title: Mode of action of steroid desmolase and reductases synthesized by Clostridium "scindens" (formerly Clostridium strain 19).
abstract: A recently isolated hitherto unknown Clostridium from human feces, designated Clostridium "scindens" (formerly strain 19), synthesizes at least two enzymes active on the side-chain of the steroid molecule and two enzymes active on the hydroxyl groups of the 7-position of bile acids. Steroid desmolase, responsible for side-chain cleavage of corticoids, and 20 alpha-hydroxysteroid dehydrogenase have not been detected in any other bacterial species of the resident colonic flora. Steroid desmolase is Eh-dependent (optimum ca. -130 mV), requires a hydroxy group at C-17, and preferably an alpha-ketol group in the side-chain; an alpha-hydroxy group at C-20 reduces and a beta-hydroxy group at C-20 prevents side-chain cleavage. With suitable substrates, the yield of C-19 steroids is proportional to the bacterial multiplication rate. 20 alpha-Hydroxysteroid dehydrogenase (20 alpha-HSDH) is also Eh-dependent (optimum ca. -300 mV) and reduces the C-20 keto function to an alpha-hydroxy group, regardless of the presence or absence of a hydroxy group at C-17. 7 alpha-Dehydroxylase metabolizes cholic and chenodeoxycholic acid, while 7 beta-hydroxysteroid dehydrogenase acts upon ursodeoxycholic acid. The latter two enzymes are not specific for C. scindens.
mesh_terms: D001647:Bile Acids and Salts / Q000378:metabolism; D003013:Clostridium / Q000201:enzymology* / Q000254:growth & development; D003470:Culture Media; D006863:Hydrogen-Ion Concentration; D006914:Hydroxysteroids / Q000378:metabolism*; D007700:Kinetics; D008190:Lyases / Q000378:metabolism*; D010088:Oxidoreductases / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D001647:Bile Acids and Salts; D003470:Culture Media; D006914:Hydroxysteroids; D010088:Oxidoreductases; D008190:Lyases