Membrane effects on drug monoxygenation activity in hepatic microsomes |
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Authors: | Wilfried Duppel Volker Ullrich |
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Affiliation: | Department of Physiological Chemistry, University of the Saarland, 665 Homburg/Saar G.F.R. |
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Abstract: | The temperature dependence of drug monooxygenation in phenobarbital-induced rat liver microsomes has been investigated. With 7-ethoxycoumarin as a substrate the activity of the microsomes could be measured down to 0°C by the increase in fluorescence of the dealkylated reaction product 7-hydroxycoumarin (umbelliferone).Arrhenius plots of the activities at various temperatures between 0°C and 45°C showed a break in the activation energy around 20°C.Addition of deoxycholate or high concentrations of glycerol, known to solubilize membrane-bound enzymes, abolished the break of the activation energy. Cholesterol, incorporated into the microsomal membrane in amounts equimolar to the microsomal phospholipid content led to a decrease of the activation energy at low temperatures and to an increase at higher temperatures, resulting in a loss of the break.The activity of microsomal NADPH-cytochrome reductase with the water-soluble electron acceptor dichlorophenolindophenol showed no discontinuity in the Arrhenius plot. In addition the cumene hydroperoxide-mediated and cytochrome O-dealkylation of 7-ethoxycoumarin proceeded without a break in the activation energy.It is concluded that phospholipid phase transitions affect the electron transfer from the reductase to cytochrome . |
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Keywords: | TEMPO letramethylpiperidine nitroxide |
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