Dehydroxylase

Dehydroxylase Dehydroxylation of C- and N-hydroxy Bile acids, N-hydroxyfluorenyl-... 

Redox reaetions and hydrolysis are the predominant metabolie eonversions triggered by the intestinal microflora. The main reductive enzymes produeed by the intestinal mieroflora are nitroreductase, deaminase, urea dehydroxylase, and azoreduetase the hydrolytic enzymes are P-glucoronidase, P-xylosidase, P-galaetosidase, and a-L-arabinosidase. Studies conducted by Macfarlane and co-workers have shown that proteolysis ean also happen in the colon [31]. More recent findings by this group indieate that bacterial fermentation of proteins in humans could account for 17% of... 

C24 bile acids are synthesised in the liver from cholesterol. The bacterial enzyme 7-a dehydroxylase converts CDCA into LCA and converts CA into DCA. UDCA a tertiary bile acid is also shown. 

Graph 8.10 7a-dehydroxylase activity in stone-free controls and patients with cholesterol GBS. Data taken from references 40 and 41. 

An NADH-flavin oxidoreductase was also induced by cholic acid in this bacterium [61]. The physiological function of this enzyme in 7-dehydroxylation is presently unknown. Moreover, it is not known which of the induced polypeptides constitute 7-dehydroxylase however, the polypeptides of 56 000 and 27 000 molecular weight co-purify with enzymatic activity upon anaerobic high-performance gel filtration. 

Both 7a//S-dehydroxylase and A -steroid reductase activities are stimulated by adding NAD to anaerobically dialyzed cell extracts of Eubacterium sp. V.P.I. 12708. NADH inhibited all 3 activities at high concentrations (0.5 mM) but stimulated 7a-dehydroxylase activity at lower concentrations (< 0.10 mM). Double reciprocal plots of NADH inhibition of 7a-dehydroxylase activity suggested negative cooper-ativity. The effect of low concentrations of NADH on the kinetics of 7/8-dehydroxyl-ase and A -steroid reductase activity is presently unknown. The activity of 7a-dehy-... 

Dietary fat has been implicated as a risk factor for colon cancer, possibly due to the enhanced levels of bile acids that are released. Bacterial 7a-dehydroxylase acts on primary bile acids to release secondary bile acids, which have a cytotoxic effect on the... 

Hill and Drasar (110) have isolated bacterial strains that possess 12a-dehydroxylase, and it is thus possible that some lithocholic acid in feces is derived from cholic acid via 12a-dehydroxylation of deoxycholic acid. See text for references. 

TABLE III. Distribution of Taurocholate Amidase and Cholate-7-dehydroxylase Among Various Bacterial Genera... 

Species Number of strains tested Percentage showing taurocholate amidase Percentage possessing cholic acid-7-dehydroxylase... 

Assuming that the decreased j8-hydroxydehydro-genase and 11-, 17-, and 22-hydroxylase activities result from the decreased production of molecules responsible for catalyzing each of the reactions, rather than from the elaboration of inhibitors or from the development of increased activities of antagonistic enzymes (e.g., 21-dehydroxylases), several different macromolecular mechanisms can be invoked to explain the changes in enzyme activities. 

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