Targeted Synthesis and Characterization of a Gene Cluster Encoding NAD(P)H-Dependent 3α-, 3β-, and 12α-Hydroxysteroid Dehydrogenases fromEggerthellaCAG:298, a Gut Metagenomic Sequence

ABSTRACTGut metagenomic sequences provide a rich source of microbial genes, the majority of which are annotated by homology or unknown. Genes and gene pathways that encode enzymes catalyzing biotransformation of host bile acids are important to identify in gut metagenomic sequences due to the importance of bile acids in gut microbiome structure and host physiology. Hydroxysteroid dehydrogenases (HSDHs) are pyridine nucleotide-dependent enzymes with stereospecificity and regiospecificity for bile acid and steroid hydroxyl groups. HSDHs have been identified in several protein families, including medium-chain and short-chain dehydrogenase/reductase families as well as the aldo-keto reductase family. These protein families are large and contain diverse functionalities, making prediction of HSDH-encoding genes difficult and necessitating biochemical characterization. We located a gene cluster inEggerthellasp. CAG:298 predicted to encode three HSDHs (CDD59473, CDD59474, and CDD59475) and synthesized the genes for heterologous expression inEscherichia coli. We then screened bile acid substrates against the purified recombinant enzymes. CDD59475 is a novel 12α-HSDH, and we determined that CDD59474 (3α-HSDH) and CDD59473 (3β-HSDH) constitute novel enzymes in an iso-bile acid pathway. Phylogenetic analysis of these HSDHs with other gut bacterial HSDHs and closest homologues in the database revealed predictable clustering of HSDHs by function and identified several likely HSDH sequences from bacteria isolated or sequenced from diverse mammalian and avian gut samples.IMPORTANCEBacterial HSDHs have the potential to significantly alter the physicochemical properties of bile acids, with implications for increased/decreased toxicity for gut bacteria and the host. The generation of oxo-bile acids is known to inhibit host enzymes involved in glucocorticoid metabolism and may alter signaling through nuclear receptors such as farnesoid X receptor and G-protein-coupled receptor TGR5. Biochemical or similar approaches are required to fill in many gaps in our ability to link a particular enzymatic function with a nucleic acid or amino acid sequence. In this regard, we have identified a novel 12α-HSDH and a novel set of genes encoding an iso-bile acid pathway (3α-HSDH and 3β-HSDH) involved in epimerization and detoxification of harmful secondary bile acids.

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Ursodeoxycholic Acid and Its Taurine- or Glycine-Conjugated Species Reduce Colitogenic Dysbiosis and Equally Suppress Experimental Colitis in Mice

Applied and Environmental Microbiology ◽

10.1128/aem.02766-16 ◽

2017 ◽

Vol 83 (7) ◽

Cited By ~ 25

Author(s):

Lien Van den Bossche ◽

Pieter Hindryckx ◽

Lindsey Devisscher ◽

Sarah Devriese ◽

Sophie Van Welden ◽

...

Keyword(s):

Community Structure ◽

Bile Acid ◽

Bile Acids ◽

Experimental Colitis ◽

Acid Therapy ◽

Content Type ◽

Bile Acid Therapy ◽

Inflammatory Bowel ◽

The Impact ◽

Secondary Bile Acids

ABSTRACT The promising results seen in studies of secondary bile acids in experimental colitis suggest that they may represent an attractive and safe class of drugs for the treatment of inflammatory bowel diseases (IBD). However, the exact mechanism by which bile acid therapy confers protection from colitogenesis is currently unknown. Since the gut microbiota plays a crucial role in the pathogenesis of IBD, and exogenous bile acid administration may affect the community structure of the microbiota, we examined the impact of the secondary bile acid ursodeoxycholic acid (UDCA) and its taurine or glycine conjugates on the fecal microbial community structure during experimental colitis. Daily oral administration of UDCA, tauroursodeoxycholic acid (TUDCA), or glycoursodeoxycholic acid (GUDCA) equally lowered the severity of dextran sodium sulfate-induced colitis in mice, as evidenced by reduced body weight loss, colonic shortening, and expression of inflammatory cytokines. Illumina sequencing demonstrated that bile acid therapy during colitis did not restore fecal bacterial richness and diversity. However, bile acid therapy normalized the colitis-associated increased ratio of Firmicutes to Bacteroidetes. Interestingly, administration of bile acids prevented the loss of Clostridium cluster XIVa and increased the abundance of Akkermansia muciniphila, bacterial species known to be particularly decreased in IBD patients. We conclude that UDCA, which is an FDA-approved drug for cholestatic liver disorders, could be an attractive treatment option to reduce dysbiosis and ameliorate inflammation in human IBD. IMPORTANCE Secondary bile acids are emerging as attractive candidates for the treatment of inflammatory bowel disease. Although bile acids may affect the intestinal microbial community structure, which significantly contributes to the course of these inflammatory disorders, the impact of bile acid therapy on the fecal microbiota during colitis has not yet been considered. Here, we studied the alterations in the fecal microbial abundance in colitic mice following the administration of secondary bile acids. Our results show that secondary bile acids reduce the severity of colitis and ameliorate colitis-associated fecal dysbiosis at the phylum level. This study indicates that secondary bile acids might act as a safe and effective drug for inflammatory bowel disease.

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Metabolism of Oxo-Bile Acids and Characterization of Recombinant 12α-Hydroxysteroid Dehydrogenases from Bile Acid 7α-Dehydroxylating Human Gut Bacteria

Applied and Environmental Microbiology ◽

10.1128/aem.00235-18 ◽

2018 ◽

Vol 84 (10) ◽

Cited By ~ 12

Author(s):

Heidi Doden ◽

Lina A. Sallam ◽

Saravanan Devendran ◽

Lindsey Ly ◽

Greta Doden ◽

...

Keyword(s):

Phylogenetic Analysis ◽

Bile Acid ◽

Bile Acids ◽

Cholic Acid ◽

Deoxycholic Acid ◽

Gut Bacteria ◽

Dietary Lipids ◽

Human Gut ◽

Content Type ◽

Low Activity

ABSTRACTBile acids are important cholesterol-derived nutrient signaling hormones, synthesized in the liver, that act as detergents to solubilize dietary lipids. Bile acid 7α-dehydroxylating gut bacteria generate the toxic bile acids deoxycholic acid and lithocholic acid from host bile acids. The ability of these bacteria to remove the 7-hydroxyl group is partially dependent on 7α-hydroxysteroid dehydrogenase (HSDH) activity, which reduces 7-oxo-bile acids generated by other gut bacteria. 3α-HSDH has an important enzymatic activity in the bile acid 7α-dehydroxylation pathway. 12α-HSDH activity has been reported for the low-activity bile acid 7α-dehydroxylating bacteriumClostridium leptum; however, this activity has not been reported for high-activity bile acid 7α-dehydroxylating bacteria, such asClostridium scindens,Clostridium hylemonae, andClostridium hiranonis. Here, we demonstrate that these strains express bile acid 12α-HSDH. The recombinant enzymes were characterized from each species and shown to preferentially reduce 12-oxolithocholic acid to deoxycholic acid, with low activity against 12-oxochenodeoxycholic acid and reduced activity when bile acids were conjugated to taurine or glycine. Phylogenetic analysis suggests that 12α-HSDH is widespread amongFirmicutes,Actinobacteriain theCoriobacteriaceaefamily, and human gutArchaea.IMPORTANCE12α-HSDH activity has been established in the medically important bile acid 7α-dehydroxylating bacteriaC. scindens,C. hiranonis, andC. hylemonae. Experiments with recombinant 12α-HSDHs from these strains are consistent with culture-based experiments that show a robust preference for 12-oxolithocholic acid over 12-oxochenodeoxycholic acid. Phylogenetic analysis identified novel members of the gut microbiome encoding 12α-HSDH. Future reengineering of 12α-HSDH enzymes to preferentially oxidize cholic acid may provide a means to industrially produce the therapeutic bile acid ursodeoxycholic acid. In addition, a cholic acid-specific 12α-HSDH expressed in the gut may be useful for the reduction in deoxycholic acid concentration, a bile acid implicated in cancers of the gastrointestinal (GI) tract.

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Bile Acid Administration Elicits an Intestinal Antimicrobial Program and Reduces the Bacterial Burden in Two Mouse Models of Enteric Infection

Infection and Immunity ◽

10.1128/iai.00942-16 ◽

2017 ◽

Vol 85 (6) ◽

Cited By ~ 18

Author(s):

Sarah Tremblay ◽

Guillaume Romain ◽

Mélisange Roux ◽

Xi-Lin Chen ◽

Kirsty Brown ◽

...

Keyword(s):

Bile Acid ◽

Bile Acids ◽

Mouse Models ◽

Bacterial Infections ◽

Immune Cell ◽

Paneth Cell ◽

Farnesoid X Receptor ◽

Enteric Infection ◽

Content Type ◽

Acid Administration

ABSTRACT In addition to their chemical antimicrobial nature, bile acids are thought to have other functions in the homeostatic control of gastrointestinal immunity. However, those functions have remained largely undefined. In this work, we used ileal explants and mouse models of bile acid administration to investigate the role of bile acids in the regulation of the intestinal antimicrobial response. Mice fed on a diet supplemented with 0.1% chenodeoxycholic acid (CDCA) showed an upregulated expression of Paneth cell α-defensins as well as an increased synthesis of the type-C lectins Reg3b and Reg3g by the ileal epithelium. CDCA acted on several epithelial cell types, by a mechanism independent from farnesoid X receptor (FXR) and not involving STAT3 or β-catenin activation. CDCA feeding did not change the relative abundance of major commensal bacterial groups of the ileum, as shown by 16S analyses. However, administration of CDCA increased the expression of ileal Muc2 and induced a change in the composition of the mucosal immune cell repertoire, decreasing the proportion of Ly6G+ and CD68+ cells, while increasing the relative amount of IgGκ+ B cells. Oral administration of CDCA to mice attenuated infections with the bile-resistant pathogens Salmonella enterica serovar Typhimurium and Citrobacter rodentium, promoting lower systemic colonization and faster bacteria clearance, respectively. Our results demonstrate that bile acid signaling in the ileum triggers an antimicrobial program that can be potentially used as a therapeutic option against intestinal bacterial infections.

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Influence of the structure of bile acids on their partition coefficient in dibutyl ether and chloroform

Zbornik Matice srpske za prirodne nauke ◽

10.2298/zmspn1528077s ◽

2015 ◽

pp. 77-85 ◽

Cited By ~ 1

Author(s):

Ana Sebenji ◽

Mihalj Pоsa ◽

Nevena Grujic-Letic ◽

Kosta Popovic

Keyword(s):

Bile Acid ◽

Partition Coefficient ◽

Bile Acids ◽

Partition Coefficients ◽

Ph Value ◽

Organic Layer ◽

Hydroxyl Groups ◽

Dibutyl Ether ◽

Ph Values ◽

Acid Salts

Bile acids are well known natural surfactants able to modify the per?meability of biological membranes. The logarithm of partition coefficient between, tradi?tionally used, n-octanol and water is a measure of lipophilicity as a predictor of solute membrane partitioning. The aim of this work was to determine partition coefficients of bile acids in a mixture of water and chloroform and dibutyl ether at different pH values and with addition of different concentrations of sodium ions, and to examine the influence of the structure of bile acid nucleus on measured partition coefficients. Partition coefficients of three bile acid salts were determined using shake-flask method and the concentration of bile acids was determined after twelve hours of shaking at the room temperature in aqueous and organic layer using reversed phase HPLC with DAD detector on 210 nm. For all three analysed bile acid salts values of logP are lower in dibutyl ether than in chloroform. At certain pH values, curves representing the dependence of partition coeffi?cient on pH value intersect, and these are the pH values for which partition coefficients are the same for both solvents. Increasing the solution ionic strength, this intersection is shifted toward lower pH values. It is found that, for both organic solvents, after the addition of hy?droxyl group in the steroid nucleus (i.e. if the bile acid is less hydrophobic) the value of logP falls, especially if more hydroxyl groups are present. With chloroform as a solvent, system quickly comes to excess with electrolyte ions than with dibutyl ether.

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Reexamining the Germination Phenotypes of Several Clostridium difficile Strains Suggests Another Role for the CspC Germinant Receptor

Journal of Bacteriology ◽

10.1128/jb.00908-15 ◽

2015 ◽

Vol 198 (5) ◽

pp. 777-786 ◽

Cited By ~ 28

Author(s):

Disha Bhattacharjee ◽

Michael B. Francis ◽

Xicheng Ding ◽

Kathleen N. McAllister ◽

Ritu Shrestha ◽

...

Keyword(s):

Bile Acid ◽

Clostridium Difficile ◽

Bile Acids ◽

Spore Germination ◽

Germination Rate ◽

Taurocholic Acid ◽

Receptor Complex ◽

Health Care Environment ◽

Content Type ◽

Downstream Processes

ABSTRACTClostridium difficilespore germination is essential for colonization and disease. The signals that initiateC. difficilespore germination are a combination of taurocholic acid (a bile acid) and glycine. Interestingly, the chenodeoxycholic acid class (CDCA) bile acids competitively inhibit taurocholic acid-mediated germination, suggesting that compounds that inhibit spore germination could be developed into drugs that prophylactically preventC. difficileinfection or reduce recurring disease. However, a recent report called into question the utility of such a strategy to prevent infection by describingC. difficilestrains that germinated in the apparent absence of bile acids or germinated in the presence of the CDCA inhibitor. Because the mechanisms ofC. difficilespore germination are beginning to be elucidated, the mechanism of germination in these particular strains could yield important information on howC. difficilespores initiate germination. Therefore, we quantified the interaction of these strains with taurocholic acid and CDCA, the rates of spore germination, the release of DPA from the spore core, and the abundance of the germinant receptor complex (CspC, CspB, and SleC). We found that strains previously observed to germinate in the absence of taurocholic acid correspond to more potent 50% effective concentrations (EC50values; the concentrations that achieve a half-maximum germination rate) of the germinant and are still inhibited by CDCA, possibly explaining the previous observations. By comparing the germination kinetics and the abundance of proteins in the germinant receptor complex, we revised our original model for CspC-mediated activation of spore germination and propose that CspC may activate spore germination and then inhibit downstream processes.IMPORTANCEClostridium difficileforms metabolically dormant spores that persist in the health care environment. In susceptible hosts,C. difficilespores germinate in response to certain bile acids and glycine. Blocking germination byC. difficilespores is an attractive strategy to prevent the initiation of disease or to block recurring infection. However, certainC. difficilestrains have been identified whose spores germinate in the absence of bile acids or are not blocked by known inhibitors ofC. difficilespore germination (calling into question the utility of such strategies). Here, we further investigate these strains and reestablish that bile acid activators and inhibitors of germination affect these strains and use these data to suggest another role for theC. difficilebile acid germinant receptor.

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Dexamethasone regulates bile acid synthesis in monolayer cultures of rat hepatocytes by induction of cholesterol 7α-hydroxylase

Biochemical Journal ◽

10.1042/bj2620341 ◽

1989 ◽

Vol 262 (1) ◽

pp. 341-348 ◽

Cited By ~ 48

Author(s):

H M Princen ◽

P Meijer ◽

B Hofstee

Keyword(s):

Enzyme Activity ◽

Bile Acid ◽

Bile Acids ◽

Steroid Hormones ◽

Mrna Level ◽

Rat Hepatocytes ◽

Acid Synthesis ◽

Bile Acid Synthesis ◽

Hydroxylase Activity ◽

Acid Pathway

To study the effect of steroid hormones on bile acid synthesis by cultured rat hepatocytes, cells were incubated with various amounts of these compounds during 72 h and conversion of [4-14C]cholesterol into bile acids was measured. Bile acid synthesis was stimulated in a dose-dependent way by glucocorticoids, but not by sex steroid hormones, pregnenolone or the mineralocorticoid aldosterone in concentrations up to 10 microM. Dexamethasone proved to be the most efficacious inducer, giving 3-fold and 7-fold increases in bile acid synthesis during the second and third 24 h incubation periods respectively, at a concentration of 50 nM. Mass production of bile acids as measured by g.l.c. during the second day of culture (28-52 h) was 2.2-fold enhanced by 1 microM-dexamethasone. No change in the ratio of bile acids produced was observed during this period in the presence of dexamethasone. Conversion of [4-14C]7 alpha-hydroxycholesterol, an intermediate of the bile acid pathway, to bile acids was not affected by dexamethasone. Measurement of cholesterol 7 alpha-hydroxylase activity in homogenates of hepatocytes, incubated with 1 microM-dexamethasone, showed 10-fold and 90-fold increases after 48 and 72 h respectively, as compared with control cells. As with bile acid synthesis from [14C]cholesterol, no change in enzyme activity was found in hepatocytes cultured in the presence of 10 microM steroid hormones other than glucocorticoids. Addition of inhibitors of protein and mRNA synthesis lowered bile acid production and cholesterol 7 alpha-hydroxylase activity and prevented the rise of both parameters with dexamethasone, suggesting regulation at the mRNA level. We conclude that glucocorticoids regulate bile acid synthesis in rat hepatocytes by induction of enzyme activity of cholesterol 7 alpha-hydroxylase.

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Antibiotic-Induced Alterations of the Gut Microbiota Alter Secondary Bile Acid Production and Allow for Clostridium difficile Spore Germination and Outgrowth in the Large Intestine

mSphere ◽

10.1128/msphere.00045-15 ◽

2016 ◽

Vol 1 (1) ◽

Cited By ~ 154

Author(s):

Casey M. Theriot ◽

Alison A. Bowman ◽

Vincent B. Young

Keyword(s):

Bile Acid ◽

Clostridium Difficile ◽

Gut Microbiota ◽

Bile Acids ◽

Large Intestine ◽

Spore Germination ◽

Secondary Bile Acid ◽

Content Type ◽

Secondary Bile Acids

ABSTRACT Antibiotics alter the gastrointestinal microbiota, allowing for Clostridium difficile infection, which is a significant public health problem. Changes in the structure of the gut microbiota alter the metabolome, specifically the production of secondary bile acids. Specific bile acids are able to initiate C. difficile spore germination and also inhibit C. difficile growth in vitro, although no study to date has defined physiologically relevant bile acids in the gastrointestinal tract. In this study, we define the bile acids C. difficile spores encounter in the small and large intestines before and after various antibiotic treatments. Antibiotics that alter the gut microbiota and deplete secondary bile acid production allow C. difficile colonization, representing a mechanism of colonization resistance. Multiple secondary bile acids in the large intestine were able to inhibit C. difficile spore germination and growth at physiological concentrations and represent new targets to combat C. difficile in the large intestine. It is hypothesized that the depletion of microbial members responsible for converting primary bile acids into secondary bile acids reduces resistance to Clostridium difficile colonization. To date, inhibition of C. difficile growth by secondary bile acids has only been shown in vitro. Using targeted bile acid metabolomics, we sought to define the physiologically relevant concentrations of primary and secondary bile acids present in the murine small and large intestinal tracts and how these impact C. difficile dynamics. We treated mice with a variety of antibiotics to create distinct microbial and metabolic (bile acid) environments and directly tested their ability to support or inhibit C. difficile spore germination and outgrowth ex vivo. Susceptibility to C. difficile in the large intestine was observed only after specific broad-spectrum antibiotic treatment (cefoperazone, clindamycin, and vancomycin) and was accompanied by a significant loss of secondary bile acids (deoxycholate, lithocholate, ursodeoxycholate, hyodeoxycholate, and ω-muricholate). These changes were correlated to the loss of specific microbiota community members, the Lachnospiraceae and Ruminococcaceae families. Additionally, physiological concentrations of secondary bile acids present during C. difficile resistance were able to inhibit spore germination and outgrowth in vitro. Interestingly, we observed that C. difficile spore germination and outgrowth were supported constantly in murine small intestinal content regardless of antibiotic perturbation, suggesting that targeting growth of C. difficile will prove most important for future therapeutics and that antibiotic-related changes are organ specific. Understanding how the gut microbiota regulates bile acids throughout the intestine will aid the development of future therapies for C. difficile infection and other metabolically relevant disorders such as obesity and diabetes. IMPORTANCE Antibiotics alter the gastrointestinal microbiota, allowing for Clostridium difficile infection, which is a significant public health problem. Changes in the structure of the gut microbiota alter the metabolome, specifically the production of secondary bile acids. Specific bile acids are able to initiate C. difficile spore germination and also inhibit C. difficile growth in vitro, although no study to date has defined physiologically relevant bile acids in the gastrointestinal tract. In this study, we define the bile acids C. difficile spores encounter in the small and large intestines before and after various antibiotic treatments. Antibiotics that alter the gut microbiota and deplete secondary bile acid production allow C. difficile colonization, representing a mechanism of colonization resistance. Multiple secondary bile acids in the large intestine were able to inhibit C. difficile spore germination and growth at physiological concentrations and represent new targets to combat C. difficile in the large intestine.

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Deciphering a Marine Bone-Degrading Microbiome Reveals a Complex Community Effort

mSystems ◽

10.1128/msystems.01218-20 ◽

2021 ◽

Vol 6 (1) ◽

Author(s):

Erik Borchert ◽

Antonio García-Moyano ◽

Sergio Sanchez-Carrillo ◽

Thomas G. Dahlgren ◽

Beate M. Slaby ◽

...

Keyword(s):

Gene Cluster ◽

Marine Environment ◽

Field Experiments ◽

Bone Matrix ◽

Taxonomic Diversity ◽

Bone Surface ◽

Metagenomic Sequence ◽

Content Type ◽

Bone Degradation ◽

Genes Encoding

ABSTRACT The marine bone biome is a complex assemblage of macro- and microorganisms; however, the enzymatic repertoire to access bone-derived nutrients remains unknown. The bone matrix is a composite material made up mainly of organic collagen and inorganic hydroxyapatite. We conducted field experiments to study microbial assemblages that can use organic bone components as nutrient source. Bovine and turkey bones were deposited at 69 m depth in a Norwegian fjord (Byfjorden, Bergen). Metagenomic sequence analysis was used to assess the functional potential of microbial assemblages from bone surface and the bone-eating worm Osedax mucofloris, which is a frequent colonizer of whale falls and known to degrade bone. The bone microbiome displayed a surprising taxonomic diversity revealed by the examination of 59 high-quality metagenome-assembled genomes from at least 23 bacterial families. Over 700 genes encoding enzymes from 12 relevant enzymatic families pertaining to collagenases, peptidases, and glycosidases putatively involved in bone degradation were identified. Metagenome-assembled genomes (MAGs) of the class Bacteroidia contained the most diverse gene repertoires. We postulate that demineralization of inorganic bone components is achieved by a timely succession of a closed sulfur biogeochemical cycle between sulfur-oxidizing and sulfur-reducing bacteria, causing a drop in pH and subsequent enzymatic processing of organic components in the bone surface communities. An unusually large and novel collagen utilization gene cluster was retrieved from one genome belonging to the gammaproteobacterial genus Colwellia. IMPORTANCE Bones are an underexploited, yet potentially profitable feedstock for biotechnological advances and value chains, due to the sheer amounts of residues produced by the modern meat and poultry processing industry. In this metagenomic study, we decipher the microbial pathways and enzymes that we postulate to be involved in bone degradation in the marine environment. We here demonstrate the interplay between different bacterial community members, each supplying different enzymatic functions with the potential to cover an array of reactions relating to the degradation of bone matrix components. We identify and describe a novel gene cluster for collagen utilization, which is a key function in this unique environment. We propose that the interplay between the different microbial taxa is necessary to achieve the complex task of bone degradation in the marine environment.

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Bile acid inhibition of taurocholate uptake by rat hepatocytes: role of OH groups

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.1987.252.3.g339 ◽

1987 ◽

Vol 252 (3) ◽

pp. G339-G344 ◽

Cited By ~ 2

Author(s):

S. Bellentani ◽

W. G. Hardison ◽

P. Marchegiano ◽

G. Zanasi ◽

F. Manenti

Keyword(s):

Bile Acid ◽

Bile Acids ◽

Competitive Inhibition ◽

Silicone Oil ◽

Nonlinear Least Squares ◽

Rat Hepatocytes ◽

Hydroxyl Groups ◽

Oh Groups ◽

Least Squares Fit ◽

Uptake Velocity

To define further the structural specificity of the taurocholate uptake site, we studied the ability of a variety of taurine-conjugated bile acids with differing hydroxyl substituents on the sterol moiety to inhibit [14C]taurocholate uptake. Rat hepatocytes isolated by collagenase perfusion were incubated in a tris(hydroxymethyl)aminomethane-phosphate buffer containing [14C]taurocholate (2.5-100 microM) in the presence or absence of inhibitor bile acid. Stronger inhibitors were studied at a fixed concentration of 5 microM, weaker ones at 25 microM. Initial uptake velocity was measured by sedimenting an aliquot of cells through silicone oil into 3 N KOH every 15 s for 1 min. Uptake velocity (nmol X mg protein-1 X min-1) could then be related to taurocholate concentration and a Vmax and Km could be determined by applying a nonlinear least squares fit to the data obtained with or without inhibitor. The kinetic parameters allowed the determination of the type of inhibition and of inhibition constants (Ki) of the various test bile acids. The data indicate that bile acids containing a 6- or 7-OH group exhibit competitive inhibition, whereas bile acids with no 6- or 7-OH group exhibit noncompetitive inhibition. Of the compounds exhibiting competitive inhibition, Ki varied with the number of hydroxyl groups on the sterol moiety. We conclude that the presence or absence of a 6- or 7-OH group dictates the mechanism of inhibition; the number of hydroxyl substituents determines the potency of competitive inhibition.

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Intestinal Absorption of Bile Salts

Canadian Journal of Gastroenterology ◽

10.1155/1990/624985 ◽

1990 ◽

Vol 4 (2) ◽

pp. 79-84 ◽

Cited By ~ 1

Author(s):

Karen Madsen

Keyword(s):

Bile Acid ◽

Bile Salt ◽

Bile Acids ◽

Intestinal Bacteria ◽

Dietary Lipids ◽

Side Chain ◽

Hydroxyl Groups ◽

Carrier Mechanism ◽

Major Route ◽

The Individual

Bile acids are secreted from the liver into the duodenum where they aid in the digestion and absorption of dietary lipids. Absorption of bile acids occurs through both ionic and nonionic diffusion in the jejunum and colon and through an active sodium ion-dependent carrier mechanism in the ileum. The prima, y bile acids synthesized in the liver can be converted by intestinal bacteria into secondary and tertiary bile acids. Bile acids may also be conjugated with glycine or taurine which results in an increase in the hydrophilicity and solubility of these compounds at physiological pH. The amount of passive diffusion of bile acids that occurs across the brush border membrane along the length of the entire intestine depends upon the ratio of ionized to nonionized bile acids coupled with the bile salt concentration and the individual permeability coefficients of monomers. Active transport of both conjugated and nonconjugated species of bile acids depends upon the presence of a single negative charge on the side chain. Maximal transport rates for bile acids are related to the number of hydroxyl groups present while the Michaelis-Menten constant for transport is dependent upon whether or not the bile acid is conjugated. Although active uptake of bile acids from the ileum has been considered the major route for bile salt absorption in the small intestine, the mechanism may actually be responsible for only a small proportion of the total bile acid pool absorbed from the lumen.

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