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R P Mason

Publications and source records attributed to R P Mason.

10 recordsLinked to original sources
PubMed1979

Oxygen-sensitive and -insensitive nitroreduction by Escherichia coli and rat hepatic microsomes.

Nitrofurazone is shown to undergo an initial 1-electron (oxygen-sensitive) or 2- or more electron (oxygen-insensitive) reduction by partially purified nitroreductases from Escherichia coli. Nitrofurazone (50 micronM) is reduced by the oxygen-sensitive reductase to a nitro anion free radical as indicated by ESR and visible spectroscopy. The visible spectrum of the nitro anion free radical is characterized by an increase in absorption at 406 nm. In the presence of the oxygen-sensitive reductase, nitrofurazone stimulates superoxide formation and oxygen consumption. This enzyme gives a steady state radical concentration which is proportional to the square root of the enzyme concentration, suggesting that the nitrofurazone anion radical is an obligate intermediate in the reduction and that the radical decays by a nonenzymatic second order process. The oxygen-insensitive reductase does not form the nitro anion free radical nor in the presence of nitrofurazone does it stimulate oxygen consumption. Visible spectroscopy shows that nitrofurazone is reduced by the oxygen-sensitive reductase to a species with an absorption maximum at 335 nm, which has been previously identified as the amine. The oxygen-insensitive reductase reduces nitrofurazone to a previously identified cyano derivative with an absorption maximum at 280 nm. Rat hepatic microsomes appear to metabolize nitrofurazone in a manner similar to the oxygen-sensitive E. coli reductase.

Aerobiosis↗
PubMed1978

Spin-label and deuterium order parameter discrepancies in bilayers: one possible explanation.

We have simulated electron spin resonance spectra of anisotropically immobilized spin labels of the type seen in lipid and soap-like bilayers using a rigorous formalism which explicitly includes the effects of spin-label motion. In most bilayer systems, spin-label experiments have shown lower order parameters then deuterium-label experiments. In the past this apparent decrease in the order parameters was thought to reflect the distortion of the bilayer by the doxyl ring of the spin probes. We wish to report that this type of discrepancy may be due to the neglect of important motional effects in the time-independent effective Hamiltonian formalisms used in previous interpretations of anisotropically immobilized spin label spectra. That the true order parameters may be the same can be shown by including slow motional corrections in the effective Hamiltonian formalism. The larger volume of the doxyl ring may change the apparent order parameter by increasing the importance of the slow motional effects, as opposed to causing a real decrease in the order parameter, as previously proposed.

Chemical Phenomena↗
PubMed1977

Complement-induced decrease in membrane mobility: introducing a more sensitive index of spin-label motion.

We have used spin-labeling to investigate complement-induced changes in lipid organization of antibody-sensitized sheep erythrocyte membranes. The spectrum of methyl 5-doxylstearate incorporated into the lipid component of sheep erythrocyte membranes is typical of a membrane bilayer. The membranes from complement-lysed erythrocytes have a small, but statistically significant, reduction in fluidity when compared to membranes from osmotically-lysed erythrocytes, as indicated by a small increase in T'. In theory, measurements of the widths of the outer hyperfine extrema should be more sensitive to motion than the separation of the outer hyperfine extrema (2T'). Our results indicate that the half-width at half-height of the outer hyperfine extrema show a severalfold greater percentage change than T'. The sign and magnitude of these changes are in general agreement with previous predictions. Our results imply that motional corrections to the S formalism of Hubbell, Gaffney, and McConnell are necessary because spin-label motion appears to be explicitly represented in this type of electron spin resonance spectra.

Animals↗
PubMed1975

The mechanism of microsomal and mitochondrial nitroreductase. Electron spin resonance evidence for nitroaromatic free radical intermediates.

Electron spin resonance spectra are observed during the enzymatic reduction of many nitrophenyl derivatives by rat hepatic microsomes or mitochondria. The spectra indicate that nitroaromatic anion radicals are present and are freely rotating in aqueous solution at a steady-state concentration of 0.1-6 muM. The rate of formation of p-nitrobenzoate (NBZO) dianion radical in microsomal incubates is consistent with the radical being an obligate intermediate in the reduction of NBZO to p-aminobenzoic acid. A model system consisting of NBZO, NADPH, and FMN, but no heme-containing compounds, also reduced NBZO to the NBZO dianion free radical. The steady-state concentration of the anion radicals in microsomal systems is not altered by CO. This observation, together with the results from the model system, suggests that the formation of nitroaromatic anion radicals is mediated through a flavine and not cytochrome P-450. The oxidation of the anion radical intermediate by O2 to the parent nitro compound is proposed to account for the well-known O2 inhibition of microsomal nitroreductase.

Animals↗