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

Spectrophotometric measurements, using the rapid-mixing technique, have been made of the initial rate of egress of oxygen from HbO₂ in sheep erythrocytes after mixing with dithionite. This rate is $\text{18}\text{sec}{}^{\mathrm{-1}}\text{± 20\%}$ at 25° and the activation energy is $\text{14}\text{kcal}\text{± 20\%}$ for HbB. No variation was found with a change of oxygen concentration from 0.075–0.35 mM at 10°, nor with a change of Na₂S₂O₄ concentration from 4–12 g/l. The rate of dissociation was found to be greater, during the later part of the reaction, with the stopped-flow system as compared with the constant-flow technique. An analysis of the results suggests that the cell membrane has negligible resistance to the egress of oxygen and that the initial rate is determined by the chemical reaction of $\text{HbO}{}_2\text{→}\text{Hb}$. It is also suggested that the internal contents of the erythrocyte may be effectively mixed.

Electron spin resonance spectra showed that radicals produced in Escherichia coli B/r and Micrococcus radiodurans by exposure to γ-rays at −195° undergo a series of changes on warming. Cysteamine modifies these reactions in a characteristic manner, but the full effect is only seen if the bacteria are ruptured by mechanical methods prior to the addition of cysteamine. With intact M. radiodurans cysteamine has almost no effect on the ESR spectra seen after irradiation, while with E. coli B/r cysteamine already has some influence before irradiation possibly because the cells are damaged to some extent by freeze-drying. The data are interpreted as showing that cysteamine does not penetrate into the interior of non-metabolizing bacteria, yet it protects against the lethal action of ionizing radiations under these conditions. Physicochemical experiments had shown earlier that main-chain scission of DNA extracted from irradiated bacteria was not protected by cysteamine. The hypothesis is advanced that cysteamine protects a metabolic repair system of DNA which is situated at or near the cell membrane, and in this way makes the restitution of DNA lesions more efficient in irradiated bacteria.

The effect of glucose, succinate, sorbitol, glycerol and arabinose on the rate of uptake of α-[¹⁴C]methylglucoside by Escherichia coli K12 was studied in comparison with the effect of these substances on the rate of α-methylglucoside exit from cells preloaded with the radioactive compound.

Glucose was a very potent inhibitor of uptake and greatly accelerated exit, its effect on the former reaction exceeding four-fold that on the latter. Each of the other compounds tested was much less active than glucose and affected both uptake and exit of methylglucoside to a similar extent.

The inhibition of uptake and the acceleration of exit by glucose were not affected by sodium azide. On the other hand, azide practically abolished the effect of succinate and glycerol, both on the uptake and exit reactions and reduced to 50% the effect of sorbitol.

These results provide further support for the conclusion reached by Hoffeeet al. that α-methylglucoside is transported by a glucose permease and that the inhibitory effect of metabolizable compounds on its uptake is due to energy supply for the exit reaction. Another possible mechanism for acceleration of exit in the presence of hexitols and sugars is also discussed.

• 1.

  1. Plasma membranes have been isolated from rat liver and the ATPase (EC 3.6.1.4), (Na⁺-K⁺)-ATPase and 5′-nucleotidase (EC 3.1.3.5; substrate: AMP) activities of the preparations have been studied.

• 2.

  2. The pH-, Mg²⁺-, temperature- and concentration-dependences and the nucleotide specificity of the ATPase and nucleotidase were established.

• 3.

  3. The ATPase and (Na⁺-K⁺)-ATPase of the isolated plasma membranes hydrolyzed the terminal phosphate bond of ATP. The following effects on the two enzymes were obtained: Ca²⁺, ouabain, cysteine, p-chloromercuribenzoate, Myleran and oligomycin inhibited the (Na⁺-K⁺)-ATPase while having no effect on the ATPase. Ultrasound, oleic acid, vitamin A alcohol and deoxycholate inhibited both ATPases. Bile, deoxycholate (at low concentration), Lubrol-W, deoxycorticosterone acetate and progesterone inhibited ATPase and activated the (Na⁺-K⁺)-ATPase. Activation of the ATPase and inhibition of the (Na⁺-K⁺)-ATPase could be obtained by saponin and preincubation of the membranes at 37°.

• 4.

  4. None of the treatments inhibited the nucleotidase; ultrasound, deoxycholate, saponin and Lubrol-W activated the enzyme.

• 5.

  5. The results are discussed in regard to the relation between membrane structure and enzymic function.

• 1.

  1. The amount of extractable, photoconvertible, protein-chlorophyll complex CP 668 from Chenopodium album leaves was found to be very variable, depending on site and day of leaf collection.

• 2.

  2. Photoconversion of CP 668 into a 740-nm absorbing form probably does not occur in intact leaves; CP 668 in its extracted form is concluded to be an artifact.

• 3.

  3. CP 668 may be separated from other “soluble”, non-photoconvertible material absorbing at about 668 nm by precipitation with polyethyleneglycol

• 4.

  4. CP 740, the phototransformed form of CP 668, does not show any clear absorption band in ethanol. It is suggested that the 740-nm band arises from interaction of photo-oxidized chlorophyll with protein.

• 5.

  5. Photoconversion of CP 668 into CP 740 is promoted by lowering the pH from about 8 to 6.

• 6.

  6. Photoconversion of CP 668 is inhibited by β-mercaptoethanol and KCN; it is slightly inhibited in Tris buffer as compared with phosphate buffer.

• 7.

  7. In discussing the experiments it is suggested that CP 668 is a denatured form of a native chlorophyll-protein complex containing essential SH groups.

A new continuous-source, monochromatic ultraviolet optical system designed for use in the Spinco Model-E ultracentrifuge was constructed and its properties described. Using this system, the flotation density of native and denatured DNA from various T-bacteriophages in Cs₂SO₄ was determined. It was found that all the denatured DNA's examined gave rise to two density bands, except for T6 DNA which possibly had three bands, in Cs₂SO₄ but not in CsCl. The relevance of these findings to the data of others was discussed.