Conjugated Bile Acid Hydrolase Research Articles

A catalytic mechanism for cysteine N-terminal nucleophile hydrolases, as revealed by free energy simulations.

The N-terminal nucleophile (Ntn) hydrolases are a superfamily of enzymes specialized in the hydrolytic cleavage of amide bonds. Even though several members of this family are emerging as innovative drug targets for cancer, inflammation, and pain, the processes through which they catalyze amide hydrolysis remains poorly understood. In particular, the catalytic reactions of cysteine Ntn-hydrolases have never been investigated from a mechanistic point of view. In the present study, we used free energy simulations in the quantum mechanics/molecular mechanics framework to determine the reaction mechanism of amide hydrolysis catalyzed by the prototypical cysteine Ntn-hydrolase, conjugated bile acid hydrolase (CBAH). The computational analyses, which were confirmed in water and using different CBAH mutants, revealed the existence of a chair-like transition state, which might be one of the specific features of the catalytic cycle of Ntn-hydrolases. Our results offer new insights on Ntn-mediated hydrolysis and suggest possible strategies for the creation of therapeutically useful inhibitors.

Improved annotation of conjugated bile acid hydrolase superfamily members in Gram-positive bacteria

Most Gram-positive bacteria inhabiting the gastrointestinal tract are capable of hydrolysing bile salts. Bile salt hydrolysis is thought to play an important role in various biological processes in the host. Therefore, correct annotation of bacterial bile salt hydrolases (Bsh) in public databases (EC 3.5.1.24) is of importance, especially for lactobacilli, which are considered to play a major role in bile salt hydrolysis in vivo. In the present study, all enzymes listed in public databases that belong to the Bsh family and the closely related penicillin V acylase (Pva; EC 3.5.1.11) family were compared with the sequences annotated as Bsh in Lactobacillus plantarum WCFS1, as an example. In Gram-positive bacteria, a clear distinction was made between the two families using sequence alignment, phylogenetic clustering, and protein homology modelling. Biochemical and structural data on experimentally verified Bsh and Pva enzymes were used for validation of function prediction. Hidden Markov models were constructed from the sequence alignments to enable a more accurate prediction of Bsh-encoding genes, and their distinction from those encoding members of the Pva family. Many Pva-related sequences appeared to be annotated incorrectly as Bsh in public databases. This refinement in the annotation of Bsh family members influences the prediction of the function of bsh-like genes in species of the genus Lactobacillus, and it is discussed in detail.

Conjugated Bile Acid Hydrolase Is a Tetrameric N-Terminal Thiol Hydrolase with Specific Recognition of Its Cholyl but Not of Its Tauryl Product,

Bacterial bile salt hydrolases catalyze the degradation of conjugated bile acids in the mammalian gut. The crystal structures of conjugated bile acid hydrolase (CBAH) from Clostridium perfringens as apoenzyme and in complex with taurodeoxycholate that was hydrolyzed to the reaction products taurine and deoxycholate are described here at 2.1 and 1.7 A resolution, respectively. The crystal structures reveal close relationship between CBAH and penicillin V acylase from Bacillus sphaericus. This similarity together with the N-terminal cysteine classifies CBAH as a member of the N-terminal nucleophile (Ntn) hydrolase superfamily. Both crystal structures show an identical homotetrameric organization with dihedral (D(2) or 222) point group symmetry. The structure analysis of C. perfringens CBAH identifies critical residues in catalysis, substrate recognition, and tetramer formation which may serve in further biochemical characterization of bile acid hydrolases.

Continuous spectrophotometric assay of conjugated bile acid hydrolase.

Conjugated bile acid hydrolase (CBAH) refers to a class of enzymes which catalyze the cleavage of the amino acid moieties from conjugated bile acids. These enzymes are significant because of their role in providing substrates for further microbial metabolism in the gastrointestinal tract. They also are used in research laboratories for the deconjugation of bile acids prior to structural analyses. A continuous spectrophotometric assay for CBAH activity was developed using a conjugate of cholic acid and the chromophore, 5-amino-2-nitro-benzoic acid. The free chromophore is detected by virtue of its absorbance at 410 nm. The CBAH from Clostridium perfringens displayed a Km for this substrate of 120 microM. These results demonstrate that this new compound functions as an effective substrate of the enzyme and forms the basis for a convenient and rapid method to monitor CBAH activity.

Cloning and characterization of a conjugated bile acid hydrolase gene from Clostridium perfringens

The gene encoding a conjugated bile acid hydrolase (CBAH) from Clostridium perfringens 13 has been cloned and expressed in Escherichia coli, and its nucleotide sequence has been determined. Nucleotide and predicted amino acid sequence analyses indicated that the gene product is related to two previously characterized amidases, a CBAH from Lactobacillus plantarum (40% identity) and a penicillin V amidase from Bacillus sphaericus (34% identity). The product is apparently unrelated to a CBAH from C. perfringens for which N-terminal sequence information was determined. The gene product was purified from recombinant E. coli and used to raise antibody in rabbits. The presence of the protein in C. perfringens was then confirmed by immunoblot analysis. The protein was shown to have a native molecular weight of 147,000 and a subunit molecular weight of 36,100, indicating its probable existence as a tetramer. Disruption of the chromosomal C. perfringens CBAH gene with a chloramphenicol resistance cartridge resulted in a mutant strain which retained partial CBAH activity. Polyacrylamide gel electrophoresis followed by enzymatic activity staining and immunoblotting indicated that the mutant strain no longer expressed the cloned CBAH (CBAH-1) but did express at least one additional CBAH (CBAH-2). CBAH-2 was immunologically distinct from CBAH-1, and its mobility on native polyacrylamide gels was different from that of CBAH-1. Furthermore, comparisons of pH optima and substrate specificities of CBAH activities from recombinant E. coli and wild-type and mutant C. perfringens provided further evidence for the presence of multiple CBAH activities in C. perfringens.

Gene disruption in Lactobacillus plantarum strain 80 by site-specific recombination: isolation of a mutant strain deficient in conjugated bile salt hydrolase activity

A chloramphenicol-resistance gene (cml) was introduced into the Lactobacillus plantarum gene encoding conjugated bile acid hydrolase (cbh) on a ColE1 replicon. This plasmid which is nonreplicative in Lactobacillus was used to transform L. plantarum strain 80. A homologous double cross-over recombination event resulted in replacement of the chromosomal cbh gene by the cml-containing cbh gene. The transformants obtained were unable to synthesize active conjugated bile acid hydrolase (Cbh). The Cbh- CmlR phenotype was stably maintained for more than 100 generations under nonselective conditions.

Cloning and expression of a conjugated bile acid hydrolase gene from Lactobacillus plantarum by using a direct plate assay.

The conjugated bile acid hydrolase gene from the silage isolate Lactobacillus plantarum 80 was cloned and expressed in Escherichia coli MC1061. For the screening of this hydrolase gene within the gene bank, a direct plate assay developed by Dashkevicz and Feighner (M. P. Dashkevicz and S. D. Feighner, Appl. Environ. Microbiol. 53:331-336, 1989) was adapted to the growth requirements of E. coli. Because of hydrolysis and medium acidification, hydrolase-active colonies were surrounded with big halos of precipitated, free bile acids. This phenomenon was also obtained when the gene was cloned into a multicopy shuttle vector and subsequently reintroduced into the parental Lactobacillus strain. The cbh gene and surrounding regions were characterized by nucleotide sequence analysis. The deduced amino acid sequence was shown to have 52% similarity with a penicillin V amidase from Bacillus sphaericus. Preliminary characterization of the gene product showed that it is a cholylglycine hydrolase (EC 3.5.1.24) with only slight activity against taurine conjugates. The optimum pH was between 4.7 and 5.5. Optimum temperature ranged from 30 to 45 degrees C. Southern blot analysis indicated that the cloned gene has similarity with genomic DNA of bile acid hydrolase-active Lactobacillus spp. of intestinal origin.

Pigment Gallstone Disease

Black and brown pigment gallstones are morphologically, compositionally, and clinically distinct. Black stones form primarily in the gallbladder in sterile bile and are associated with advanced age, chronic hemolysis, alcoholism, cirrhosis, pancreatitis, and total parenteral nutrition. Brown stones form not only within the gallbladder but also within the intrahepatic and extrahepatic ducts; they are uniformly infected with enteric bacteria and are usually associated with ascending cholangitis. Brown stones are related to juxtapapillary duodenal diverticula and are the predominant type of de novo common bile duct stones. Cholecystectomy is usually curative in black pigment stone disease, whereas stones often recur after cholecystectomy for brown stone disease. The pathogenesis of black stones is probably related to nonbacterial, nonenzymatic hydrolysis of bilirubin conjugates. At the pH of bile, this results in two monohydrogenated bilirubin anions that precipitate with calcium ions. Bilirubin monoconjugates that are increased in several conditions, such as Gilbert's syndrome and chronic hemolysis, may play a pivotal role in black stone formation as a source of unconjugated monohydrogenated bilirubin and as a possible co-precipitant with calcium. The precipitation of calcium carbonate and phosphate is influenced by local gallbladder factors. Brown pigment stones are formed in bile infected with enteric bacteria that elaborate hydrolytic enzymes: beta-glucuronidase, phospholipase A, and conjugated bile acid hydrolase. The resulting anions of bilirubin and fatty acids form insoluble calcium salts. We used nb/nb mice with a chronic hemolytic anemia as a model of hemolysis-induced black stone disease. The presence of 40% bilirubin monoconjugates in mouse gallstones indicated the importance of this moiety in the pathogenesis of black stones. Other data obtained by marrow transplantation experiments in mice revealed the relative importance of genotype versus the hemolytic anemia on determinants such as biliary bile acid composition and mucin secretory glands in the mouse gallbladder neck. Additional physical chemical studies of the interaction of unconjugated bilirubin in model bile solutions will be helpful in further delineating the pathogenesis of both black and brown pigment gallstones.

Pigment gallstone disease.

Black and brown pigment gallstones are morphologically, compositionally, and clinically distinct. Black stones form primarily in the gallbladder in sterile bile and are associated with advanced age, chronic hemolysis, alcoholism, cirrhosis, pancreatitis, and total parenteral nutrition. Brown stones form not only within the gallbladder but also within the intrahepatic and extrahepatic ducts; they are uniformly infected with enteric bacteria and are usually associated with ascending cholangitis. Brown stones are related to juxtapapillary duodenal diverticula and are the predominant type of de novo common bile duct stones. Cholecystectomy is usually curative in black pigment stone disease, whereas stones often recur after cholecystectomy for brown stone disease. The pathogenesis of black stones is probably related to nonbacterial, nonenzymatic hydrolysis of bilirubin conjugates. At the pH of bile, this results in two monohydrogenated bilirubin anions that precipitate with calcium ions. Bilirubin monoconjugates that are increased in several conditions, such as Gilbert's syndrome and chronic hemolysis, may play a pivotal role in black stone formation as a source of unconjugated monohydrogenated bilirubin and as a possible co-precipitant with calcium. The precipitation of calcium carbonate and phosphate is influenced by local gallbladder factors. Brown pigment stones are formed in bile infected with enteric bacteria that elaborate hydrolytic enzymes: beta-glucuronidase, phospholipase A, and conjugated bile acid hydrolase. The resulting anions of bilirubin and fatty acids form insoluble calcium salts. We used nb/nb mice with a chronic hemolytic anemia as a model of hemolysis-induced black stone disease. The presence of 40% bilirubin monoconjugates in mouse gallstones indicated the importance of this moiety in the pathogenesis of black stones. Other data obtained by marrow transplantation experiments in mice revealed the relative importance of genotype versus the hemolytic anemia on determinants such as biliary bile acid composition and mucin secretory glands in the mouse gallbladder neck. Additional physical chemical studies of the interaction of unconjugated bilirubin in model bile solutions will be helpful in further delineating the pathogenesis of both black and brown pigment gallstones.