Abstract
The temperature dependence of the initial burst in the NADH-dependent reduction of rat liver microsomal cytochrome b5 in situ in the membrane was examined by means of stopped-flow spectrophotometry. The rate of cytochromeb5 reduction increased only moderately with increasing temperature, in contrast to the larger increase in a pure, delipidated reconstituted system in free solution. Treatment with a low concentration (0.05%) of sodium deoxycholate, which perturbs the membrane integrity without causing disassembly, abolished the characteristically weak temperature dependence of cytochrome b5 reduction observed in intact microsomes. Despite the striking difference in the behavior of intact and deoxycholate-treated microsomes as regards temperature dependence, the microviscosity in these preparations was found to be essentially the same by means of fluorescence polarization studies. The microsomal NADH-ferricyanide reductase and NADH-cyto-chrome c reductase activities with a water-soluble electron acceptor showed a usual linear dependency on temperature, even in deoxycholate-treated microsomes. It is concluded that the microenvironment (membrane structure) of the microsomal enzyme system strongly influences the electron transfer from the reductase to cytochrome b5. A model explaining the results is proposed and discussed.
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