出版社：American Society for Biochemistry and Molecular Biology
摘要：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.
