The mechanism of the selective inhibitions of succinate- and NADH oxidase activities of Escherichia coli membrane vesicles by zinc ions were studied.
Zinc ions strongly inhibited the succinate oxidase activity, but this inhibition was prevented by the presence of β-mercaptoethanol. p-Chloromercuribenzoate and N-ethylmaleimide also inhibited the activity. The succinate oxidase activity was slightly inhibited by photooxidation of the membrane vesicles with rose bengal. However, this was prevented by the presence of dithiothreitol.
The succinate dehydrogenase [EC 1. 3. 99. 1] activity of the membrane vesicles was found to be inhibited by zinc ions, even under anaerobic assay conditions. It was therefore concluded that zinc ions selectively inhibit the succinate oxidase system by blocking an active -SH residue(s), and that the site of the inhibition is located on succinate dehydrogenase. The mechanism of the inhibition is discussed briefly.
The NADH oxidase activity of the membrane vesicles was strongly inhibited by zinc ions and by photooxidation with rose bengal. The inhibition by zinc ions was not prevented by β-mercaptoethanol and N-ethylmaleimide was not inhibitory. The extent of inactivation of the oxidase activity by photooxidation depended on the pH and was not prevented by the presence of dithiothreitol. These, and other findings suggest that zinc ions inhibit the NADH oxidase activity by attacking a histidine residue(s) in the system, and that the site of the inhibition is before cytochrome b₁ in the respiratory chain.
Other properties of the succinate-, NADH-, and D-(-)-lactate oxidase activities of the membrane vesicles, including their K_m values for substrates and pH optima are reported.