Catalyzes the oxidation of sulfides, such as hydrogen sulfide, with the help of a quinone. Has the highest activity with caldariella quinone and decylubiquinone, and lower activity with naphtoquinones. Consecutive reaction cycles lead to the accumulation of a polysulfide product on the active site Cys residues; these products are released when they exceed a critical length, typically as cyclooctasulfur. n a quinone + n H(+) + n hydrogen sulfide = n a quinol + n sulfur Binds 1 FAD per subunit. Inhibited by the quinone analog 2-heptyl-4-hydroxyquinolone N-oxide (HQNO). Inactivated by iodoacetamide treatment. Inhibited by KCN. Optimum pH is about 6. Optimum temperature is above 70 degrees Celsius. Monomer. Belongs to the SQRD family. Was originally identified as FMN-containing NADH dehydrogenase (PubMed:12417325), based on the NADH oxidase activity of the C-terminally truncated protein. Was then shown to function as sulfide:quinone reductase. The full-length protein does not have NADH oxidase activity (PubMed:19438211).