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

The properties of some model systems for transcellular active transport (translocation) are described. The functioning of the models depends upon, firstly the occurrence of solute (substrate) pumping across the opposing bordering membranes of a unit cell, the membranes also being leaky to the substrate and secondly, an intrinsic asymmetry of the opposing membranes. The systems are characterized by the maintenance in the steady state of substrate concentration differences between the contents of the unit cell and its environment.

Making assumptions as to the properties of the individual faces of the cell equations have been written describing the rate of translocation in terms of the parameters of substrate transport at the faces of the cell and the extracellular concentrations. Using assumed values for these parameters and concentrations, translocation rates have been calculated for various conditions. From study of the models the conditions under which translocation will occur are demonstrated and the kinetic properties of the model systems compared with experimental data.

It is also shown that changes in the rates of leaking and of pumping of a substrate across the opposing faces of the unit cell can, under appropriate conditions, change the direction as well as the rate of substrate translocation. The induction of a net counterflow of one substrate through the cell (countertranslocation) can be a consequence of the presence of a second substrate which is also capable of being pumped across the bordering membranes of the cell in the same direction as the first.

Spectroscopic studies indicate that the anionic dye Biebrich scarlet binds and stacks to polycations in an analogous way to cationic dyes on polyanions. Both exciton and dispersion interactions are needed to account for the spectral shifts and intensity changes. The low stacking coefficients are suggestive but not strong evidence for helical structure in the polycations examined. Calculations based on exciton theory using the observed spectral shifts, show that the dyes lie on the outside of the polymers, probably in a radial orientation.

The dielectric dispersion of G-actin has been measured and the value of its dipole moment at neutral pH in a salt-free solution was estimated to be about 660 Debye. The dependence of the dielectric properties on protein concentration, pH and solvent viscosity, and the effect of urea denaturation were examined.

The results suggest that the main mechanism of dielectric relaxation is the orientation of the permanent dipole, while the proton fluctuation makes a considerable contribution to the dielectric increment.

Spinach chloroplasts, when stored in vitro in 50% glycerol at −20° have been found to retain the capacity for cyclic and non-cyclic photophosphorylation for several weeks, whereas this activity is quickly lost in control experiments.

A time study made of the decline in light-triggered ATP hydrolysis and photophosphorylation for 3 electron-flow systems revealed that ATP synthesis was more labile than hydrolysis. Photosynthetic electron flow (NADP+ reduction) and light-induced comformational changes are lost much more slowly upon storage of glycerinated chloroplasts than ATP hydrolysis or synthesis. Hence light-induced conformational changes are more a sensitive test for assessing the occurrence of the high-energy state of spinach chloroplasts than the reactions involving ATP hydrolysis or synthesis.

• 1.

  1. Individuals of the Australian marsupial species, the possum (Trichosurus vulpecula), differ in erythrocyte cation concentration. ATPase activity was measured in the fragmented membranes of erythrocytes in which potassium predominates (HK cells) and in which sodium predominates (LK cells). A comparison was made with erythrocyte membrane ATPase activity in the marsupial quokka (Setonyx brachyurus) in which sodium and potassium are present in approximately equal concentration.

• 2.

  2. All erythrocyte preparations showed basal ATPase activity. Significant (Na⁺-K⁺)-activated ATPase was present only in HK possum and quokka cells, absolute values and values relative to basal ATPase being higher in HK possum than in quokka cells.

• 3.

  3. These results suggest that the low erythrocyte potassium concentrations in LK possums may be due to the absence of (Na⁺-K⁺)-activated ATPase and pump activity in their cell membranes.