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

An oxidase, which catalyzes mainly the deamination of thyroxine, 3,3′,5-triiodothyronine and 3,5-diiodothyronine, has been partially purified from rat-kidney mitochondrial extracts by successive (NH₄)₂SO₄ fractionation and diethylaminoethyl-cellulose-column treatment. The enzyme preparation was contaminated with catalase (H₂O₂: H₂O₂ oxidoreductase, EC 1.11.1.6) in relatively high concentration, but contained no thyroid-hormone transaminase. The enzyme preparation had no effect on either diiodotyrosine or tyrosine, leucine and lactic acid were well oxidized with the formation of keto acids.

The elution pattern of the partially purified oxidase on diethylaminoethyl-cellulose column indicates that the enzyme oxidizing thyroid hormone is identical to the mammalian l-amino acid oxidase (l-amino acid: O₂ oxidoreductase, EC 1.4.3.2) which catalyzes chiefly the deamination of leucine and the dehydrogenation of lactic acid. Furthermore, the prosthetic group of the purified enzyme oxidizing thyroid hormone was identified as riboflavin 5′-phosphate, which is well known as the prosthetic group of the mammalian l-amino acid oxidase.

• 1.

  1. An enzyme capable of hydrolysing the dipeptide l-cysteinyl-l-tyrosine has been purified 60-fold from bovine-thyroid tissue by means of acetone fractionation, precipitation with zinc acetate and chromatography on calcium phosphate and diethylaminoethyl-cellulose.

• 2.

  2. The purified “cysteinyltyrosinase” (l-cysteinyl-l-tyrosine hydrolase) was found to hydrolyse a wide range of peptides but was substantially free from proteinase activity as tested against haemoglobin.

• 3.

  3. Preliminary studies with cysteinyltyrosine, leucyltyrosine and leucylglycine suggest that only one enzyme is involved in the hydrolysis of these peptides.

• 4.

  4. Enzymic activity can be inhibited by ethylenediamine tetraacetate and restored by the addition of Zn²⁺ and to a lesser extent by Mn²⁺.

• 5.

  5. No evidence was found for the hydrolysis of thyroglobulin by cysteinyltyrosinase leading to the formation of iodinated amino acids, although an enzyme preparation showing high proteinase and low cysteinyltyrosinase activities did effect such release.

Effects of changes in the structures of flavin phosphates were studied to assess the coenzyme specificity of nicotinamide-adenine dinucleotide phosphate cytochrome c reductase (NADPH: cytochrome c oxidoreductase, EC 1.6.2.3) from brewer's yeast.

• 1.

  1. Among analogues of riboflavin 5′-phosphate which are symmetrically substituted in positions 6 and 7 of the isoalloxazine ring system, diethylriboflavin 5′-phosphate is as active as riboflavin 5′-phosphate, but decreasing activities are seen with the dibromo-, dichloro-, and diiodo-analogues.

• 2.

  2. With analogues which are substituted in position 6 of the ring system, satisfactory activities of the 5′-phosphates of methyl- and, somewhat less, methyl-pyridinoriboflavin are observed; however carboxy and ethoxy substituents markedly diminish activity.

• 3.

  3. Activities of 6,7-dimethylflavin phosphates bearing different side chains are progressively decreased with the shorter chains of the glycityl series of d-ribityl, d-erythrityl, and dl-glyceryl, respectively. Similarly with the alkyl chains, 2′, 3′, 4′-trideoxyribityl is better than 2′-deoxyglyceryl.

• 4.

  4. Secondary hydroxyl groups on the chain confer greater activity, but are not obligatory as seen by the activities of 2′,3′,4′-trideoxyriboflavin 5′-phosphate and 2′-deoxyglyceroflavin 3′-phosphate. A reduction in activity is also seen when the 2′-hydroxy function is missing, as in 2′-deoxyriboflavin 5′-phosphate, or in l-configuration, as in d-araboflavin 5′-phosphate.

• 1.

  1. Acetic thiokinase (acetate: CoA ligase (AMP), EC 6.2.1.1) has been partially purified from yeast and the effects of Mg²⁺ and univalent cations on the overall reaction catalyzed by the enzyme and the individual steps of the reaction have been investigated. In addition of Mg²⁺ the dialyzed enzyme required K⁺, NH₄⁺ or Rb⁺ for the catalysis of the overall reaction. The concentrations of univalent cations required for optimum activity were in the range from 0.01 to 0.05 M. The addition of Cs⁺ to reaction mixtures resulted in an intermediate stimulation of activity whereas Na⁺ or Li⁺ were relatively ineffective.

• 2.

  2. It has been confirmed that the first step of the overall acetic thiokinase reaction, assayed by determining the rate of synthesis of ATP from adenyl acetate and Tris pyrophosphate, requires Mg²⁺. This reaction does not require univalent cations. In contrast, the second step of the overall reaction in which acetyl-CoA and AMP are synthesized from adenyl acetate and CoA, requires univalent cations but is inhibited by Mg²⁺ at a concentration of 0.01 M.

• 3.

  3. In most experiments it has not been possible to dialyze the yeast acetic thiokinase sufficiently to show an absolute univalent cation requirement for either the overall reaction or the step in which acetyl-CoA and AMP are synthesized from adenyl acetate and CoA. Dialyzed enzyme extracts generally contain from 6·10⁻³ to 8·10⁻³ M K⁺ which is sufficient to account for the activity in the absence of added univalent cations.

• 4.

  4. The addition of KCl to the dialyzed acetic thiokinase greatly influences the maximum velocity of the reaction in which acetyl-CoA and AMP are formed from adenyl acetate and CoA. The addition of KCl to reactions, however, fails to measurably affect the K_m values for either adenyl acetate or CoA. It is concluded that KCl influences either the rate of breakdown of an enzyme-adenyl acetate-CoA complex or the number of active enzyme sites involved in the synthesis of acetyl-CoA and AMP from adenyl acetate and CoA.

The energy-dependent reduction of NAD⁺ by succinate catalyzed by submitochondrial particles derived from beef heart is demonstrated to be supported energetically not only by the addition of ATP but also by a high-energy intermediate generated by the flow of electrons through the cytochrome oxidase portion of the respiratory chain. As substrate for the energy generation is used ascorbate and tetramethyl-p-phenylenediamine. Phenazinemethosulfate but not cytochrome c can replace the phenylenediamine. The reduction does not require the addition of Mg²⁺, phosphate or nucleotides, it is not inhibited by EDTA or oligomycin and by arsenate only in the absence of oligomycin, when the generated high-energy intermediate is the energy source. Rotenone is inhibitory whereas antimycin A is an obligatory addition to avoid reoxidation of the formed NADH.

• 1.

  1. In the cells of Pseudomonas aeruginosa, the cytochrome oxidase studied by Yamanakaet al. (Pseudomonas cytochroom C-551 : nitrite O₂ oxidoreductase) is only induced in the presence of KNO₃. This enzyme is probably an important factor in the utilization of nitrates by this organism.

• 2.

  2. A second cytochrome oxidase (cytochrome c: O₂ oxidoreductase, EC 1.9.3.1) is present in these cells. This enzyme is characterized by a strong absorption at 590 mμ and oxidizes the reduced cytochrome c of mammalian heart.

• 3.

  3. When the cells are growing on glucose this enzyme appears in very small amounts. Apart from the fact that glucose induces a third type of respiratory system, which has not been determined but which is characterized by its insensibility to 10⁻⁴ M cyanide, glucose is a repressor of the formation of this enzyme.

Carbamate kinase (ATP:carbamate phosphotransferase, EC 2.7.2.2) has been prepared from an arginine-adapted strain of Streptococcus faecalis. Approximately a 500-fold increase in specific activity of the enzyme was achieved. Certain properties of the purified enzyme were studied.

• 1.

  1. The apparent molecular weight as measured by sedimentation equilibrium appears to be about 46 000.

• 2.

  2. The pH optimum for the formation of carbamyl phosphate from ATP and ammonium carbamate is 8.4.

• 3.

  3. Studies with p-chloromercuribenzoate and silver-tris(hydroxymethyl)aminomethane indicate that sulfhydryl groups are essential for enzyme activity.

• 4.

  4. Michaelis constants estimated for adenosine 5′-triphosphate (in the formation of carbamyl phosphate) and for adenosine 5′-diphosphate in the reverse reaction, carbamyl phosphate degradation, were about the same, 1·10⁻³ M. No such determinations were possible for carbamyl phosphate or ammonium carbonate; when these substrates were studied under comparable conditions non-linear relationships were obtained for Lineweaver-Burk plots.

All treatments which increased liver glucose-6-phosphatase (d-glucose-6-phosphate phosphohydrolase, EC 3.1.3.9) activity also caused an increase in the glucose-6-phosphatase activity of the kidney cortex. Adrenalectomy and hypophysectomy both caused marked decreases in liver and kidney-cortex glucose-6-phosphatase activities. Three treatments increased liver glutaminase (l-glutamine amidohydrolase, EC 3.5.1.2) activity; however, only one of these, a high-protein diet, increased kidney glutaminase activity. Fasting decreased both total available liver glutaminase and kidney glutaminase activities. The removal of the adrenals or pituitary caused large decreases in glutaminase activity in both the liven and cidney. It appears that factors which affected the activity of liver glucose-6-phosphatase activity had a similar effect on kidney-cortex activity. The correlations were more difficult with the glutaminase activity since alterations of glutaminase activity were not noted with as many treatments as affected glucose-6-phosphatase activity.