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

• 1.

  1. The concentration of choline acetylase (acetyl-CoA: choline O-acetyltransferase, EC 2.3.1.6) in the nerve fibers of the walking leg of the lobster has been determined. In sensory bundles, dissected from the whole nerve trunk, the activity of the enzyme was found to be on the average 2 μm/g wet wt./h; the values of the whole nerve' trunk were similar.

• 2.

  2. The sensory bundle is formed by approx. 800 axons, each about 5 μ in diameter, estimated on the basis of photomicrographs; the total surface area per g nerve is therefore approx. 400 cm². Referred to cm³/sec the enzyme activity amounts to 1.5·10⁻⁶ μm of acetylcholine formed.

• 3.

  3. Recent information on the concentration of acetylcholinesterase (acetylcholine acetyl-hydrolase, EC 3.1.1.7) in lobster nerves per unit time and surface area permits an estimate of the amount of acetylcholine which may be hydrolyzed per cm². On this basis the amount of acetylcholine formed in the sensory axons would be adequate for more than 400 000 impulses/h.

• 4.

  4. The objection was raised to the proposal role of acetylcholine in conduction, that the concentration of choline acetylase in sensory fibers is too low to be compatible with the theory. This difficulty has now been eliminated.

The thermal denaturation of the nucleic acid is considered as a first-order phase change, the transition temperature of which is determined only by the ionic strength of the solvent and the free energy. An equation is derived which relates the transition temperature to ΔG_E, the differene in electrostatic free energy between the native and denatured nucleic acids. The adoption of cylindrical models for both native and denatured nucleic acid enables ΔG_E to be estimated at various ionic strengths. Application of the theory then yields estimates for the enthalpy and entropy of denaturation and the free energy of the stabilising forces in the helix.

• 1.

  1. The synthesis of a new fluorescent reagent (4-acetamido,4′-isothiocyanostilbene-2,2′-disulphonic acid) for the specific labelling of the outer components of the plasma membrane is described.

• 2.

  2. The reaction of the reagent with ox erythrocytes is examined. It is concluded that the reagent reacts with a fixed number of sites on the surface of the cell.

• 3.

  3. The problems of designing reagents of this type, and the nature of the reaction between the stilbene isothiocyanate and erythrocytes are discussed.

The light-induced cytochrome reactions and dark steady state changes were studied in photosynthetic bacteria, Chromatium and Rhodospirillum rubrum. The effects of external substrates, reducing agents, inhibitors, O₂ tension etc. on the kinetics and the steady state changes of the cytochrome reactions are presented.

The pattern of electron transfer in the bacterial systems changed markedly under different conditions. O₃ was one of the key factors to control the patterns. In general, apparently “non-cyclic” cytochrome response was observed under the growing conditions; on the other hand, ‘cyclic” or “closed-chain” type characteristics were observed under non-growing aerobic conditions. Kinetic analysis indicated that heptylhydroxyquinoline-N-oxide had an interaction with the reaction of cytochrome c and cytochrome b as had been postulated from difference spectra. Cross-over of the photochemical oxidation and photochemical reduction was observed in the presence of the inhibitor. It was indicated that ascorbate and trichlorophenolidophenol (added singly or combined) did not compete effectively with the internal photochemical reducing system and did not change the steady state of cytochromes. Under certain conditions, however, ascorbate plus trichlorophenolindophenol shifted the steady state of cytochrome b. The formation of an electron-transfer by-pass by N-methylphenazonium methosulfate was confirmed.

Direct interaction of bacteriochlorophyll and cytochrome molecules was postulated in the heptylhydroxyquinoline-N-oxide-insensitive rapid “light-off” reaction or aerobic cells, as well as in the rapid phase of the light-induced oxidation of cytochrome. The possibility of the control of electron transfer by concentration of adenosine phosphates was also discussed.

Fresh chloroplasts treated with appropriate amounts of digitonin give light-induced absorption changes in the Soret-band and far-red regions. The absorption changes are enhanced by ascorbate and eliminated by ferricyanide. The light-minus-dark difference spectrum of digitonin-treated chloroplasts agrees closely with that of the pigment complex P 700 reported by Kok. Dichlorophenolindophenol does not appear to mediate in the electron transfer between ascorbate and the photooxidized chlorophyll complex, whereas phenazine methosulfate, depending on its concentration, greately modifies the kinetics of the re-duction of the pigment complex by ascorbate.

Fluorescence spectra were determined at temperatures between 20° and −196° for a number of photosynthetic organisms. Below −90° the single fluorescence maximum around 685 mμ was replaced by a system of three bands, at 686, 696 and 717–720 mμ in algal cells. Cooling usually resulted in a decrease of the 685-mμ band. In young cultures of blue-green and red algae the three bands were of about equal intensity; in old cultures and in green algae the 717-mμ band was dominant, while in the latter the 696-mμ bans was weak.

In green leaves and chloroplast also, three bands were present at low temperatures, at 686, 696 and 735–740 mμ. Here too, the 740-mμ band was by far the major one.

During cooling of both diluted and concentrated chlorophyll a solutions and chlorphyll adsorbed to filter paper, the height of the 677-mμ fluorescence band and the 730-mμ vibrational level were increased by a factor of about two, provided no increased reabsorption due to increased scattering could occur. In concentrated chlorophyll a solutions no extra bands could be detected.

The three fluorescence bands measured invivo at low temperatures are assumed to belong to three chlorophyll a forms: C_a 670-F 686; C_a 695-F 717 in algal cells. Apart from an increase in intrinsic fluorescence yield of C_a 695, the marked increase in 717-mμ fluorescence during cooling is suggested to be due to increased energy transfer from C_a 670 and C_a 680 to C_a 695 as a result of shrinkage, when the temperature is lowered.

Transient absorption changes at 430 mμ are induced in aged chloroplasts by red light flashes, The absorption change occurs in 10⁻⁴ sec or less and has a half life of approx. 10⁻² sec. The enhancement of the reaction by ascorbate and the abolishment by ferricyanide suggest that an oxidation reaction is responsible for the absorption change. In the presence of 3-(3,4-dichlorophenyl)-I,I-dimethylurea the absorption change is not observed, but the signal can be restored completely by adding ascrobate.

Available evidence from the decay kinetics of the 430-mμ transient absorption changes in the presence of ascorbate suggests that dichlorophenolindophenol at all concentrations reacts with cytochrome while phenazine methosulfate at low concentrations reacts with cytochrome and at concentrations greater than 3.10⁻⁵ M with P700 only. This in accord with the difference spectra observed in the blue region (where also cytochrome changes are observed) and also with the concominant and similarly decaying absorption changes at 430 and 703 mμ.

Light-intensity dependency of the complex reaction in aged chloroplast containing ascorbate and trace amount of phenazine methosulfate showed that at low intensity only the oxidation of the chlorophyll complex takes place. The coupling reaction between the pigment complex and cytochrome takes place only at higher light intensities.