Leiurus quinquestriatus venom inhibits different kinds of Ca2+-dependent K+ channels

A minor protein component of Leiurus quinquestriatus venom has been reported to inhibit selectively the apamin-insensitive Ca2+-dependent K+ channels of mammalian skeletal muscle (Miller, C., Moczydlowski, E., Latorre, R. and Phillips, M. (1985) Nature 313, 316-318). We report the effect of the venom on both the apamin-insensitive channels of the human erythrocyte, the Ehrlich cell and the rat thymocyte and the apamin-sensitive channel of the guinea pig hepatocyte. The venom inhibited Ca2+-dependent K+ transport in all the cases with a Ki value within the range of 1 to 10 micrograms/ml, similar to that reported previously in muscle. Valinomycin-induced K+ transport was also antagonized by the venom but its sensitivity was about 1/10 as much as that of the Ca2+-dependent K+ channel.     

The effect of pH on the phase transition temperature of dipalmitoylphosphatidylcholine-palmitic acid liposomes

The shift in the gel-liquid crystal phase transition temperature (tm) of dipalmitoylphosphatidylcholine liposomes induced by incorporation of 10 mol% palmitic acid, was measured by 90 degrees light scattering at different bulk pH values. It has been found that the tm shift decreases sigmoidally from 4.7 to -0.3 degrees C as the bulk pH is raised from 5 to 11. Since it is in this range that the carboxyl group of a membrane-bound fatty acid should ionize, our results can be interpreted to mean that there is relationship between the tm shift and the degree of dissociation of palmitic acid, the uncharged fatty acid increasing tm and its conjugate, anionic form, slightly decreasing the transition temperature of dipalmitoylphosphatidylcholine liposomes. The experimental results are fitted by a modified form of the Henderson-Hasselbach equilibrium expression which takes into account the effect of the anionic fatty acid on the surface potential and hence, on the surface pH of liposomes, according to Gouy-Chapman and Boltzmann equations, respectively. Best fit between theory and experiments is found when the intrinsic interfacial pK of palmitic acid is set equal to 7.7. This high pK value can be explained as due to the effect of the lower dielectric constant of the interfacial region, as compared to bulk water, on the acid-base dissociation of the carboxyl group. The results presented here show that upon incorporation of palmitic acid, the phase transition of dipalmitoylphosphatidylcholine bilayers becomes extremely sensitive to changes of pH in the vicinity of the physiological range. This property is not shown by the pure phospholipid bilayers in the same pH range.     

Stimulation of gibberellin activity in winter wheat by metribuzin

Sublethal doses of metribuzin applied to wheat plants at the stage of ear emergence increased endogenous gibberellin levels in the ears. The activation of hormonal systems in connection with “chemical stress” is briefly discussed.     

Determination of ribonuclease activity in coloured extracts of citrus leaves

A rapid method is presented for the determination of ribonuclease activity in coloured extracts from citrus leaves. At the same time, the influence exercised by several precipitating reagents and the storage time at 4 °c on the activity of the enzymatic system are studied. In addition the enzyme stability against heat is studied.     

X-ray crystal structure of galabiose, O-alpha-D-galactopyranosyl-(1---4)-D-galactopyranose

O-alpha-D-Galactopyranosyl-(1---4)-D-galactopyranose, C12H22O11, Mr = 342.30, crystallises in the orthorhombic space group P2(1)2(1)2(1), and has alpha = 5.826(1), b = 13.904(3), c = 17.772(4) A, Z = 4, and Dx = 1.579 g.cm-3. Intensity data were collected with a CAD4 diffractometer. The structure was solved by direct methods and refined to R = 0.063 and Rw = 0.084 for 2758 independent reflections. The glycosidic linkage is of the type 1-axial-4-axial with torsion angles phi O-5' (O-5'-C-1'-O-1'-C-4) = 98.1(2) degrees, psi C-3 (C-3-C-4-O-1'-C-1') = -81.9(3) degrees, phi H (H-1'-C-1'-O-1'-C-4) = -18 degrees, and psi H (H-4-C-4-O-1'-C-1') = 35 degrees. The conformation is stabilised by an O-3 . . . O-5' intramolecular hydrogen-bond with length 2.787(3) A and O-3-H . . . O-5' = 162 degrees. The glycosidic linkage causes a folding of the molecule with an angle of 117 degrees between the least-square planes through the pyranosidic rings. The crystal investigated contained 56(1)% of alpha- and 44(1)% of beta-galabiose as well as approximately 70% of the gauche-trans and approximately 30% of the trans-gauche conformers about the exocyclic C-5'-C-6' and C-5-C-6 bonds. The crystal packing is governed by hydrogen bonding that engages all oxygen atoms except the intramolecular acceptor O-5' and the glycosidic O-1' oxygen atoms.