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Biomedical subjects

D G Wallace

Publications and source records attributed to D G Wallace.

32 records · Page 2Linked to original sources
PubMed1988

The effects of heparin on the physicochemical properties of reconstituted collagen.

Pepsin-solubilized bovine dermal collagen was reconstituted in 0.02 M sodium phosphate (pH 7.2), concentrated to 30-40 mg/ml, and adjusted to physiological ionic strength by addition of sodium chloride. These preparations, at 4-15 degrees C, are fibrillar suspensions composed of fibrils of varying diameters and nonassociated molecules. Addition of heparin to these suspensions promoted a dose-dependent increase in average fibril diameter as measured by turbidimetry and electron microscopic analyses. These effects were relatively specific for heparin and heparin-like glycosaminoglycans. Chondroitin sulfate and hyaluronic acid had little or no effect on fibrillar diameters under these conditions, whereas dermatan sulfate had an intermediate effect on fibrillar reorganization. Differential scanning calorimetry revealed that addition of optimal concentrations of heparin generated fibrils of higher stability and that this effect was associated with the disappearance of structures of lower stability, including nonassociated molecules and thin fibrils. Light microscopic analyses of the fibrillar collagen/heparin matrix showed it to be a more open network of distinct collagen fibers than was observed with the fibrillar collagen preparation alone. Binding experiments indicated that heparin bound to fibrillar collagen in a saturable fashion with a Kd of approximately 4 X 10(-7) M. Creep experiments provided evidence that the addition of heparin to fibrillar collagen suspensions greatly reduces the gelation phenomenon that is normally observed when such suspensions are warmed to 37 degrees C. These differences in fibrillar architecture may be in part responsible for differences noted in the biological response to fibrillar collagen and fibrillar collagen/heparin implants in vivo (McPherson et al., 1988).

Animals↗
PubMed1987

Shear creep of injectable collagen biomaterials.

Shear creep experiments in a parallel-plate apparatus were carried out on two injectable forms of collagen: semidilute nonfibrillar solutions (32 and 45 mg protein/mL) and aqueous fibrillar suspensions (at 35 and 65 mg/mL). Nonfibrillar solutions were more easily deformed than fibrillar suspensions, and both materials continued to deform slowly, even at long times (up to 6 X 10(4) sec). When the creep compliance, J(t), was plotted versus t1/3, regions of linear dependence were identified, which indicated that the materials were undergoing Andrade creep. At higher stress levels, J(t) diverged from t1/3, and true viscous flow occurred. A delayed viscosity could be calculated which agreed with viscosities derived from measurements under conditions of steady shear Couette flow. The time of onset of viscous flow was identified with a specific strain for each material, suggesting yield strain behavior. An analysis of recovery curves showed that at low stress levels or for short experimental times, the Boltzmann superposition was approximately obeyed, indicating linear behavior. At high stress levels and times greater than 10(3) sec, both nonfibrillar solutions and fibrillar suspensions showed marked curvature of stress-strain curves, which indicated that the materials are nonlinear in creep.

Animals↗
PubMed1986

The preparation and physicochemical characterization of an injectable form of reconstituted, glutaraldehyde cross-linked, bovine corium collagen.

Pepsin-solubilized bovine corium collagen was purified, reconstituted, and treated with various levels of glutaraldehyde. Treatment of suspensions of fibrillar collagen with low concentrations of glutaraldehyde appeared to have little effect on the gross morphology of fibrils, as judged by electron microscopy, but did have a significant impact on their physicochemical stability. Fibrillar collagen treated with glutaraldehyde at a concentration equal to or greater than 0.0075% demonstrated significant decreases in neutral solubility at elevated temperatures as compared to noncross-linked controls. Differential scanning calorimetry provided a convenient and quantitative means to correlate increases in melting temperature with increases in glutaraldehyde treatment concentration. Fibrillar collagen cross-linked with glutaraldehyde concentrations as low as 0.0075% demonstrated a significantly greater resistance to proteolytic degradation than did noncross-linked fibrillar collagen samples. The residual, extractable aldehyde content of such preparations was between 1 and 3 ppm. Rheological measurements on such cross-linked suspensions demonstrated that they were non-Newtonian, shear-thinning fluids, and that they were two- to threefold more viscous than corresponding preparations of noncross-linked collagen.

Amino Acids↗
PubMed1985

Collagen fibrillogenesis in vitro: a characterization of fibril quality as a function of assembly conditions.

Pepsin-solubilized bovine corium collagen was reconstituted by rapid neutralization in dilute phosphate buffer at temperatures ranging from 10 degrees C-25 degrees C. The resultant fibrils were harvested by centrifugation and resuspended in physiological buffer to a constant protein concentration. The optical density of such suspensions, measured at 410 nm in a 1 mm path length cuvette, exhibited a strong inverse correlation with temperature of fibrillogenesis. The absorbance values of fibrillar suspensions prepared from intact collagen were greater than those observed with suspensions prepared from pepsin-solubilized collagen under similar conditions and demonstrated a reduced dependence on temperature of fibril assembly. The nature of the variation in opacity of fibrillar suspensions prepared from pepsin-solubilized material was further investigated using transmission electron microscopy, trypsin sensitivity, SDS gel electrophoresis and polarimetry. Reconstitution conditions that favored more rapid precipitation (e.g., higher incubation temperatures) tended to produce fibril suspensions of lower opacity (translucent). These translucent suspensions exhibited fibrils that were small in diameter when compared to fibril suspensions of higher opacity. Translucent preparations also contained higher levels of a trypsin sensitive, early melting component and displayed a higher proportion of peptides migrating faster than alpha 2(I) on SDS polyacrylamide gels. Collagen preparations depleted of the early melting component continued to demonstrate the correlation between increased temperature and decreased fibrillar opacity, suggesting that the two phenomena were independent. It is proposed that the unstable components are nicked or shortened collagen helices, presumably generated by pepsinization or the action of endogenous proteases of the bovine corium, which are differentially incorporated into fibrils depending on the conditions of fibril assembly.

Animals↗
PubMed1976

Prediction of the secondary and tertiary structure of plastocyanin.

The amino acid sequences of nine plastocyanins were examined using four published methods for the prediction of secondary structure in proteins. The results of the four methods were combined in such a way as to maximize agreement, and the position of alpha helices, beta sheets, and beta turns in plastocyanin was predicted. From this result and other information, such as the position of conserved residues and the requirements for coordination of copper, a preliminary model for the mainchain folding of the molecule was presented.

Amino Acid Sequence↗
PubMed1971

Albumin evolution in frogs: a test of the evolutionary clock hypothesis.

Frogs are an ancient group compared to placental mammals. Yet, although there are about as many species of frogs as there are of mammals, zoologists consider that frogs have undergone only limited morphological divergence, while placental mammals have diversified greatly in morphology and way of life. The serum albumins of numerous frog species were compared by the quantitative microcomplement fixation technique. Frogs that are morphologically similar enough to merit taxonomic distinction at only the species level often exhibit differences in the serological properties of their albumins larger than those usually seen between mammals placed in distinct families or suborders. Thus, there seems to be a contrast between albumin evolution and evolution at the organismal level. The large differences between albumins among frogs can be explained by the hypothesis that albumin evolution has proceeded at the same rate in frogs as in mammals.

Acrylamides↗
PubMedDate not supplied

The relative contribution of electrostatic interactions to stabilization of collagen fibrils.

Electrostatic energies of interaction between type I collagen molecules were calculated, using models developed by Timasheff and Hill. These energies, along with a contribution from hydrophobic forces, were then incorporated into an equation due to Flory that described phase equilibria of rod-like polymers. The Flory formalism in turn permitted a calculation of the overall free energy of fibril formation (delta Ff), and an assessment of the relative contribution of electrostatic and hydrophobic forces to delta Ff. Lastly, delta Ff was used in a nucleation-growth model relating halftimes of fibril formation (t1/2) to ionic strength (I) and temperature. Because the theory provided no basis for setting absolute levels of the energetic contributions, five parameters in the model had to be derived from experimental data. Based on the fit of theory to experimental results both for intact and pepsinized collagen, it was found that very low electrostatic energies (about -1 kcal/mole per collagen molecule) were sufficient to explain experimental t1/2 vs I relationships. This energy is equivalent to 1 close charge-pair interaction per molecule and appears to be lower than the energy assignable to hydrophobic interactions.

Chemical Phenomena↗
PubMedDate not supplied

Ten years later: an update of the case for teaching breast self-examination.

Breast self-examination (BSE) remains an appropriate area for continuing research. Nonetheless, it is now established that instruction by either a nurse or a physician, whether offered individually or in groups, and regardless of the details of the content or the teaching materials used, will result in a reported increase in frequency and proficiency. Less traditional instruction by means of the mass media also has been reported as effective. The actual competence in the practice of BSE remains questionable both because of inconsistency in the standards of practice and of reliance on self-reporting. The ultimate question related to the effectiveness of BSE in detecting tumors at a stage where the diagnosis is favourable remains uncertain. There have been some gains in knowledge during the last decade and the case for teaching BSE remains strong but its effectiveness is not yet unequivocally established.

Breast↗