HOQNO interaction with cytochrome b in succinate:menaquinone oxidoreductase from Bacillus subtilis

2-n-Heptyl 4-hydroxyquinoline-N-oxide (HOQNO) inhibits the succinate:quinone oxidoreductase activity of isolated and membrane-bound succinate:menaquinone oxidoreductase of B. subtilis. The inhibition pattern resembles closely that observed for alpha-thenoyltrifluoroacetone and carboxins in the mitochondrial succinate:ubiquinone oxidoreductase: ca. 90% of the activity is highly sensitive to HOQNO (Ki ca. 0.2 microM for the isolated enzyme) whereas the rest 10% proves to be resistant to the inhibitor. HOQNO binding is shown to perturb the absorption spectrum of the ferrous di-heme cytochrome b of the B. subtilis succinate:quinone oxidoreductase both in the alpha and Soret bands. In addition, the inhibitor is shown to bring about a negative shift of Em of the low-potential heme b. It is suggested that HOQNO interacts with a menasemiquinone binding site near the low-potential heme and suppresses the MQ.(-)-to-MQH2 step of the quinone reductase reaction but allows partly for the MQ-to-MQ.- transition to occur; dismutation of MQ. formed in the latter reaction to MQ and MQH2 may account for the 10% of the enzyme activity insensitive to HOQNO.