从吸收光谱的变化看H2O2对豆血红蛋白的定位损伤

豆血红蛋白（Ｌｂ）是一种含铁的蛋白质,在大豆（Ｇｌｙｃｉｎｅｍａｘ）的根瘤中含量很。从新鲜根瘤中提取制备的氧合豆血红蛋白（ＬｂＯ２,含二价铁的蛋白质）是Ｌｂ的活性形式。ＬｂＯ２在可见光区５７７ｎｍ和５４０ｎｍ有吸收峰,这两个峰与ＬｂＯ２中血红素（铁卟啉）的结构密切相关;另外,ＬｂＯ２在紫外区２８０ｎｍ处还有一个吸收峰,此峰与珠蛋白（ＬｂＯ２的蛋白质部分）的构象有关．ＬｂＯ２在一定浓度的Ｈ２Ｏ２作用下,可见光的特异吸收变化迅速,而紫外吸收相对稳定,因而推测Ｈ２Ｏ２对ＬｂＯ２的损伤是有着固定的位点,这个位点是在含二价铁的卟啉环上,而不在珠蛋白中．     

Oxidation of Dimethylsulphoxide to Formaldehyde by Oxyhaemoglobin and Oxyleghaemoglobin in the Presence of Hydrogen Peroxide is Not Mediated by “Free” Hydroxyl Radicals

In the presence of excess hydrogen peroxide. human oxyhaemoglobin and oxyleghaemoglobin from soybean root nodules cause oxidation of dimethylsulphoxide to formaldehyde. This reaction is inhibited by thiourea but not by phenylalanine. HEPES. mannitol or arginine. It is concluded that dimethylsulphoxide oxidation is not mediated by “free” hydroxyl radicals. consistent with previous conclusions that intact haemoglobin, leghaemoglobin or myoglobin molecules do not react with H₂O₂ to form hydroxyl radicals detectable outside the protein.     

Consequences of Cultivar Diffusion

Consequences of Cultivar Diffusion. Leonard Plotnicov and Richard Scaglion. eds. Pittsburgh, PA: University of Pittsburgh, 1999. 151 pp.     

Diffusion inside microtubules

Recent high-resolution analysis of tubulin's structure has led to the prediction that the taxol binding site and a tubulin acetylation site are on the interior of microtubules, suggesting that diffusion inside microtubules is potentially a biologically and clinically important process. To assess the rates of transport inside microtubules, predictions of diffusion time scales and concentration profiles were made using a model for diffusion with parameters estimated from experiments reported in the literature. Three specific cases were considered: 1) diffusion of αβ-tubulin dimer, 2) diffusion/binding of taxol, and 3) diffusion/binding of an antibody specific for an epitope on the microtubule's interior surface. In the first case tubulin is predicted to require only ∼1 min to reach half the equilibrium concentration in the center of a 40 μm microtubule open at both ends. This relatively rapid transport occurs because of a lack of appreciable affinity between tubulin and the microtubule inner surface and occurs in spite of a three-fold reduction in diffusivity due to hindrance. By contrast the transport of taxol is much slower, requiring days (at nm concentrations) to reach half the equilibrium concentration in the center of a 40 μm microtubule having both ends open. This slow transport is the result of fast, reversible taxol binding to the microtubule's interior surface and the large capacity for taxol (∼12 mm based on interior volume of the microtubule). An antibody directed toward an epitope in the microtubule's interior is predicted to require years to approach equilibrium. These results are difficult to reconcile with previous experimental results where substantial taxol and antibody binding is achieved in minutes, suggesting that these binding sites are on the microtubule exterior. The slow transport rates also suggest that microtubules might be able to serve as vehicles for controlled-release of drugs. Received: 2 March 1998 / Revised version: 3 May 1998 / Accepted: 3 May 1998     

Diffusion with attrition

This article treats the problem of the sharp front observed when a diffusing substance interacts irreversibly with binding sites within the medium. The model consists of two simultaneous partial differential equations that are nonlinear and cannot be solved in closed form. The parameters are the diffusion coefficient D in the direction under consideration (x), the interaction constant k, the binding-site concentration μ and the boundary concentration of the diffusing ion c ₀. Our aim is to develop methods to enable the estimation of these parameters from the experimental data. An analytical solution for the case k → ∞, as found by others, is given first and then a finite element analysis package is used to obtain numerical solutions for the general case. Graphs are presented to illustrate the effects of the various parameters. Simple graphical procedures are described to compute μ and c ₀. The position of the advancing front ξ then provides, together with μ, a way to estimate D. A mathematical identity relating D and x and a second one involving D, k and t help to reduce the complexity of the problem. A new, measurable quantity S(t) is defined as where f is the total concentration (free + bound) of the diffusing ion at time t, and detailed plots are furnished that permit the computation of k directly from S(t), μ and D. The accuracy with which such methods can be expected to determine the various parameters of the model is considered at some length. Finally, in a concluding section, we simulate typical experimental data, examine the validity of our methods, and see how their accuracy is affected by controlled amounts of various kinds of noise.     

Theory of Diffusion in Gels

It has been shown that when concentration of solute is expressed as amount per unit volume of gel—solvent—solute system, Fick's laws for diffusion in a gel take the same form as for diffusion in solvent alone, except that the usual coefficient must be replaced by a new coefficient, D′, equal to D(1 - α)/(1 - ), where is the effective volume fraction of the gel substance and α is a coefficient of obstruction equal to 5/3 if the gel substance can be considered to be made up of randomly oriented rods. An equation was derived for the total amount of solute entering the gel, which is analogous to but not identical with the equation for the total amount of solute crossing the initial boundary in free diffusion. The effect of slice thickness was investigated by a mathematical procedure involving the solutions of approximate differential equations. It was shown that even for slices so thick that 95 per cent of the solute in the gel is contained in the first two, a correction factor equal to the square of the slice thickness divided by 48D't permits one to obtain accurate measurement of D′ from the mean concentration and the position of the midplane of the slice.     

Diffusion limited immunochemical sensing

The time-dependent surface coverage of antigen-antibody complexes for a sensor in which antigens are bound to surface immobilized antibodies is determined analytically. Assuming a reversible first order reaction between the antigens and antibodies, a model is derived describing the dynamical response of the sensor. The surface coverage is related explicitly to the antigen concentration which is of special interest in experimental situations. The stationary state and short time behaviour are determined explicitly. Several illustrations of the full solution are provided.     

Diffusion in immobilized-cell gels

Summary The relationship between the diffusion coefficient, the effective diffusion coefficient and the partition coefficient for a solute in a cell-containing gel is discussed. The use of correlation equations that are based on some kind of physical model is recommended when the effect of cell concentration on diffusion is interpreted.     

Diffusion in colloidal media

When the molecules of a solute diffuse through a medium containing large colloidal particles, which absorb the diffusing molecules, the latter are transported in the diffusion flow not as free molecules, but as absorbtion compounds: solute+colloid. When the colloidal particle is much larger than the molecule of the solute, and has therefore a much smaller mobility, this results in a reduction of the apparent diffusion coefficient for the solute. The biological implications of this are discussed.     

Diffusion of molecules on biological membranes of nonplanar form. II. Diffusion anisotropy.

Molecules diffusing on nonplanar membranes, which have different amounts of corrugation in different directions, may experience dissimilar diffusion coefficients in each direction. Smith et al. (1979, Proc. Natl. Acad. Sci. USA, 76:5641-5644) measured diffusion anisotropy on fibroblast cell membranes in which the ratio of the diffusion coefficients, in different directions, was 0.27. In the present work we calculate the effect of anisotropic corrugation on the rate of diffusion of molecules on membranes. We find that part of the anisotropy reported by Smith et al. (1979, Proc. Natl. Acad. Sci. USA, 76:5641-5644) can be explained by the membrane nonplanarity, and we present the way of calculating this geometric factor.     

The Diffusion of KCI from Micro-electrodes

The diffusion of salt from the tips of 3 M KCI filled micro-electrodesis measured. The leakage of KCI in 3 min is found to be 5.70x 10^–12–1.08 x 10^–11 mol for tips of 0.2–0.6µm diameter, and 2.70 x 10^–11 mol for tips of 0.6–1.0µm diameter. The observed leakages compare with estimatesbased on the diffusion coefficient of KCI and micro-electrodegeometry. The results confirm that the use of 3 M KCI in micro-electrodesmay lead to serious contamination of impaled cells providedthe electrode tips remain unoccluded.     

Diffusion in gas exchange of insects

The air-filled tracheal system constitutes the organ for gas exchange in terrestrial insects-its finest branches, the tracheoles, contacting individual cells. In the pupal stage, in which the animal lacks significant ventilatory movement, diffusion in the gas phase of the tracheal system constitutes the only mechanism for gas transfer between the environment and the tissues, transport in the hemolymph being insignificant. We have attempted to identify the main sites of diffusional resistance in the tracheal gas system by measuring the evolution of inert gases of low solubility from the pupa of the giant silkworm moth (Hyalophora cecropia). The results are compatible wih a single model in which the resistance to diffusional gas transfer in the tracheal system is concentrated at its opening at the body surface (spiracle).