Search PubMed⌕ Search

Biomedical subjects

K Shikama

Publications and source records attributed to K Shikama.

51 records · Page 3Linked to original sources
PubMed1981

Autoxidation of oxymyoglobin. A nucleophilic displacement mechanism.

In the presence of a salt in an excess amount, the oxidation of native MbO2 was enhanced considerably above the normal autoxidation in buffr alone with the formation of the corresponding MetMb . anion complex. This anion-induced oxidation of MbO2 was measured for each salt at some 30 different values of pH in 0.1 M buffer at 25 degrees C. The anions examined were SCN-, F-, OCN-, N3- and CN-. The resulting pH dependence shows that the reaction involves two types of the displacement processes of O2- from MbO2 by the anion, i.e. those with and without proton assistance. The Brønsted plots for the rate constants versus the pKa values of the conjugate acids of anions indicate that the displacing oxidation of MbO2 proceeds by way of a nucleophilic attack of anions on the iron center and that both H2O and OH- can react with native MbO2 as the most common nucleophiles in vivo. These findings lead to a view that the proton-catalyzed nucleophilic displacement of O2- from MbO2 by an entering water molecule, or SN2 mechanism with proton assistance, is the basis for most of the autoxidation reaction under normal conditions.

Animals↗
PubMed1980

Autoxidation of native oxymyoglobin. Thermodynamic analysis of the pH profile.

A complete kinetic description has been made on the pH profile for the autoxidation rate in terms of displacement of superoxide anion, O2- from MbO2 by the entering water molecule or hydroxyl ion. Using the equation, the effect of temperature on the autoxidation rate has been studied over the pH range 4.8-12.6 in 0.1 M buffer at 15 degrees, 25 degrees and 35 degrees C. The resulting thermodynamic parameters characterize the dissociation groups involved in the reaction as histidyl and tyrosyl residues. Despite the fact that each elementary process of the reaction is primarily protected against autoxidation by the high energy barrier of approximately 85-150 kJ . mol-1, the catalytic proton participates not only in decreasing the value of delta H degrees not equal to but also in increasing the value of delta S degrees not equal to to facilitate the formation of the activated complex, thereby promoting most of the autoxidation reaction of MbO2. The proton-catalyzed process is therefore of primary importance and a mechanistic detail of the reaction is discussed.

Animals↗
PubMed1978

Autoxidation of native oxymyoglobin. Kinetic analysis of the pH profile.

The rate of autoxidation of native oxymyoglobin to metmyoglobin has been examined over the pH range of 4.8--12.6 in 0.1 M buffer at 25 degrees C, and some 40 values of the observed first-order rate constant, kobs, are plotted against pH of the solution. In order to understand the kobs--pH profile thus obtained, some mechanistic models are proposed for the autoxidation reaction. The fitting of their rate equations as a function of pH has been examined to the experimental kobs-pH plot by a least-squares method with the use of a digital computer. The complicated pH-profile can be best explained by the 'acid-base catalyzed three states model', which reveals not only the catalytic role of hydrogen ions and hydroxyl ions, but also the involvement of two dissociation groups of myoglobin molecule in the autoxidation reaction.

Animals↗
PubMed1976

Equilibrium studies on the formaldehyde reaction with native DNA.

Equilibrium analysis of the reaction of formaldehyde with native calf thymus DNA was carried out at temperatures below the thermal transition zone by the spectrophotometric method. The apparent equilibrium constant, Kconf., for the conformational opening and closing reaction of base pairs along the double-helical chain, was measured in various concentrations of formaldehyde, and these values were extrapolated to the zero concentration. The value of KOconf. thus obtained in the absence of the chemical probe was 0.12 in 5 mM 2-(N-morpholino)ethane sulfonic acid buffer (Mes), and 0.003 in 0.15 M NaCl plus 5 mM Mes, pH 7.0, at 50 degrees C. These results make it possible to calculate roughly that 230 base pairs and 7 base pairs are open respectively, at 50 degrees C in the native DNA molecule composed of 2300 base pairs, using Mr = 1.5 x 10(6) for the sample used. This conformational reaction was also characterized by the following thermodynamic parameters: deltaGOconf. = 1.36 kcal - mol-1 (5.68 kJ - mol-1), deltaHOconf. = 36.8 kcal - mol-1 (154 kJ - mol-1), and deltaSOconf. = 110 cal - mol-1 - K-1 (460 J - mol-1 - K-1) in 5 mM Mes, pH 7.0, at 50 degrees C, and the nature of the 'breathing' of base pairs was discussed.

Animals↗
PubMed1976

Generation of the superoxide radical during autoxidation of oxymyoglobin.

Autoxidation of bovine oxymyoglobin to metmyoglobin induces co-oxidation of epinephrine to adrenochrome. This co-oxidation is markedly inhibited by superoxide dismutase [EC 1.15.1.1]. Electron transfer from oxymyoglobin to ferricytochrome c is partially inhibited by superoxide dismutase. These results indicate that autoxidation of oxymyoglobin results in generation of superoxide radicals. Autoxidation of oxymyoglobin is accelerated by superoxide dismutase and partially inhibited by catalase [EC 1.11.1.6].

Adrenochrome↗