Pulse radiolytic investigation of single heme group reduction in hum an methemoglobin

Radiolysis of myoglobin concentrated gels by protons: specific changes in secondary structure and production of carbon monoxide

While particle therapy has been used for decades for cancer treatment, there is still a lack of information on the molecular mechanisms of biomolecules radiolysis by accelerated ions. Here, we examine the effects of accelerated protons on highly concentrated native myoglobin, by means of Fourier transform infrared and UV-Visible spectroscopies. Upon irradiation, the secondary structure of the protein is drastically modified, from mostly alpha helices conformation to mostly beta elements at highest fluence. These changes are accompanied by significant production of carbon monoxide, which was shown to come from heme degradation under irradiation. The radiolytic yields of formation of denatured protein, carbon monoxide, and of heme degradation were determined, and found very close to each other: G+denatured Mb ≈ G+CO ≈ G-heme = 1.6 × 10-8 ± 0.1 × 10-8 mol/J = 0.16 ± 0.01 species/100 eV. The denaturation of the protein to a beta structure and the production of carbon monoxide under ion irradiation are phenomena that may play an important role in the biological effects of ionizing radiation.

Intra- and intermolecular oxidation of oxymyoglobin and oxyhemoglobin induced by hydroxyl and carbonate radicals

The mechanism of the reactions of myoglobin and hemoglobin with *OH and CO3*- in the presence of oxygen was studied using pulse and gamma-radiolysis. Unlike *NO2, which adds to the porphyrin iron, *OH and CO3*- form globin radicals. These secondary radicals oxidize the Fe(II) center through both intra- and intermolecular processes. The intermolecular pathway was further demonstrated when BSA radicals derived from *OH or CO3*- oxidized oxyhemoglobin and oxymyoglobin to their respective ferric states. The oxidation yields obtained by pulse radiolysis were lower compared to gamma-radiolysis, where the contribution of radical-radical reactions is negligible. Full oxidation yields by *OH-derived globin radicals could be achieved only at relatively high concentrations of the heme protein mainly via an intermolecular pathway. It is suggested that CO3*- reaction with the protein yields Tyr and/or Trp-derived phenoxyl radicals, which solely oxidize the porphyrin iron under gamma-radiolysis conditions. The *OH particularly adds to aromatic residues, which can undergo elimination of H2O forming the phenoxyl radical, and/or react rapidly with O2 yielding peroxyl radicals. The peroxyl radical can oxidize a neighboring porphyrin iron and/or give rise to superoxide, which neither oxidize nor reduce the porphyrin iron. The potential physiological implications of this chemistry are that hemoglobin and myoglobin, being present at relatively high concentrations, can detoxify highly oxidizing radicals yielding the respective ferric states, which are not toxic.

Temperature effects on malonyl-CoA inhibition of carnitine palmitoyltransferase I

Malonyl-CoA inhibition of hepatic mitochondrial carnitine palmitoyltransferase I and malonyl-CoA binding were measured at temperatures ranging from 0 degrees C to 37 degrees C. Protease treatment of mitochondria resulted in greatly diminished malonyl-CoA binding, indicating that the method used detected malonyl-CoA binding sites located on the outer surface of the mitochondrial outer membrane as expected. The apparent Ki for malonyl-CoA inhibition was found to increase with increasing temperature. Arrhenius plots for the initial velocity of the enzymatic reaction and for the Ki for malonyl-CoA both indicated a transition temperature between 20 and 25 degrees C with the transition for the malonyl-CoA interaction being more pronounced. Total specific binding of malonyl-CoA to mitochondrial proteins increased with increasing temperature, and Kd values decreased. The opposite effect of temperature on Kd values and Ki values was surprising because it was expected that these equilibrium constants would be identical. These observations indicate that Kd values for malonyl-CoA binding and Ki values for inhibition of carnitine palmitoyltransferase I by malonyl-CoA represent two significantly different binding phenomena. These data suggest that either: (a) malonyl-CoA binding measurements are unrelated to malonyl-CoA inhibition, or (b) inhibition of carnitine palmitoyltransferase I by malonyl-CoA involves more complex relationships than binding of malonyl-CoA to a single protein.

The interaction of alcohol radicals with human hemoglobin. I. Spectral properties of hemoglobin in the visible range

Aqueous deoxyhemoglobin solutions (2 mg/ml) were gamma-irradiated by a 60Co source in the presence of methanol, ethanol, 1-butanol and t-butanol under N2O or argon. The effects of the interaction of the particular alcohol radical species with hemoglobin were determined according to the detected spectral alterations in the visible range. The amounts of stable final products in the form of methemoglobin (MetHb) and the sum of hemichromes and cholehemichromes (Hemichr) were estimated in irradiated preparations. For preparations irradiated under N2O, the radiation yield for MetHb formation was three-fold lower in the presence of ethanol and 1-butanol [G(MetHb) = 0.33] compared with preparations irradiated in the presence of t-butanol or without alcohol [G(MetHb) = 1.00]. The yield of hemichromes and cholehemichromes in preparations irradiated under N2O increased in the order: ethanol (G = 0.38), 1-butanol (G = 0.52), t-butanol (G = 0.59), and in the absence of alcohol (G = 0.72). The high effectivity of t-butanol radicals for iron oxidation and Hb destruction is apparently due to their oxidative properties, compared with the other radicals. It was also shown that ethanol radicals reduce MetHb 10 times more effectively [G(Fe(II) = 2.5] compared with t-butanol radicals [G(Fe(II)) = 0.24]. For samples irradiated under argon all the observed changes were similar, regardless of the presence of alcohols. This effect can be attributed to reconstruction reactions of Hb molecules in the presence of both oxidizing (OH or t-but.) and reducing agents (e-aq). The following sequence of effectivities of water radiolysis products and secondary alcohol radicals for hemoglobin destruction has been identified: meth; eth.-->1-but.-->e-aq-->t-but.-->.OH.

The reduction of flavocytochrome b2 by carboxylate radicals. A pulse radiolysis study

The reaction of flavocytochrome b2 with carboxylate radicals has been studied by the pulse-radiolysis technique. The absorbance difference spectra observed 2, 10 and 90 mus after the pulse were very similar to the (reduced-minus-oxidized) difference spectrum of the cytochrome b2 core, a deflavoderivative prepared from flavocytochrome b2. The heme b2 group was reduced in a bimolecular reaction with a rate constant of (2.1 +/- 0.2) X 10(8) M-1 X s-1. Simulation data allowed us to assign this reduction mainly to a direct reaction of CO-2 on heme b2 without flavin involvement. However, this heme b2 reduction was accelerated in a flavocytochrome b2 sample of low molar activity. This observation seemed to reflect modifications of the heme b2 environment, in particular a closer contact with the solvent. Moreover, in such samples the flavin became reducible to the semiquinone state as deduced from absorbance difference spectra in the 440-490 nm region. The agreement between experimental and computed time-courses allowed to estimate the reaction rate of bound flavin to be equal to 2 X 10(8) M-1 X s-1 in this studied sample. Thus the reactivity of bound heme b2 and flavin within flavocytochrome b2 with CO-2 seems to be sensitive to the physical alterations of the polypeptide chain.

Chain inequivalence in bovine methemoglobin

Using pulse radiolysis, a single heme in the tetramer of bovine methemoglobin was reduced within a few microseconds to the ferro state, producing a valence intermediate. The kinetics of oxygen binding to the valence intermediate as well as the re-oxidation of the ferro-heme to the ferric state were studied as a function of pH. The kinetics of the oxygenation revealed the existence of two species, characterized by high and low affinities for oxygen that are associated with two quaternary structures (R and T, respectively). A sigmoidal curve representing a transition between the two states as a function of pH was derived. Above pH 7.7 only the R state could be observed, while below pH 6.5 the T state was dominant. The reaction between the valence intermediate and ferricyanide at pH 7.75 (R state) consisted of two (about) equal contributions (k1 = 23 x 10(4) M-1 S-1; k2 = 2.1 x 10(4) M-1 S-1) attributed to the beta and alpha subunits within the tetramer, respectively. At pH 6.3 (T state) a similar phenomenon was observed (k1 = 69 x 10(4) M-1 S-1; k2 = 3.7 x 10(4) M-1 S-1), indicating chain inequivalences both in the T and the R states of methemoglobin. In the presence of inositol hexakisphosphate the T leads to R transition, as monitored by oxygenation of the valence intermediate, was shifted up to a higher pH by about 0.35. Yet similar rate constants exhibiting similar chain inequivalences have been measured.

Applications of pulse radiolysis to protein chemistry

Since its introduction, pulse radiolysis has been an important technique for examining the properties of organic and inorganic radicals, and for enumerating those reactions responsible for cellular damage by ionizing radiation. Biochemists, and biophysicists outside the area of radiation biology appear, perhaps for historical reasons, to have an incomplete appreciation of the technique's potential. Protein chemists in particular, have been only dimly aware of the numerous reports of, and the significant results obtained from pulse radiolysis studies of proteins. Our purpose here is to bring some of these results together in order to emphasize the power and usefulness of pulse radiolysis experiments both for elucidating enzyme reaction mechanisms, and for gaining information on the structure of proteins in aqueous solutions. Reviews containing related, or in part the same material to be covered here have appeared previously; for example, Land (1970), Adamset al.(1972a), Shafferman & Stein (1975), Adams & Wardman (1977). This review updates these earlier works, but more importantly approaches the topic of protein pulse radiolysis with a different emphasis.

Transient intermediates in the reduction of Fe(III) myoglobin-ligand complexes by electrons at low temperature

1. The reductions of a number of sperm-whale Fe(III) myoglobin-ligand complexes by electrons generated by gamma-irradiation in ethylene glycol/water glass, have been investigated by using low-temperature spectrophotometry. The ligands are azide, fluoride, imidazole and water. 2. The reduction of the Fe(III) myoglobin-ligand complexes at 77 K leads to the formation of low-spin liganded Fe(II) myoglobin, in the case of the azide, imidazole and water derivatives, while the reduction of the fluoride derivative proceeds both by a pathway involving prior dissociation of the ligand and with the ligand in position. 3. Investigation of the effect of temperature on the stability of the Fe(II) myoglobin-ligand complexes indicates that more than one bound states exists in dissociation of the ligand molecule from the ferrous heme iron of the reduced azide and imidazole derivatives. 4. The results are discussed in terms of the possible structure of the Fe(II) myoglobin complexes and it is suggested that the low-spin state is created by a strained configuration of the heme center with the iron atom in an intermediate position relative to the heme plane.

The spin-state transition of the hemochrome non-equilibrium conformation in partially reduced human methemoglobin. A pulse-radiolysis study of aqueous-methanol solutions of methemoglobin

The effect of external parameters on the relaxation process of the hemochrome-type non-equilibrium conformation in partially reduced methemoglobin has been investigated. The relaxation of the intermediate ferrous low-spin state to the high-spin equilibrium conformation of hemoglobin appears to be facilitated particularly by protons and phosphate ions. In addition to studying the spin-state transition in aquomethemoglobin we have also studied it in complexes of the heme group in methemoglobin with fluoride, azide and cyanide anions.

Kinetics of carbon monoxide binding to fully and partially reduced human hemoglobin valency hybrids

The kinetics of carbon monoxide binding following fast reduction of the valency hybrids alpha2+betaCO2 and alphaCO2beta+2 by hydrated electrons have been studied at different degrees of reduction. The results show that at pH 6.0 and 7.0 reduction of one heme group yields a species which reacts fast with carbon monoxide (rate constant of the order of 10(6) M-1S-1). At pH 6.0 the intermediates alphaCO2beta2 and alpha2betaCO2 bind carbon monoxide with a rate characteristic of the T state. At pH 7.0 alphaCO2beta2 is for the greater part in the T state, while in the case of alpha2betaCO2 the R and the T state are about equally populated.

Heterogeneity in the kinetics of oxygen binding to partially reduced human methemoglobin. A pulse-radiolysis study of oxygenated solutions of methemoglobin

The pulse-radiolysis technique has been introduced because it permits a rapid reduction (in a few microseconds) of one heme group of the methemoglobin tetramer by hydrated electrons. The kinetics of the binding of oxygen to this particular valence intermediate (Hb3+) with one reduced alpha or beta subunit has been studied. It appears that the hydrated electrons preferentially reduce one type of subunit of methemoglobin at acid and neutral pH-values as is shown by the biphasic behaviour of Hb3+ on oxygenation. The second-order on-rate constants measured for the binding of oxygen to Hb3+ are 14 +/- 3 mM-1 ms-1 and 56 +/- 9 mM-1 ms-1, respectively. The relative contribution of the faster fraction is about 0.63 +/- 0.08 of the total oxygenation process. A comparison of the kinetic absorbance difference spectrum for the reduction of methemoglobin with the static difference spectrum of deoxyhemoglobin and methemoglobin in the Soret-region revealed a decreased absorbance of the unliganded subunit of Hb3+ at 430 nm. This fact suggests that Hb3+ is in the relaxed quaternary conformation, which is in agreement with the observed on-rate constants.

One electron reduction of metmyoglobin and methemoglobin and the reaction of the reduced molecule with oxygen

We have used the pulse radiolysis technique to reduce with solvated electrons (see article) a single Fe(III) site in methemoglobin and metmyoglobin. The reduction process was followed spectrophotometrically and the reactions rate constants were measured: (see article) =6.5 +/- 1-10(10) M-1-S-1. (see article)=2.5 +/- 0.3-10(10) M-1-S-1. Approx. 60% of the (see article) have reacted with the hemin group, and the rest of the (see article) have probably reacted with the globin moiety. We followed the reaction of the reduced proteins to yield the oxyderivatives and measured the rate constants of the oxygenation process k reduced methemoglobin + O2 = 2.6 +/- 0.6-10(7) M-1-S-1 and k myoglobin + O2 = 1.8 +/- 0.2-10(7) M-1-S-1, all the rate constants were measured at pH = 6.8, I = 0.004, T = 22 +/- 2 degrees C. The high rate constant for reduced methemoglobin indicates that one-site-reduced methemoglobin is probably in the R state, as predicted for methemoglobin from X-ray analysis. The spectra of the reduced and oxygenated species were measured under similar conditions at gamma = 450-650 nm. We were able to follow slight changes in the micro-second time scale, these changes were attributed to conformational changes. We were not able to detect any reaction between the radical (see article) and the hemin group (which would result in a complex such as heme O-2). This may be due to kinetic reasons.