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

The kinetics of the influx of PO₄³⁻ into erythrocytes reveal a first-order process, with a half-time of 140 min at 37°.

The exchange measured for short times of incubation (0–300 sec) revealed that the incorporation of ³²P begins immediately and increases linearly. Inorganic phosphate is the precursor of phosphate in organic fractions: the highest initial specific activity was constantly found in this inorganic fraction. Incorporation of labelled phosphate into ATP and ADP is not so rapid and phosphate esters exchange very slowly.

The flux through the membrane remains proportional to the extracellular concentration and is not affected by a ten-fold increase above physiological conditions. This lack of saturation effect and null value for the affinity constant, excludes any participation of an active site located in the membrane.

The kinetics of permeability are not modified by competitive anions (SO₄²⁻, AsO₄³⁻). The AsO₄³⁻ ions decreased the uptake slowly; however, this effect is an indirect consequence of the inhibition of glycolysis.

The above results are in accordance with a process of simple diffusion.

• 1.

  1. Everted sacs of rat jejunum, ileum, and colon were used to investigate the possibility of active F⁻ transport. Fluoride was placed in equal concentration in both compartments at the initial levels of 0.083, 0.167, and 1.670 mM. A secretion of F⁻ against its electrochemical gradient was observed across ileum at 0.167 mM F⁻ and either 1.9 or 1.0 mM Ca²⁺, and across jejunum and ileum at 0.167 and 1.670 mM F⁻ in the presence of 0.5 mM Ca²⁺.

• 2.

  2. When 0.167 mM F⁻ was placed initially on either the mucosal or the serosal| side, at 1.9 mM Ca²⁺, fluoride crossed each section of the intestine freely in both the absorptive and secretory directions. The translocation of F⁻, however, was greater in the direction of secretion with jejunal and ileal sacs.

• 3.

  3. With equal initial concentration of F⁻ in both compartments, alteration of| the exogenous Ca²⁺ concentration and also the addition of 2,4-dinitrophenol affected only the net movement of F⁻ in the mucosal compartment. Since the loss of F⁻ from the serosal compartment was not influenced, it appeared that this effect was localized at the mucosal surface.

• 4.

  4. The net movement of F⁻ against its electrochemical gradient was inhibited| by the addition of 1 mM dinitrophenol at 0° confirming the existence of active F⁻ transport in jejunum and ileum. A less active F⁻ pump also appeared to exist in the colon.

The effects of the denaturing agents, urea and guanidine·HCl, and detergents, sodium dodecyl sulphate and hexadecyltrimethylammonium bromide (HTAB), on the optical rotatory dispersion of fetuin have been investigated. It was concluded that fetuin has a high affinity for the cationic detergent HTAB which seems to bind, at one type of binding site only, as demonstrated by both an optical rotatory and Optical density study. The same detergent also increased by the parameters λ_c and $\text{b}{}_{\mathrm{<mtext>o</mtext>}}$, determined from Yang-Doty plots and the Moffitt equation, respectively. The conformation-dependent Cotton trough also shows an increase in amplitude and in addition undergoes a shift to higher wavelengths upon addition of HTAB. Although these observations suggest an increase in helical structure, caution was exercised in interpreting the results in this way since optical artifacts (e.g., higher refractive indexforprotein-micelle complex) may contribute. Modification of the fetuin molecule by removal of sialic acid or cleavage of the disulfide bonds results in a reduction in helical content of the molecule but does not alter the affinity of the protein for HTAB. The effect of HTAB on the various fetuin preparations was opposed by guanidine·HCl but not by urea; this was attributed to a charge effect by the former ions. The red shifts of the Cotton curve and the absorption spectra, upon the addition of HTAB to fetuin solutions are gradual and seem to be related, and it is tempting to attribute both phenomena to possible micelle formation in which tyrosine chromophores are also included. Such protein-micellar complexes would have a higher refractive index than the bulk solvent, and this in itself would cause a red shift of the $\text{π → π}{}^{\mathrm{*}}$ absorption band of the helix.

On the basis of solvent perturbation and spectrophotometric studies, it was concluded that fetuin contains no buried tyrosine groups. A structural similarity of both bovine albumin and fetuin was proposed as a result of the appearance in both cases of a second Cotton trough in the far-ultraviolet region after cleaving the disulfide bonds, indicative in both cases of a more random conformation.

• 1.

  1. The conformation in solution of eye-lens proteins obtained from cortical extracts of adult bovine lenses, was studied by means of the optical rotatory dispersion method.

• 2.

  2. The rotatory data were analysed according to the modified two-term Drude equation, the Moffitt-Yang equation and the one-term Drude equation.

• 3.

  3. The lens proteins appeared to consist of a mixture of α-helix, random coil and some other structure, which disappeared upon denaturation by acid, alkali or 8 M urea, and also in 2-chloroethanol.

• 4.

  4. From a comparison with other proteins it seems probable that this other structure has a β conformation.

• 1.

  1. In Lemma gibba L. the reversible increase in the activity of NADP⁺-dependent glyceraldehyde-3-phosphate dehydrogenase by illumination is inhibited not only by chloramphenicol (cf. ref. 1) but also by amino acid analogues like $\text{dl}\text{-p-}\text{fluorophenyl-alanine}$ (10⁻², 10⁻³ M) and dl-ethionine (10⁻² M). This block is removed to a great extent by equimolar concentrations of the corresponding amino acids (phenyl-alanine or methionine).

• 2.

  2. In Lemma the same concentrations of the inhibitory substances strongly reduce the photosynthetic O₂ production. The inhibition by the amino acid analogues is more pronounced at the commencement of the light phase and is then diminished, probably under the influence of the photosynthetically produced amino acids. This reduction of inhibition is also brought about by the addition of the corresponding amino acids via the medium.

• 3.

  3. The possibilities for a relationship between the intensity of the photosynthesis and the increase in activity of the enzyme in the light are discussed, emphasizing a feed-back control of the photosynthetic capacity by light.

The influence of certain intermediary metabolites of the aldehyde and benzoquinone type on the formation of collagen fibrils from a solution of an acid-soluble collagen (at 30° and pH 7.2) has been studied. Turbidimetric measurements made it possible to differentiate the nucleation process from the growth process during fibril formation with a reasonable degree of certainty. The increase in intermolecular cross-linkages results in a quicker and quantitatively increased formation of nucleation centers which determine the final characteristic of the fibrils. Among the aldehydes studied glutaraldehyde and acrolein were found to enhance the nucleation process most markedly, while formaldehyde and glyceraldehyde were less active. Of the benzoquinone derivatives homogentisic acid was highly effective, and gentisic acid was less effective. It can be concluded that these effective cross-linking agents are also involved in the formation of cross-linkages in connective tissues in vivo.

• 1.

  1. A simple model mechanism for photosynthetic CO₂ fixation which incorporates the autocatalytic features of the carbon cycle and a mechanism for bleeding off the intermediates has been postulated to account for the variation in and high values of the activation energy (Q₁₀) of CO₂ fixation.

• 2.

  2. The steady-state rate equation for the model is derived and its properties discussed. It is shown that the model can account for the effects of temperature on the rate of CO₂ fixation using reasonable values for the activation energies and heats of reaction of the kinetic constants.

• 3.

  3. The proposed type of mechanism is compatible with the lag period observed after rewarming whole cells, the lack of a marked effect of temperature on the lag period of isolated chloroplasts and the transient oscillations in the levels of intermediates observed after alteration of the CO₂ concentration.

• 4.

  4. The importance of the reaction in which intermediates are bled off as a control mechanism in systems involving stoichiometric amplification is considered.

• 1.

  1. Optical rotatory dispersion and light-scattering measurements have been used for an investigation of the protein-denaturing ability of the water-soluble carcinogens, dimethylnitrosamine, diethylnitrosamine, ethylcarbamate, and dioxane.

• 2.

  2. It is found that each of these carcinogens produces a helix-coil transition in ovalbumin, as revealed by the change in b₀ of the Moffitt-Yang equation. The reduced, non-carcinogenic derivative of dimethylnitrosamine, dimethylhydrazine, is unable to produce this unfolding in the ovalbumin molecule. The helix content of ovalbumin in dioxane measured as a function of time indicates an unfolding and partial refolding of the protein.

• 3.

  3. Calculations of molecular weight from the light-scattering measurements show the formation of aggregates consisting of 2–9 ovalbumin molecules, for the range of denaturant concentrations used here. The magnitude of the time-dependent refolding of ovalbumin in dioxane appears to be limited by the aggregation process, since the helix content of ovalbumin in concentrated dioxane solution is not as high as is generally observed for other proteins and polypeptides in this solvent.

• 4.

  4. Alteration of hydrophobic bonding alone in ovalbumin cannot account for the denaturing ability of these carcinogens, rather the results support the proposal of hydrogen bonding between the agent and functional groups of the protein.