Comparison of Some Catalytic Properties of the Purified and Membrane-Bound Reduced Nicotinamide Adenine Dinucleotide-Cytochrome o Reductase of Vitreoscilla

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The NADH oxidase system of Vitreoscilla was localized in the membrane as shown by three assays of respiratory activity: NADH oxidation, oxygen consumption and NADH dehydrogenase activity using p-iodonitrotetrazolium violet (INT) as electron acceptor. The purified metalloflavoprotein component of the NADH-cytochrome o reductase system catalyzes the aerobic reduction of impure cytochrome o but not of the pure cytochrome. The purified enzyme exhibited hyperbolic kinetics with NADH when impure cytochrome o was used as electron acceptor but sigmoid kinetics with INT as electron acceptor. The kinetics of INT reduction became more hyperbolic when assayed in the presence of impure cytochrome o and the Hill coefficient decreased from 1.8 to 1.1. In contrast, both the NADH-INT reductase and NADH oxidase activities of membrane vesicles showed hyperbolic kinetics with NADH. The NADH oxidase activity of membrane vesicles was subject to substrate inhibition at NADH concentrations greater than 150μM which was not observed for the NADH-cytochrome o reductase activity of the purfied enzyme under aerobic conditions. Respiratory inhibitors such as rotenone, cyanide, and salicylhydroxamic acid inhibited the oxidation of NADH by membrane vesicles with oxygen as electron acceptor but not with INT as electron acceptor. These comparative studies of some catalytic properties of the purified and membrane bound enzyme provided evidence that the NADH-cytochrome o oxidase system of Vitreoscilla consists of cytochrome o, the flavoprotein reductase, and at least one other component which may regulate the catalytic properties of the reductase.

Comparison of Some Catalytic Properties of the Purified and Membrane-Bound Reduced Nicotinamide Adenine Dinucleotide-Cytochrome o Reductase of Vitreoscilla

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