Biochimica et Biophysica Acta (BBA) - Specialized Section on Biophysical Subjects最新文献

The polymerization of actin was followed by the measurement of electric birefringence. Small elementary polymers of actin which appeared at very low salt concentrations or at the initial stage of polymerization showed a positive electric birefringence. They were oriented with the long axis parallel to the electric field. This was mainly due to a permanent dipole moment in the direction of the long axis, the magnitude of which was estimated from the Kerr constant to be about 75 Debye units per actin monomer. The elementary polymers grew into normal F-actin which showed a large negative electric birefringence. That is, F-actin filaments were oriented in the direction perpendicular to the field. Therefore, the electric structure of F-actin filaments was completely different from that of elementary small polymers. The negative electric birefringence of F-actin was cancelled by the addition of H-meromyosin.

The fluorescence emitted by slices of rat liver and sea-gull salt gland which were irradiated with light at 366 mμ, showed a peak at 468 mμ under aerobic conditions. The intensity of the fluorescence decreased during the early stages of incubation of 38°, due largely to the effects of the ultraviolet exciting radiation. Under aeobic conditions of incubation the fluorescence intensity fell by 40–60, and the remained relatively constant for a period of several hours; in anaerobic conditions the fall in intensity continued to lower levels. During the period in which the fluorescence of aerobic slices constant, quantitatively reproducible increases in tissue fluorescence were caused by the onset of tempory anoxia, by the addition of succinate to the medium and by the treatments which stimulated ion transport in the slices.

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  1. Wild-type Neurospora crassa, growing logarithmically in minimal medium, contains 180 ± 16 mM intracellular K⁺ and 14 ± 2 mM Na⁺. It requires K⁺ but not Na⁺ for growth. Over a wide range of extracellular K⁺ concentrations (0.3 to 100 mM), the cells maintain a constant, high level of intracellular K⁺ (180 mM) and rapid growth occurs. Below 0.3 mM, the internal K⁺ concentration falls and growth is inhibited.

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  2. When cells are harvested from logarithmic growth and resuspended in distilled water or in 1 mM sodium azide, they show no significant loss of internal K⁺ for at least 1 h. Exposure to the polyene antibiotic nystatin (30 μg/ml) or the decapeptide antibiotic tyrocidine (20 μg/ml) causes most of the internal K⁺ to be lost within 2 min. Both antibiotics are thought to attack the plasma membrane of sensitive organisms.

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  3. Neurospora exchanges its intracellular K⁺ quite rapidly for extracellular K⁺ or Rb⁺, but only slowly for Na⁺.

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  4. These experiments were undertaken to provide a backgroun for microelectrode studies with Neurospora and for the isolation of a K⁺-transport mutant.

$\text{2,3,4,6-}\text{tetra}\text{[O-}\text{acetyl}\text{]-β-}\text{d}\text{-}\text{glucopyranose}$ penetrated into human erythrocytes following a transport mechanism different from that of glucose. However, TAG interacted with the glucose-carrier ssytem, inhibiting the transport of glucose to a variable extent, depending on experimental conditions.

Comparison of the inhibitory effect of TAG on exchange and non-exchange transport showed that it affected a suppression of glucose transport similar to that obtained by lowering the temperature.

The presence of subtances in the matrix of the membrane which regulate the movement of carriers, interacting similarly to TAG, is tentatively suggested.

The ability of everted sacs of rat small intestine to transport neutral, dibasic and N-methyl-substituted amino acids has been determined. The data obtained support the suggestion that three separate amino acid-transport systems are present in rat intestine.

A concerted analysis of sedimentation, light scattering and small-angle X-ray-scattering data on high-molecular-weight Ehrlich ascites tumor cell RNA has been carried out. The three sets of data have been found to be in quantitative agreement with a compact structure consisting of short double-helical roldlike subunits, which account for 90% of the RNA mass.

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  1. Irradition of urocanic acid or urocanate frozen in aqueous solution with 280–320-mμ light produced a photodimer which was isolated by gradient elution from an anion-exchange resin.

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  2. A similar substance was shown to be present in dilute acid extracts of guinea-pig epidermis after irradiation with 280–320-mμ light.

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  3. The photodimer formed with 280–320-mμ light when irradiated in solution with 254-mμ light was converted to urocanic acid and smaller amounts of several other products.

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  4. Irradiation of crystalline urocanic acid or its solutions does not produce an accumulation of the photodimer.