Inhibition of electron transfer in the cytochromeb-c ₁ segment of the mitochondrial respiratory chain by a synthetic analogue of ubiquinone

Abstract

A synthetic analogue of ubiquinone, 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole, inhibits oxidation of succinate and NADH-linked substrates by rat liver mitochondria. Inhibition occurs both in the presence (state 3) and absence (state 4) of ADP. With isolated succinate-cytochromec reductase complex from bovine heart mitochondria the quinone analogue inhibits succinate-cytochromec reductase and ubiquinol-cytochromec reductase activities but does not inhibit succinate-ubiquinone reductase activity. Inhibition of cytochromec reductase activities is markedly dependent on pH in the range pH 7–8. At pH 7.0 inhibition occurs with an apparentK _i≤1×10⁻⁸ M, while at pH 8.0 the apparentK _i is more than an order of magnitude greater than this. Spectrophotometric titrations of 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole show a visibly detectable pK _a at pH 6.5 attributable to ionization of the 6-hydroxy group. These results indicate that this quinone derivative is a highly specific and potent inhibitor of electron transfer in theb-c ₁ segment of the respiratory chain. Because of the structural analogy, it is likely that the mechanism of inhibition involves disruption of normal ubiquinone function. In addition, this inhibition depends on protonation of the ionizable hydroxy group of the inhibitory analogue or on protonation of a functional group in theb-c ₁ segment.