Perturbation of hydrogen bonds in the adenine ... thymine base pair by Na+, Mg2+, Ca2+ and NH4+ cations

Perturbation of the hydrogen bonds in the adenine ... thymine base pair by Na+, Mg2+, Ca2+ and NH4+ cations has been investigated by means of ab initio SCF calculations with the STO-3G basis set. The geometry of adenine...thymine, as well as those of the perturbed pairs were optimized. Approach of any cation to thymine at O6 leads to destabilization of the adenine...thymine pair; divalent cations (Mg2+, Ca2+) have a profound effect on the structure of the base pair. The approach of a cation to other available sites (thymine: O2, adenine N1 and N3) leads, on the other hand, to stabilization of the base pair. If a water molecule is placed between the cation and the base pair, the structure and stability of the base pair are changed only negligibly.     

A rapid and safe method of estimating nanomole quantities of P, K+, Na+, Ca2+, and Mg2+ in plant material by perchloric acid digestion

A method for the determination of nanomole amounts of P and major cations in samples of dried plant tissue is described. Samples weighing 2-7 mg were digested by refluxing tissue in a mixture of perchloric and nitric acids, under conditions which minimized the hazards associated with perchloric acid digestion. In most tissues analyzed, reproducibility between triplicate samples was less than +/- 5%. However, where the amount of P in the tissue was very low (samples contained about 50 nmol P each), the variation between triplicates was greater.     

抗寒剂CR-4 对冬小麦幼苗质膜钙泵（Ca2 +-ATPase） 的稳定作用

通过磷酸铈沉淀的细胞化学观察揭示,常温下生长的冬小麦幼苗的Ca2+ -ATP酶活性主要定位在质膜上,同时,水浸种和抗寒剂浸种的小麦质膜Ca2+ -ATP酶活性没有差异。然而,小麦幼苗经-7℃冰冻处理12小时和24小时后,则表现明显的区别：水浸种的小麦幼苗质膜Ca2+ -ATP酶活性明显下降,直至完全失活,细胞的精细结构也同时被破坏;而经抗寒剂浸种的小麦幼苗质膜Ca2+ -ATP酶仍维持较高的活性,细胞结构也保持完整,显示抗寒剂对质膜Ca2+ -ATPase酶起着明显的稳定作用。     

Erythrocyte membrane in rats fed high erucic acid-containing mustard oil: osmotic fragility, lipid composition, and (Na+, K+)- and (Ca2+, Mg2+)-ATPases

Erythrocyte hemolytic properties, cholesterol/phospholipid ratios, fatty acid composition, and activities of the membrane-bound enzymes (Na+, K+)- and (Ca2+, Mg2+)-ATPase were studied in male and female rats fed low erucic acid rapeseed (LEAR) and high erucic acid mustard oils (HEAM) for a period of 16 months. Rats receiving groundnut oil (GNO) served as controls. Erythrocytes from HEAM-receiving male and female rats showed increased resistance to hypotonic hemolysis. In male rats this was associated with an 85% increase (P less than 0.07) in the cholesterol/phospholipid molar ratio. The fatty acid double-bond index showed an increase in male rats receiving HEAM as well as LEAR oils. In the erythrocytes from female rats, the cholesterol/phospholipid molar ratio and double bond index remained unaffected. Specific activity of ouabain-sensitive (Na+, K+)-ATPase showed a small (+20%) but significant (P less than 0.05) increase in male but not female rats of HEAM group. Total (Na+, K+)-ATPase, ouabain-insensitive component, and (Ca2+, Mg2+)-ATPase were not altered in rats receiving LEAR or HEAM.     

Changes in microsomal Na+, K+-, Mg2+- and Ca2+-ATPase activities during proliferation of Chinese hamster V-79 and human HeLaS-3 cells

Changes in microsomal Na+, K+-, Mg2+- and Ca2+-ATPase activities during cell proliferation were examined in Chinese hamster V-79 (V-79) cells (normal cells) and human HeLaS-3 (HeLaS-3) cells (malignant cells). For V-79 cells, the Mg2+-ATPase activity per cell (pmol Pi/h/cell) in the confluent phase was higher than that in the logarithmically growing (log) phase. The amount of microsomal protein per cell was also high in the confluent phase. Specific activities (mumol Pi/h/mg protein) of Na+, K+-, Mg2+- and Ca2+-ATPase were significantly lower in the confluent phase than in the log phase. For HeLaS-3 cells, an increase in Ca2+-ATPase activity per cell was observed. The amount of microsomal protein per cell did not change between the log and confluent phase. The specific activity of Ca2+-ATPase in the confluent phase was also markedly higher than in the log phase. The relation between changes in ATPase activities and cell proliferation is discussed.     

NH4+ currents across the peribacteroid membrane of soybean. Macroscopic and microscopic properties, inhibition by Mg2+, and temperature dependence indicate a SubpicoSiemens channel finely regulated by divalent cations

The control of ammonium (NH(4)(+)) transport is critical in preventing futile cycles of NH(4)(+)/ammonia transport. An unusual nonselective cation channel with subpicoSiemens single-channel conductance permeable to NH(4)(+) had previously been identified in the peribacteroid membrane (PBM) of symbiosomes from soybean (Glycine max) nodules. Here, we investigate the proposed channel mechanism and its control by luminal magnesium. Currents carried by NH(4)(+) were measured in inside-out PBM patches by patch clamp. NH(4)(+) transport corresponding to the physiological direction of net transfer showed time-dependent activation and associated single-channel-like events. These could not be resolved to discrete conductances but had the same selectivity as the total current. The voltage dependence of the steady-state current was affected by temperature consistent with the rate constant of channel opening being reduced with decreased temperature. This resulted in steady-state currents that were more temperature sensitive at voltages where the current was only partially activated. When fully activated, the current reflected more the ion conduction through open channels and had an activation energy of 28.2 kJ mol(-1) (Q10 = 1.51, 8 degrees C-24 degrees C). Increased Mg(2+) on the symbiosome lumen side blocked the current (ID(50) = 351 microm, with 60 mm NH(4)(+)). Complete inhibition with 2 mm Mg(2+) was relieved with a small increase in NH(4)(+) on the lumen side of the membrane (shift of 60-70 mm). With Mg(2+) the selectivity of the transport for divalent cations increased. From these features, we propose a divalent-dependent feedback regulation of the PBM-nonselective cation channel that could maintain a constant NH(4)(+) gradient across the membrane.     

Syntheses and structures of M(L)(X)BPh4 complexes {M=Co(II), Zn(II); L=cis-1,3,5-tris[3-(2-furyl)prop-2-enylideneamino]cyclohexane, X=OAc, NO3}: structural models of the active site of carbonic anhydrase

The metal coordination geometries in the structures of the zinc(II) and cobalt(II) complexes of the ligand cis-1,3,5-tris[3-(2-furyl)prop-2-enylideneamino]cyclohexane (fr-protach) and with the anions nitrate and acetate are structural models for the active site of carbonic anhydrase. The acetate structures show a striking structural correlation with the metal coordination environments in the known bicarbonate forms of the enzyme. Such structures provide a basis for understanding the marked effect of different metal substitution on the catalytic rate of the enzyme.