Kinetic Theory Model for Ion Movement through Biological Membranes: III. Steady-State Electrical Properties with Solution Asymmetry

An electrodiffusion model for plasma membrane ion transport, which takes into account the influence of high electric field strengths and ion-membrane molecule interactions, is presented and analyzed. A generalized Nernst-Planck equation for steady-state situations is derived which has electric field-dependent mobility and diffusion coefficients. Under the assumption of a constant electric field within the membrane, this equation is integrated to give a more general form of the Goldman equation. Based on this equation numerical computations of ionic chord conductance as a function of applied electric field strength were carried out for several permeant ion concentration ratios. The model is capable of yielding significantly larger rectification ratios than is the Goldman equation. Further, high field asymptotes to the current vs. electric field strength curve do not generally intersect at the origin.     

Peroxide sensitivity of cold-shocked Salmonella typhimurium and Escherichia coli and its relationship to minimal medium recovery

Cold-shocked Salmonella typhimurium displayed minimal medium recovery (MMR), viable counts on M9 minimal agar being much higher than those on tryptone soya yeast extract agar (TSYA). The addition of catalase to TSYA restored counts to the level found on M9 agar. Peroxide concentrations between 12 and 30 mumol/l were measured in TSYB but none was detected in M9 medium. Cold-shocked cells were sensitive to reagent hydrogen peroxide at a concentration similar to that found in TSYB. The minimal medium recovery phenomenon of cold-shocked cells is thus a manifestation of peroxide sensitivity. Changing the composition of growth media affected both cellular catalase activity and the magnitude of the MMR effect but the two properties were not directly related. Factors additional to cellular catalase activity must therefore affect susceptibility to peroxide following cold shock. Mutational loss of catalase, exonuclease III or recA-dependent DNA repair functions all increased the sensitivity of cold-shocked Escherichia coli to the inhibitory effects of peroxide present in rich medium. The peroxide resistant fraction of a cold-shocked population of Salm. typhimurium (i.e. those cells able to grow on TSYA) was more resistant to gamma radiation than the population as a whole. Cold shock thus sensitizes cells to more than one form of oxidative stress. Prior exposure of growing cells to 30 mumol/l hydrogen peroxide abolished their sensitivity to rich medium following cold shock implying that Salm. typhimurium contains an inducible system protecting against oxidative stress.     

Enzymatic hydrolysis of pyridoxine-5'-beta-D-glucoside is catalyzed by intestinal lactase-phlorizin hydrolase

An obligatory step in the mammalian nutritional utilization of pyridoxine-5'-beta-D-glucoside (PNG) is the intestinal hydrolysis of its beta-glucosidic bond that releases pyridoxine (PN). This laboratory previously reported the purification and partial characterization of a novel cytosolic enzyme, designated PNG hydrolase, which hydrolyzed PNG. An investigation of the subcellular distribution of intestinal PNG hydrolysis found substantial hydrolytic activity in the total membrane fraction, of which 40-50% was localized to brush border membrane. To investigate the possible role of a brush border beta-glucosidase in the hydrolysis of PNG, lactase phlorizin hydrolase (LPH) was purified from rat small intestinal mucosa. LPH hydrolyzed PNG with a K(m) of 1.0 +/- 0.1 mm, a V(max) of 0.11 +/- 0.01 micromol/min.mg protein, and a k(cat) of 1.0 s(-1). LPH-catalyzed PNG hydrolysis was inhibited by glucose, lactose, and cellobiose but not by PN. Specific blockage of the phlorizin hydrolase site of LPH using 2',4'-dintrophenyl-2-fluoro-2-deoxy-beta-D-glucopyranoside did not reduce PNG hydrolysis. Evidence of transferase activity was also obtained. Reaction mixtures containing LPH, PNG, and lactose yielded the formation of another PN derivative that was identified as a pyridoxine disaccharide. These results indicate that LPH may play an important role in the bioavailability of PNG, but further characterization is needed to assess its physiological function.     

The segmentation clock in mice: interaction between the Wnt and Notch signalling pathways

In the last few years, the efforts to elucidate the mechanisms underlying the segmentation clock in various vertebrate species have multiplied. Early evidence suggested that oscillations are caused by one of the genes under the Notch signalling pathway (like those of the her or Hes families). Recently, Aulehla et al. [Wnt3a plays a major role in the segmentation clock controlling somitogenesis. Dev. Cell 4, 395-406] discovered that Axin2 (a gene under the Wnt3a signalling pathway) also oscillates in the presomitic mesoderm (PSM) of mice embryos and proposed some mechanisms through which the Notch and Wnt3a pathways may interact. They further suggested that a decreasing concentration of Wnt3a along the PSM may be the gradient the segmentation clock interacts with to form somites. These results were reviewed by Rida et al. [A notch feeling of somite segmentation and beyond. Dev. Biol. 265, 2-22], who introduced a complex clockwork comprising genes Hes1, Lfng (under the Notch pathway), and Axin2, as well as their multiple interactions. In the present work we develop a mathematical model based on the Rida et al. review and use it to tackle some of the questions raided by the Aulehla et al. paper: can the Axin2 feedback loop constitute a clock? Could a decreasing Wnt3a signaling constitute the wavefront, where phase is recorded and the spatial pattern laid down? What is the master oscillator?     

The role of plasticity in resource capture by plants

Comparative experiments measuring the form and extent of plant responses to environmental factors allow an ecologically important distinction to be made between morphological plasticity and cellular acclimation. The two phenomena are associated with different sets of plant traits with which they occur with highest fidelity and potency at opposite ends of productivity gradients. A trade-off between scale and precision occurs with respect to the responses of plants to patchiness in resource supply and this provides valuable predictors of the status achieved by species in plant communities.     

Molecular mapping of a new gene Rrs4 CI 11549 for resistance to barley scald (Rhynchosporium secalis)

Doubled haploid (DH) progeny from a cross between the scald susceptible barley (Hordeum vulgare L.) cultivar Ingrid and the resistant accession CI 11549 (Nigrinudum) was evaluated for resistance in the pathogen Rhynchosporium secalis (Oudem) J.J. Davis. Two linked and incompletely dominant loci confer resistance CI 11549 against isolate 4004. One is an allele at the complex Rrs1 locus on chromosome 3H close to the centromere; the other is located 22 cM distally on the long arm. The latter locus is designated Rrs4. In BC₃-lines into Ingrid from CI 2222 (another Nigrinudum) resistance seems governed by one locus close to the telomeric region of chromosome 7H, probably allelic to Rrs2. In neither case did we find any trace of the recessive gene rh8 reported to be present in Nigrinudum. Various resistance donors of Ethiopian origin designated as Nigrinudum, Jet or Abyssinian were identical to a great extent with respect to markers, but differed in resistance to different isolates of scald or in barley yellow dwarf virus (BYDV) resistance. The implications for their use as differentials in scald tests and screening of germplasm collections are discussed.