Biochimica et Biophysica Acta (BBA) - Enzymology and Biological Oxidation最新文献

• 1.

  1. Evidence is presented showing that the burst in ATP synthesis, occuring on addition of ADp to rat-liver mitochondria supplemented with β-hydroxybutyrate and phosphate, is due to the oxidation of endogenous NADH and not to a “discharg” of high-energy intermediates of oxidative phosphorylation.

• 2.

  2. The ATP burst is about 3 times the amount of NAD⁺ formed on the addition of ADP.

• 3.

  3. Accompanying the ATP burst and the oxidation of endogenous NADH there is an extra oxygen uptake equivalent to the oxidation of the NADH.

• 4.

  4. The ATP burst was abolished by 15 μM dinitrophenol.

• 5.

  5. With succinate as substrate, the ATP burst equalled the amount of NADH oxidized, indicating that reducing equivalents from NADH compete for the respiration chain with those coming from succinate.

• 1.

  1. The nature of the active site of the butyrycholinesterase (EC 3.1.1.8) of human blood serum was compared with that of the acetylcholinesterase (EC 3.1.1.7) of Torpedo marmorata electric organ using a series of carbinol acetates of pyridine and N-methylpyridine. The activities of the two cholinesterases were affected in opposite directions by quaternization of the pyridine N atom, as well as by changing the position of the carbinol substituent in the ring.

• 2.

  2. The effects of certain pyridine derivatives on the enzymatic activity differed, particularly as regards the effect of pyridyl-2- and 3-carbinols, by which acetylcholinesterase was activated and butyrylcholinesterase inhibited.

• 3.

  3. A comparison was made between the rates of hydrolysis of pyridyl-2,6-dicarbinol diacetate and its N-methyl derivative by butyrylcholinesterase.

• 4.

  4. The pH dependence of the enzymatic activity of the two esterases revealed that the charge on the pyridine N atom played a much more important role in complex formation between the enzymes and the compounds studied in the case of acetylcholinesterase than for butyrylcholinesterase.

• 5.

  5. The results presented support the view that butyrylcholinesterase contains a second “non-esteratic” site which differs from the anionic site of acetylcholinesterase, and constitutes the main difference between the two cholinesterases. The dominant type of force involved in reactions with the second site of butyrylcholinesterase are Van der Waals forces, in contrast to the Coulombic attractions which favour complex formation between the anionic site of acetylcholinesterase and substrates and inhibitors of the onium type. In addition, some evidence is presented suggesting that the esteratic sites of the two esterases also differ.

A highly active and specific Na⁺, K⁺-dependent ATPase was obtained from calf-heart muscle by succesive treatments with deoxycholate and NaI.

General properties of the enzyme were studied. Simultaneous addition of Na⁺ and K⁺ increased the activity about 20 times above the basic Mg²⁺-dependent level. The activation was completely reversed by 10⁻⁴ M ouabain. The optimal pH was 7.2; the K_m for ATP for the Na⁺,K⁺-dependent and Mg²⁺-dependent enzyme activities were 2.4·10⁻⁴ M and 5.0·10⁻⁵ M, respectively. Only CTP could be substituted for ATP, but the activity was 14% of that found with ATP.

Azide and histone, which affected the crude ATPase system, had no effect on the purified enzyme. p-Chloromercuribenzoate, N-ethylmaleimide, oligomycin, tributyltin chloride, octylguanidine as well as ouabain were found to inhibit the Na⁺, K⁺-dependent component while having little or no effect on the basic Mg²⁺-dependent activity. The K_i's were 5·10⁻⁶ M, 6·10⁻⁴ M, 7·10⁻⁶ M, 1.5·10⁻⁵ M, 3·10⁻⁴ M and 10⁻⁶ M, respectively.

To investigate what the function might be of the Zn-protoporphyrin present in yeast cells, the course of the O₂-induced biosynthesis of respiratory enzymes was studied in yeast harvested from anaerobic culture on synthetic medium, either with Zn²⁺ added at a concentration which stimulates the growth ((+) Zn yeast) or without added Zn²⁺ ((−) Zn yeast).

Zn²⁺ has an inhibitory effect of the respiratory adaptation. In (−) Zn yeast, the adaptive development of respiratory enzymes is much faster; there is an increase of cytochrome content, O₂ consumption, succinate: cytochrome c reductase and NADH₂: cytochrome c reductase activities and a decrease of fermentation.

Different hypotheses are presented to explain the inhibitory effect of Zn²⁺ on the synthesis of respiratory enzymes.

• 1.

  1. Besides the prosthetic groups, disulfide, sulfhydryl and basic groups in the protein component of catalase are active centers for the reaction with H₂O₂. The disulfide and sulfhydryl groups behave as a redox system acting upon H₂O₂. The basic groups interact with the prosthetic groups. This influences the binding strength of the peroxide, which is combined with the Fe⁺ of the prosthetic group. It is discussed which groups in the protein component could be responsible for the heme-protein interaction.

• 2.

  2. The actual activity of catalase may be derived from the measured activity-pH relationship by elimination of the five intermediate equilibria. These are: the dissociation of peroxide, the dissociation of catalase hydroxide, the dissociation of catalase peroxide, the pH dependence of the redox potential and the pH dependence of latent sulfhydryl groups. The actual catalase activity is pH dependent because of heme-protein interaction.

• 3.

  3. With the help of these results the fundamental mechanism of the catalatic reaction may be explained. The oxidation of peroxide to oxygen by the disulfide bridges of catalase is the rate-determining process, the peroxide being combined with the Fe of the heme group. The reaction velocity depends upon the size of the catalase redox potential and the strength of the heme-protein interaction. Values for the magnitude of these quantities are presented

It has been found that oxidized d-amino-acid oxidase (d-amino-acid: oxygen oxidoreductase (deaminating), EC 1.4.3.3) is transformed to a semiquinone by irradiation with visible light. The semiquinone formation was detected by measurements of the optical absorption spectrum and electron spin resonance. A fairly stable semiquinone was obtained under anaerobic conditions but not under aerobic conditions since it easily reacted with molecular oxygen. After a cycle of semiquinone formation and reoxidation, the d-amino-acid oxidase activity was found to be completely restored. Although no evidence was obtained about an electron donor for the semiquinone formation, it did not appear likely, from the measurement of the activation energy of 5 kcal per mole, that water donated electrons. The electron spin resonance signal of the semiquinone exhibited an asymmetrical shape. Possible interpretations for the phenomena are discussed.

A phosphorylase a (EC 2.4.1.1)-glutamic-pyruvic transaminase (EC 2.6.1.2) enzyme complex was purified by DEAE-cellulose and Sephadex gel chromatography. Substrate inhibition studies indicated that the substrates of one enzyme induced conformational changes on the other enzyme in the complex. Iodoacetate alkylation and methylene blue photooxidative destruction as well as substrate protection against these protein modifications provided additional evidence for this. The allosteric effector AMP induces changes in the protein structure of phosphorylase a, which in turn affected the transaminase activity of glutamic-pyruvic transaminase in the enzyme complex.

The AMP effect on the transaminase activity was abolished by raising the temperature of the system or by the addition of urea.

It is believed that the substrate or AMP induced changes in enzymic activity are a result of protein-protein interactions manifesting themselves in a conformational change of phosphorylase a which is communicated to the transaminase enzyme in the complex.

The action of rabbit cathepsins D and E on the B chain of beef insulin has been studied.

Cathepsin D hydrolyses the bonds Leu₁₁-Val₁₂, Glu₁₃-Ala₁₄, Ala₁₄-Leu₁₅, Leu₁₅-Tyr₁₆, Tyr₁₆-Leu₁₇, Leu₁₇-Val₁₈, Phe₂₄-Phe₂₅, Phe₂₅-Tyr₂₆, Tyr₂₆-Thr₂₇. Furthermore, it hydrolyses one of the bonds His₅-Leu₆ or Leu₆-Cys₇, or both.

Cathepsin E hydrolyses the bonds Leu₁₁-Val₁₂, Glu₁₃-Val₁₄, Leu₁₅-Tyr₁₆, Tyr₁₆-Leu₁₇, Phe₂₄-Phe₂₅, Phe₂₅-Tyr₂₆.