Sutherland J D, Macdonald I A
J Lipid Res. 1982 Jul;23(5):726-32.
Clostridium absonum was shown to metabolize primary bile acids to give rise to both 7-oxo bile acids and 7 beta-hydroxy (urso) bile acids. At relatively low redox potential (Eh) values, high yields of urso bile acids were achieved (60-75%). If, however, the Eh value of the culture was allowed to rise above approximately -100 mv, the 7-oxo bile acid would tend to predominate (more than 75%) and the "death phase" was accelerated. Growth of C. absonum in sterile graduated cylinders instead of in conventional Erlenmeyer flasks was effective in delaying the rise in Eh value with time (which appears largely due to diffusion of atmospheric oxygen into the medium) and in preserving a higher viable count of organisms. It is proposed that the formation of excess amounts of 7-oxo bile acid is a manifestation of oxygen toxicity and that it could be mediated by an increasing intracellular NADP:NADPH ratio. Additionally, the reaction: primary bile acid in equilibrium oxo bile acid in equilibrium urso bile acid was shown to be partially reversible. When the organisms were grown with [24-(14)C]chenodeoxycholic, -cholic, or -7-keto-lithocholic acid, this reaction could be clearly demonstrated. The addition of an equimolar concentration of deoxycholic acid (which itself is not metabolized) effectively enhanced the rate of bioconversion of cholate and 7-keto-lithocholic, but not chenodeoxycholate (whose rate of bioconversion was the fastest of the three). When the organisms were grown with urso bile acids (ursocholic or ursodeoxycholic) or with 7-keto-deoxycholic acid, very little metabolism occurred unless deoxycholic acid was added which induced formation of primary and keto bile acids. In all cases, formation of oxo bile acid from primary or urso bile acid occurred as the Eh value of the medium rose with time and could thus be delayed by the use of a cylinder instead of a flask for growing the culture. These results were rationalized by demonstrating that induction of 7 alpha- and 7 beta-hydroxysteroid dehydrogenase is strongly mediated by chenodeoxycholic and deoxycholic acids, weakly mediated by cholic and 7-keto-lithocholic acids, and ineffective with 7-keto-deoxycholic, ursocholic, and ursodeoxycholic acids.
研究表明，异常梭菌可将初级胆汁酸代谢生成7-氧代胆汁酸和7β-羟基（熊去氧）胆汁酸。在相对较低的氧化还原电位（Eh）值下，可获得较高产量的熊去氧胆汁酸（60 - 75%）。然而，如果培养物的Eh值升高至约 -100 mV以上，7-氧代胆汁酸将趋于占主导地位（超过75%），且“死亡期”会加速。在无菌量筒而非传统锥形瓶中培养异常梭菌，可有效延缓Eh值随时间的升高（这似乎主要是由于大气中的氧气扩散到培养基中所致），并保持较高的活菌数。据推测，过量7-氧代胆汁酸的形成是氧毒性的一种表现，且可能由细胞内NADP:NADPH比值的增加介导。此外，反应：初级胆汁酸⇌氧代胆汁酸⇌熊去氧胆汁酸被证明是部分可逆的。当用[24-(14)C]鹅去氧胆酸、胆酸或7-酮石胆酸培养该微生物时，这一反应可得到清晰证明。添加等摩尔浓度的脱氧胆酸（其本身不被代谢）可有效提高胆酸和7-酮石胆酸的生物转化速率，但对鹅去氧胆酸无效（其生物转化速率在三者中最快）。当用熊去氧胆汁酸（熊去氧胆酸或熊去氧鹅去氧胆酸）或7-酮脱氧胆酸培养该微生物时，除非添加脱氧胆酸诱导初级和酮胆汁酸的形成，否则几乎不发生代谢。在所有情况下，随着培养基的Eh值随时间升高，初级或熊去氧胆汁酸会形成氧代胆汁酸，因此通过使用量筒而非锥形瓶培养该培养物可延缓这一过程。通过证明7α-和7β-羟基类固醇脱氢酶的诱导强烈受鹅去氧胆酸和脱氧胆酸介导，但弱受胆酸和7-酮石胆酸介导，且对7-酮脱氧胆酸、熊去氧胆酸和熊去氧鹅去氧胆酸无效，这些结果得到了合理的解释。
