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.     

CTLA4 Autoimmunity-Associated Genotype Contributes to Severe Pulmonary Tuberculosis in an African Population

The gene of Cytotoxic T Lymphocyte-associated Antigen 4 (CTLA4), a negative regulator of T lymphocytes, contains a single-nucleotide polymorphism (SNP) at position +6230A->G (ct60A->G), which has been found associated with several autoimmune diseases and appears to reduce T-cell inhibitory activity. In Ghana, West Africa, we compared the frequencies of CTLA4 +6230 A/G and 6 haplotype-tagging SNPs in 2010 smear-positive, HIV-negative patients with pulmonary tuberculosis (TB) and 2346 controls matched for age, gender and ethnicity. We found no difference in allele frequencies between cases and controls. However, +6230A and a distinct CTLA4 haplotype and a diplotype comprising the +6230A allele were significantly less frequent among cases with large opacities in chest radiographs compared to those with small ones (P_{corrected [cor]} = 0.002, P_cor = 0.00045, P = 0.0005, respectively). This finding suggests that an increased T-cell activity associated with the CTLA4 +6230G allele contributes to pathology rather than to protection in pulmonary TB.     

Effects on Plant Growth Produced by Azotobacter paspali Related to Synthesis of Plant Growth Regulating Substances

S ummary . Treating seedling roots of several plant species with cultures of Azotobacter paspali changed plant growth and development and significantly increased weight of leaves and roots; effects were probably caused by plant growth regulators. Culture supernatant fluids contained indolyl-3-acetic acid, at least 3 gibberellins and 2 cytokinins. The added inoculum of A. paspali survived in plant rhizospheres for only a few weeks and no nitrogen was fixed in the root zone of young Paspalum notatum , the grass with which A. paspali is associated.     

Efficient flow cytometric assay for platelet-leukocyte aggregates in whole blood using fluorescence signal triggering.

BACKGROUND: Platelet-leukocyte aggregates (PLAs) may be important in thrombotic and inflammatory disease states, but accurate assessment of PLA formation in vivo is hampered by the propensity for in vitro artefacts caused by sample manipulation. A whole blood flow cytometric assay for circulating PLAs, based on minimal sample manipulation, was thus developed. METHODS: Citrated whole blood was labeled with a RPE-CD45 MAb (leukocyte marker) and an FITC-CD42a (GPIX) MAb (platelet marker). The latter was used to avoid possible influences of platelet glycoprotein proteolysis by neutrophil-derived proteases. The samples were mildly fixed with 0.5% formaldehyde saline. The cytometer was triggered by RPE-CD45 fluorescence. Leukocyte subpopulations were separated according to their typical light scattering and CD45 expression. RESULTS: Minimal sample manipulation and mild sample fixation resulted in minor in vitro artefacts and good sample stability. Fluorescence triggering increased the efficiency of the flow cytometric analysis approximately 5-fold compared with triggering with light scatter, and allowed discrimination of leukocyte subpopulations. The majority of PLAs involved monocytes and neutrophils, rather than lymphocytes, both without and with in vitro stimulation by ADP or thrombin. A cocktail of blocking MAbs to CD62P, CD15, GPIIb/IIIa and the CD11b/CD18 complex had no effect on unstimulated samples, whilst totally inhibiting aggregation induced by 10(-5) M ADP, suggesting that the PLAs in unstimulated blood were preformed in vivo. CONCLUSIONS: This whole blood flow cytometric assay for PLAs is simple and efficient, and appears to reflect closely platelet-leukocyte aggregates in circulating blood in vivo.