Mechanism and dynamics of conformational ordering in xanthan polysaccharide

The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting.     

Comparison of the effects of smooth and skeletal tropomyosin on skeletal actomyosin subfragment 1 ATPase

Chicken gizzard tropomyosin, like rabbit skeletal tropomyosin, inhibits and activates skeletal actomyosin subfragment 1 ATPase at low and high [subfragment 1], respectively, showing that both smooth and skeletal tropomyosin qualitatively produce similar cooperative effects on activity. For gizzard tropomyosin, however, the extent of the inhibition was less, and the activation curve rose more sharply at lower [subfragment 1]. In terms of a two-state cooperative activity model for the actin-tropomyosin filament (Hill, T. L., Eisenberg, E., and Chalovich, J. (1981) Biophys. J. 35, 99-112), these results qualitatively suggest that, for the gizzard tropomyosin system, more units are initially in the active state (in the absence of subfragment 1) and that the switching of units to the active state is more cooperative. The greater cooperativity indicated for the gizzard system may be a consequence of the greater rigidity of gizzard tropomyosin indicated from conformational studies.     

Internode length in Pisum. I. The effect of the Le/le gene difference on endogenous gibberellin-like substances

The allelopathic potential of the dry fruits of Washingtonia filifera (L. Linden) H. Wendl. was investigated. Leachates from fruits inhibited the germination of lettuce, wheat, red cabbage and cucumber seeds. The inhibitory effect was partly neutralized by kinetin (20 mg 1⁻¹) and gibberellic acid (50 mg 1⁻¹). The effect of kinetin was more pronounced at 25°C than at 20°C. Substances inhibiting germination were localized in the pericarp of the fruit and were resistant to high temperature.     

Cardiac myoglobin deficit has evolved repeatedly in teleost fishes

Myoglobin (Mb) is the classic vertebrate oxygen-binding protein present in aerobic striated muscles. It functions principally in oxygen delivery and provides muscle with its characteristic red colour. Members of the Antarctic icefish family (Channichthyidae) are widely thought to be extraordinary for lacking cardiac Mb expression, a fact that has been attributed to their low metabolic rate and unusual evolutionary history. Here, we report that cardiac Mb deficit, associated with pale heart colour, has evolved repeatedly during teleost evolution. This trait affects both gill- and air-breathing species from temperate to tropical habitats across a full range of salinities. Cardiac Mb deficit results from total pseudogenization in three-spined stickleback and is associated with a massive reduction in mRNA level in two species that evidently retain functional Mb. The results suggest that near or complete absence of Mb-assisted oxygen delivery to heart muscle is a common facet of teleost biodiversity, even affecting lineages with notable oxygen demands. We suggest that Mb deficit may affect how different teleost species deal with increased tissue oxygen demands arising under climate change.     

Changes in the levels of wheat- and barley-germ agglutinin during embryogenesis in vivo,in vitro and during germination

Radioimmunoassay has been used to measure levels of wheat-germ agglutinin and barley-germ agglutinin during embryogenesis and germination. The two lectins exhibited similar patterns of accumulation during grain maturation in vivo and both decreased to low levels after imbibition of harvest-ripe grains for 3 d. Precocious germination of immature wheat and barley embryos excised and cultured in vitro could be prevented either by inclusion of abscisic acid or mannitol in the culture medium. Changes in the level of wheat-germ agglutinin induced by in vitro culture depended on the maturation stage of the embryo. No direct correlation was found between application of exogenous abscisic acid and accumulation of the lectin.     

Increase of sialyltransferase activity in the serum and liver of inflamed rates

Induction of inflammation by turpentine injection caused 1.5–2-fold increase of both sialy- and galactosyltransferase activity in liver homogenates. The effect was apparent after 12 h turpentine treatment. Serum sialytransferase activity started to increase in the inflamed rats after 18 h, reaching a maximum of 4-fold at 48 h. In contrast, galactosyltransferase activity in serum showed no significant increase. The coordinated and temporal increase of sialytransferase activity in liver and serum suggest involvement of a specific mechanism for the preferential release of this enzyme into serum.