Inhibition of electron transfer in the cytochrome b-c, segment of the mitochondrial respiratory chain by a synthetic analogue of ubiquinone.

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-cytochrome c reductase complex from bovine heart mitochondria the quinone analogue inhibits succinate-cytochrome c reductase and ubiquinol-cytochrome c reductase activities but does not inhibit succinate-ubiquinone reductase activity. Inhibition of cytochrome c reductase activities is markedly dependent on pH in the range pH 7-8. At pH 7.0 inhibition occurs with an apparent Ki less than or equal to 1 x 10(-8) M, while at pH 8.0 the apparent Ki 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 pKa 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 the b-c1 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 function group in the b-c1 segment.