Catalase depression in malignant liver from chickens with myeloblastosis and Marek's disease.

In rapidly frozen livers from chickens affected with myeloblastosis and Marek's disease and from unaffected control birds there exists a strong correlation between catalase activity and catalase Electron Paramagnetic Resonance (EPR) signal intensities. The diseased chickens had activities and signals reduced to as little as 10% of control values. There were no changes in the EPR parameters in diseased liver and the data support the hypothesis that the lowering in activity is due to lowered catalase levels rather than to catalase inhibition. The rate of transformation of catalase to catalase-formate in liver was studied by freeze-clamping liver in anaesthetised chickens, then warming to 37 degrees for 1 or 2 minutes anaerobiosis, and then refreezing. The only difference of significance in this transformation between diseased and normal livers was the greater percentage of total catalase present as catalase-formate (approximately + 15%) in aerobic diseased liver, which may indicate a lowered production of hydrogen peroxide, relative to formate, in these livers. The rate of transformation was far faster in chickens (t1/2 less than 1 min) than in the rat (t1/2 = 7.7 min).

The role of the membrane-bound iron-sulphur centres A and B in the photosystem I reaction centre of spinach chloroplasts.

Photosystem I particles prepared from spinach chloroplast using Triton X-100 were frozen in the dark with the bound iron-sulphur Centre A reduced. Illumination at cryogenic temperatures of such samples demonstrated the photoreduction of the second bound iron-sulphur Centre B. Due to electron spin-electron spin interaction between these two bound iron-sulphur centres, it was not possible to quantify amounts of Centre B relative to the other components of the Photosystem I reaction centre by simulating the line-shape of its EPR spectrum. However, by deleting the free radical signal I from the EPR spectra of reduced Centre A alone or both Centres A plus B reduced, it was possible to double integrate these spectra to demonstrate that Centre B is present in the Photosystem I reaction centre in amounts comparable to those of Centre A and thus also signal I (P-700) and X. Oxidation-reduction potential titrations confirmed that Centre A had Em congruent to -550 mV, Centre B had Em congruent to -585 mV. These results, and those presented for the photoreduction of Centre B, place Centre B before Centre A in the sequence of electron transport in Photosystem I particles at cryogenic temperatures. When both A and B are reduced, P-700 photooxidation is reversible at low temperature and coupled to the reduction of the component X. The change from irreversible to reversible P-700 photooxidation and the photoreduction of X showed the same potential dependence as the reduction of Centre B with Em congruent to -585 mV, substantiating the identification of X as the primary electron acceptor of Photosystem I.

Quantitative electron-paramagnetic-resonance measurements of the electron-transfer components of the photosystem-I reaction centre.

E.p.r. spectrometry was used to investigate the quantitative relationships between the oxidized chlorophyll free-radical signal I and the reduced iron-sulphur centre-A signal generated on illuminating Photosystem-I particles at cryogenic temperatures. In Photosystem-I particles prepared by using the French press or Triton X-100, at pH8.0 in the presence and absence of ascorbate and at pH 10.0 in the presence of ascorbate, the size of the light-induced signal I and iron-sulphur centre-A signals, corresponded to equal numbers of unpaired electron spins in each component. At 77K the spin-lattice relaxation time, T1, of the free radical signal I in samples of Photosystem-I particles prepared with Triton X-100 in the absence of ascorbate was 0.68 times the T1 value in the presence of ascorbate. Such changes in relaxation time can account for the different quantitative conclusions incorrectly arrived at from measurements made at saturating microwave powers [Bearden & Malkin (1976) Biochem. Biophys. Acta 430, 538-547; Malkin & Bearden (1976) FEBS Lett. 69, 216-220]. In the presence of benzoquinone and ferricyanide the ratio of free radical to centre A was 2.96:1, and at 77K the T1 was 0.50 times the T1 for ascorbate-treated samples. Here free radicals from bulk chlorophyll are generated in addition to those from the reaction-centre chlorophyll.

Quantitative electron-paramagnetic-resonance measurements of the electron-transfer components of the photosystem-I reaction centre. The reaction-centre chlorophyll (P700), the primary electron acceptor X and bound iron-sulphur centre A.

An e.p.r. spectrum of the reduced form of the electron-transport component (X), thought to be the primary electron acceptor of Photosystem I, was obtained. By using line-shape simulations of this component and the free-radical e.p.r. signal I of the oxidized reaction-centre chlorophyll (P700), it was possible to determine the ratio of the number of electron spins to which these signals correspond in Photosystem-I particles under a variety of conditions. On illumination at cryogenic temperatures of Photosystem-I preparations, in which both bound iron-sulphur centres A and B were reduced, the measured ratio of free radical to component X varied between 1.04 and 2.23, with an average value of 1.54 +/- 0.18 where a Gaussian line-shape is assumed for the component-X signal in the simulation. The error in this measurement is estimated to be up to 50%. In a similar way component X and centre A of the bound iron-sulphur protein were quantified, the ratio between these two components varying between 1.26 and 0.61 with an average value of 0.75 +/- 0.06. These results indicate that the quantitative relationship, in terms of net electron spins, between centre A, component X and P700 is of the order to be expected if component X is indeed the primary electron acceptor in Photosystem I and a component of the photosynthetic electron-transport chain.

Changes in apparent pH on freezing aqueous buffer solutions and their relevance to biochemical electron-paramagnetic-resonance spectroscopy.

Changes in apparent pH occurring during fast freezing of aqueous buffer solutions and cooling to -196 degrees C were studied by various semiquantitative methods, including simple visual measurements of colour changes with pH indicators, as well as measurements of pH-dependent changes in the e.p.r. (electron paramagnetic resonance) spectra of solutions of three different metalloenzymes. It is concluded that apparent pH changes of up to about 3pH units may occur under particular conditions. Such changes were independent of the time taken to freeze the samples, when this was varied from about 3ms t0 20s, but were affected by the presence of some proteins in solution. Recommendations on the buffers that should be used to avoid such apparent pH changes in e.p.r. spectroscopy and other low-temperature biochemical work are made. Phosphate and pyrophosphate buffers, which gave large decreases (2-3 pH units), and Tris, which under some conditions gave increases of about the same magnitude, are to be avoided. Certain zwitterionic buffers such as Bicine [NN-bis-(2-hydroxyethyl)glycine] are satisfactory. Apparent pH effects were found to depend on buffer and protein concentration. It is therefore recommended that as a prelude to future detailed low-temperature biochemical work, appropriate tests with an indicator system should be performed.

Electron paramagnetic resonance spectra of mitochondrial and microsomal cytochrome P-450 from the rat adrenal.

The electron paramagnetic resonance (EPR) spectra of rat adrenal zona fasciculate mitochondria showed peaks corresponding to low spin ferric cytochrome P-450 with apparent g values of 2.424, 2.248 and 1.917, and weak signals due to high spin ferric cytochrome P-450 with gx values of 8.08 and 7.80. The former is attributed to cholesterol side chain cleavage cytochrome P-450, the latter to 11beta-hydroxylase cytochrome P-450. On addition of deoxycorticosterone the g = 7.80 signal was elevated and there was an associated drop in the low spinal signal. As the pH was reduced from 7.4 to 6.1, the g = 8.08 signal increased with again a drop in intensity of the low spin signal. Mitochondria from the zona glomerulosa showed similar spectral properties to those described above. Addition of succinate, isocitrate or pregnenolone caused a loss of the g = 8.08 signal. Addition of calcium increased the magnitude of the g = 8.08 signal, and caused a slight reduction in the magnitude of the low spin signal. Also, addition of deoxycorticosterone, pregnenolone, succinate or isocitrate caused slight shifts of the outer lines of the low spin spectrum. Interaction of mitochondrial cytochrome P-450 with metyrapone and aminoglutethimide modified the low spinal parameters. Adrenal microsomal cytochrome P-450 had low spin ferric g values of 2.417, 2.244 and 1.919 and a high spin ferric gxy values of 7.90 and 3.85, distinct from the values obtained with mitochondria.

Electron paramagnetic resonance studies of cytochrome P-450 and adrenal ferredoxin in single whole rat adrenal glands. Effect of corticotropin.

Low and high spin ferric cytochrome P-450 and reduced adrenal ferredoxin (adrenodoxin) have been directly studied by EPR techniques in whole rat adrenal glands. The spectra obtained correspond closely to those obtained from sub-cellular fractions except in the case of low spin ferric cytochrome P-450, where there are differences in the shape of the g = 2.41 line. The relative magnitudes of these peaks in anaerobic and aerobic rapidly frozen adrenals from control and corticotropin stimulated hypophysectomised rats were used to investigate the control and rate limiting steps in adrenal steroid biosynthesis via cytochrome P-450. All adrenals showed a close to maximal level of reduced adrenodoxin and aerobic and anaerobic glands from control rats and aerobic glands from corticotropin stimulated rats showed similar quantities of low spin ferric cytochrome P-450. On anaerobiosis the quantity of low spin ferric cytochrome in adrenals from corticotropin stimulated rats dropped to 30--40% of the aerobic level. Treatment of the rats with cycloheximide prior to administration of corticotropin prevented these changes. Approximately 0.4% of the total cytochrome P-450 was high spin ferric in control adrenals and in aerobic stimulated adrenals this rose to approximately to 0.6%. These results demonstrate that association of substrate with cytochrome P-450 is the rate limiting step in adrenal steroidogenesis via cytochrome P-450. It is suggested on the basis of these and mitochondrial optical and EPR experiments that the limiting step being observed is cholesterol binding to cholesterol side chain cleavage cytochrome P-450, and that the rate of this association is stimulated by corticotropin.

Effect of calcium(ion) uptake by rat adrenal mitochondria on pregnenolone formation and spectral properties of cytochrome P-450.

The effect of calcium on pregnenolone formation from endogenous precursors has been studied in mitochondria from rat decapsulated and capsular adrenal glands. In the presence of succinate, addition of calcium chloride in the concentration range 20-150 muM caused an inhibition of pregnenolone formation in both decapsulated and capsular adrenal mitochondria. 11beta-hydroxylation of added deoxycosticosterone in decapsulated adrenal mitochondria was also inhibited. Under these conditions, calcium inhibited the reduction of adrenodoxin, a component of the cytochrome P-450 reductase system, presumably because uptake of calcium by the mitochondria competes with energy-linked transhydrogenase for high-energy intermediates. For this reason, incubations were carried out in the presence of succinate plus isocitrate plus NADP+. Under these conditions, calcium chloride in the concentration range 120-875 muM caused a 2-4-fold stimulation of pregnenolone formation, but had no effect on corticosterone formation from added deoxycorticosterone. The effect of calcium on the optical spectra of cytochrome P-450 has also been examined in mitochondria from decapsulated and capsular rat adrenals. In the presence of succinate, calcium induced a spectral change resembling a type I difference spectrum of cytochrome P-450. Thus it appears that uptake of calcium by adrenal mitochondria can stimulate pregnenolone formation by increasing the interaction of mitochondrial cytochrome P-450 with endogenous substrate.

Induced changes in the electron paramagnetic resonance spectra of mammalian catalases.

The EPR spectra of bovine liver catalase, rat liver catalase and human erythrocyte catalase have been measured at 9.0 degrees K. In N-2-hydroxyethylpiperazine-N'-2-ethanesulphonic acid (HEPES) and Tris buffers at pH 7.0, the liver catalases show EPR spectra typical of rhombically distorted high spin ferric heme with major lines at g = 6.50, 5.35, 1.98. A number of extra lines are also seen; these are weak or absent in human erythrocyte catalase. The effect of the addition of formate, nitrite, acetate, fluoride, azide, hypophosphite and of inactivation with 3-amino-1,2,4-triazole on the degree of rhombic distortion has been studied. There is a good correlation between the low temperature EPR and room temperature optical changes for the binding of formic acid in HEPES and Tris. There is no evidence from EPR spectra for the presence of heme-heme interactions in the binding of formic acid to human erythrocyte catalase. The properties of catalase are altered in phosphate and in distilled water. This is a consequence of the low temperature of measurement.

Electron paramagnetic resonance spectra of catalase in mammalian tissues.

The relatively small number of paramagnetic species and the high concentration of catalase in mammalian liver and blood make it possible to directly study this enzyme in frozen whole tissue. The EPR spectra of catalase are dependent on the heme environment and in human blood only catalase A, gxy = 6.48, 5.36 is observed whereas in liver a second spectrum, catalase B, gxy = 6.80, 5.07 can also be seen. Using rapid freeze techniques it has been shown that in rat liver catalase A corresponds to the in vivo steady state and that after death this is largely converted into catalase B. Data from the perfusion of rat livers with oxygenated and deoxygenated blood and dextran solutions together with results from in vitro studies of catalase are interpreted as indicating that catalase B results from the interaction of catalase with an organic acid, most probably formic acid, that the acid is a peroxidative substrate for catalase in vivo and that peroxidation of the acid is not the major role for catalase in rat liver. Catalase binding with other small molecules in intact liver has been demonstrated by perfusion with nitrite-containing dextrans and by intraperitoneal injection of 3-amino-1,2,4-triazole.