Biochimica et Biophysica Acta (BBA) - Biophysics including Photosynthesis最新文献

A quantitative analysis of the structure-activity relationship in cholinesterase inhibitors was made. From the results of Metcalf and Fukuto four different classes of inhibitors are considered: methylcarbamates, diethylphenylphosphates, alkylphosphonic acid esters, $\text{2,4,5-}\text{trichloro}\text{-N-}\text{alkylphosphoramidates}$. It is shown by means of substituent constants and regression analysis that the effects of substituent groups can be factored into three groups: electronic, steric, hydrophobic. The results indicate that the phosphate esters react by a mechanism different from that of the carbamates. New examples of the value of Taft's steric constant, $\text{E}{}_{\mathrm{<mtext>S</mtext>}}$, for biological reactions are discussed.

Ion transport in the pitcher of Nepenthes henryana has been studied:

• 1.

  1. Apparent concentrations of Na⁺, K⁺, H⁺, Ca²⁺, Mg²⁺ and Cl⁻ in the pitcher tissue and liquid were determined.

• 2.

  2. By 24-h dialysis of the pitcher liquid against redistilled H₂O about 90% of the K⁺ and Cl⁻ were removed, as compared with only 10% Na⁺.

• 3.

  3. The apparent membrane potentials for Na⁺, K⁺ and Cl^- at the interphase pitcher tissue-liquid were calculated according to the Nernst equation.

• 4.

  4. Using microelectrodes, the membrane potential of the cells lining the inner surface of the pitcher was found to be $\text{35 ± 3.7}\text{mV}$ (range 17–55 mV; 48 measurements in a total of 10 plants). The tissue was negative with respect to the pitcher liquid.

• 5.

  5. In Na⁺-containing media a potential difference of 30–50 mV and a short-circuit current of 10–20 μA/cm² across the pitcher wall were found. Both of these values were decreased by 0.4 mM 2,4-dinitrophenol or 0.4 mM HgCl₂.

• 6.

  6. Na⁺ in the bathing liquid produced a short-circuit current in the direction: $\text{inner compartment → pitcher tissue → outer medium}$; K⁺ and Cl⁻ brought about currents in the opposite direction.

• 7.

  7. The mechanism of the transport of Na⁺, K⁺ and Cl⁺ in the pitcher is discussed.

The influence of a variety of mono- and polybasic alcohols, ethers and acids on the electronic absorption spectrum of tyrosine in aqueous solution was investigated.

A shift towards longer wavelength in the first ($\text{L}{}_{\mathrm{b}}$) and the second ($\text{L}{}_{\mathrm{a}}$) band is observed as a function of solvent composition. The results are compared with those obtained for phenylalanine in similar solvent systems. It is found that the effects can be discussed in terms of hydrogen bond formation between tyrosine, acting as proton donor, and the various solvent components as proton acceptors. It is suggested that the changes in spectral properties due to hydrogen bonding may be attributed to a modification in the substituent effect of the hydroxyl group on the spectrum of the aromatic system.

The implications of these effects in polypeptide conformation studies are briefly mentioned.

The dependence on Na⁺ of the active transport of sugar in small intestine, in vitro, was studied by observing the influence of various sugars on the electrical potential difference measured across the wall of everted intestinal sacs. Confirming other reports, the potential difference in rat, hamster and guinea-pig preparations is significantly increased in the presence of sugars known to accumulate against a concentration difference. Also confirming other reports, the potential difference is strongly depressed with serosal ouabain and, to a lesser extent, also with mucosal ouabain in the absence of added sugar. However, it has now been additionally found that the increased potential difference in the presence of sugars as above, is still further enhanced by ouabain. This persistent effect of ouabain is species specific, being clearly evident in rat and, though of lesser intensity, also in guinea-pig, but not in hamster. In the rat the effect is observed with the sugars, d-glucose or 6-deoxy-d-glucose, in jejunal but not in ileal sacs and with d-galactose in ileal but not in jejunal sacs. Comparison of plots of i/ΔPD versusi/(sugar) in the presence and absence of ouabain suggests that ouabain acts as a competitive activator for Na⁺ entrance into the mucosal epithelial cell.

Short-circuit techniques for the study of ion transport by isolated membrane systems have been applied to defining the mechanism for calcium ion transport in the small intestine of the rat. In the absence of phosphate, the movement of calcium is passive. There is no evidence for a membrane-bound carrier to facilitate its transport. The calcium flux is a linear function of its concentration. However, phosphate ion is actively transported from the mucosal to the serosal surface. In the presence of phosphate, calcium appears to be actively transported, possibly as a counter-ion to the phosphate. The role of chelation in the transport of calcium has also been clarified.

• 1.

  1. It has been postulated that active trans-membrane transport of metabolizable sugars has at least four steps. The overall transport is inhibited by nickelous ions and the inhibition appears to be caused by an interaction of nickelous ions with polyphosphates, which are postulated to be involved in one of the steps of the transport mechanism.

• 2.

  2. The characteristics of the transport of sorbose in normal, and of glucose in iodoacetate-poisoned cells are different from this active transport mechanism and can be described as carrier-mediated facilitated diffusion. Polyphosphates are not involved in facilitated diffusion nor is this inhibited by nickelous ions.

• 3.

  3. Ni²⁺ inhibition of active transport increases strongly with increasing temperature. This is caused by a considerable decrease of the energy of activation of the transport process by nickelous ions. Different steps of the individual transport processes appear to be rate-limiting in normal and in Ni²⁺-inhibited sugar influx.

• 4.

  4. The affinity of sugars for the transport system is not influenced by nickelous ions.

• 5.

  5. Experimental results with combinations of two metabolizable sugars in the medium indicate the existence of at least two sugar carriers in the yeast cell membrane: one for the hexoses and one for maltose.

The rate of dissociation of HbO₂ in erythrocytes and solution has been measured, using a split-beam spectrophotometric detector, after rapid mixing and $\text{1:20}$ dilution in a medium equilibrated with a high pCO. The observations were made on sheep HbA and HbB over the temperature range 5–38°. In solution, the value of $\text{k}{}_4$ was 44.5 sec⁻¹ at 20° and pH 7.4 and the activation energy 19.5 kcal for HbA; the corresponding values for HbB were 54.8 sec⁻¹ and 19 kcal. In the erythrocyte, similar measurements gave the values 38.8 sec⁻¹ and 17 kcal for HbA and 51.2 sec⁻¹ and 16 kcal for HbB. The rate constant $\text{k}{}_4$ for HbA was reduced by 10% in alkaline solutions and that of HbB by 25%. No effect of increasing the ionic strength in the cell on $\text{k}{}_4$ was observed. An analysis of the results suggests that there are two forms of HbB and the haem-haem interaction is reduced when the haemoglobin is in the cell.

• 1.

  1. Using ³²P as a tracer, phosphate uptake into the giant cells of Nitella translucens has been shown to be a strictly active process. The uptake is light-enhanced and requires a supply of ATP. When non-cyclic electron flow is prevented, the light-dependent process is supported by cyclic photophosphorylation.

• 2.

  2. As it is already known that active Cl⁻ uptake into Nitella translucens is not dependent on ATP but is linked to electron-transfer reactions, there can be no common mechanism for anion transport in these cells.

• 3.

  3. Measurements of the relative amounts of ³²P in the cytoplasm and vacuole have shown that the cytoplasm contains nearly all the organic phosphate, while a large pool of inorganic phosphate may be formed in the vacuole.