin + myelin protein (N-2).

The scattering from the model membrane system, egg lecithin + myelin protein (N-2), has been analyzed by liquid diffraction methods. It is found that by manipulating the protein-lipid ratio, the scattering domains of the protein and lipid can be identified. The multilayer contribution can also be identified by its position and concentration behavior in both the intensity pattern and its Fourier transform. When the multilayer and protein components are subtracted, the phospholipid scattering and an interaction term are left: these two can be resolved by a reasonable assessment of their relative magnitude in the boundary region where they overlap. The interaction term can then be used to determine the most probably position of the protein in the membrane. The deconvoluted protein and lipid transforms can then be combined in the proper way to obtain the electron density profiles. The resolution of the interaction term is not yet complete, but a method for accomplishing this is discussed.

Egg Yolk↗
PubMed1979

The model membrane system. Egg lecithin + myelin protein (N-2). Effect of solvent density variation on the X-ray scattering.

The electron density contrast method has been applied to the membrane system egg lecithin + myelin protein (N-2). The case treated here is for a low protein concentration. As theory predicts, the scattering from the different regions of the membrane (protein, hydrocarbon, and polar head group regions) are modulated differently by changing the contrast. It is then possible to separate out the electron pair correlation functions for the different regions, and from these to determine the membrane election density distribution for an external electron density p0 = 0.

Egg Yolk↗
PubMed1977

An analysis of the X-ray interchain peak profile in dipalmitoylglycerophosphocholine.

The primary X-ray peak profile characterizing the interchain structure in the dipalmitoylglycerophosphocholine membrane has been measured as a function of temperature. The scattering between 23 and 34.6 degrees C is characterized by an asymmetric crystalline reflection accounting for 85% of the total intensity, the remaining 15% being liquid-like in character. At a pre-transition temperature of 34.6 degrees C, the reflection profile becomes (nearly) symmetrical, indicating a change in tilt angle of the chains with respect to the membrane surface. This change is accompanied by an increase of 20% in the amount of liquid-like scattering, indicating that the pre-transition mechanism includes a partial melting of the chains. At the melting point, 41.5 degrees C, the crystalline reflection disappears, and the liquid component of the scattering increases to a point where it includes all the scattered intensity. The relative values of the integrated intensities at each temperature are tabulated, and the significance of the peak widths and shapes are discussed.

Glycerylphosphorylcholine↗
PubMed1976

X-ray diffraction studies of circular superhelical DNA at 300-10,000-A resolution.

X-ray diffraction studies on circular superhelical DNA from bacteriophage PM2 at a very low scattering angle show that it is possible to measure the superhelical scattering function of the molecule. The results suggest that in addition to the primary supercoil, a higher order of supercoiling of the DNA is present, an effect which can be interpreted with a simple analogue.

Coliphages↗