Ursodeoxycholic acid (UDCA) is a clinically used drug which has a long history of practice in prevention and treatment of hepatobiliary diseases (Luo et al., 2022; Paumgartner and Beuers, 2002). As is known, the main pharmacological active ingredient contained in bear bile is ursodeoxycholic acid, which has been proved to be efficacious in dissolving cholesterol gallstones, curing primary biliary cholangitis, alcoholic liver, drug-induced hepatitis disease, cystic fibrosis and cholelithiasis, improving liver dysfunctions like transplantation, reflux gastritis and cholestasis in patients (Goossens and Bailly, 2019; Harms et al., 2019; Stefaniwsky et al., 1985). Recently, UDCA has been confirmed endowing unique effect on FXR inhibition, which could be explored as a potential drug to block SARS-CoV-2 infection (Brevini et al., 2023). Our previous work manifested that the UDCA-zein nanoparticles exerted a prominent amelioration efficacy on acute alcoholic liver injury in mice (Wang et al., 2023). Hereby, UDCA also shows great potentialities in anti-virous and anti-inflammation applications.

Till now, the natural ursodeoxycholic acid can only be obtained from bile of live bear or bear bile powder, which is very expensive and cruel, and unsustainable (Dutton et al., 2011, Zheng et al., 2017). The extraction method of UDCA from live bears not only violates the animal protection law but also has a long lead time and is far from meeting the market demand (Li et al., 2024). Current research has focused on synthetic approaches, including chemical and microbial enzymatic synthesis (Kollerov et al., 2016; Lou et al., 2016; Studer et al., 2016; Tonin and Arends, 2018). Since several protection and deprotection steps must be performed during chemical synthesis, it requires the usage of toxic and hazardous reagents in large quantities, which generate a certain degree of wastes and lead to various environmental problems (Tonin et al., 2018). Therefore, it is urgent to develop a new green route for preparation of ursodeoxycholic acid with eco-friendly and high-efficient transformation rates (Song et al., 2023).

At present, the comprehensive utilization of cheap slaughter by-products like gall bladder has aroused attention increasingly, and it is rich in bile acids especially of chenodeoxycholic acid (CDCA), which can be further converted to more-valued ursodeoxycholic acid by means of microbial conversion or bio-enzymatic catalysis through specific microorganisms. Among which, the 7α-differentiated isomeric bacteria, belonging to Clostridium perfringens (Doden et al., 2018, Lou et al., 2016, Medici et al., 2002, Zheng et al., 2015), firstly isolated from the feces in a healthy volunteer (Lepercq et al., 2004), were confirmed to be able to convert chenodeoxycholic acid to ursodeoxycholic acid. Besides, Bakonyi and Hummel (2017) isolated NADP+ -dependent 7α-hydroxysteroid dehydrogenase (7α-HSDH) from Clostridium difficile and revealed it could convert CDCA to 7-ketocholic acid (7K-LCA) on a small scale, providing a new strategy for fabrication of intermediates of bile acid metabolism. Furthermore, Zheng et al. (2015) found a new 7β-hydroxysteroid dehydrogenase (7β-HSDH) from Clostridium tumefaciens, which could realize a high UDCA conversion (yield of 98 %) when using CDCA as the substrate via a two-step biotransformation strategy. These studies indicate UDCA’s fabrication by biocatalytic conversion has promising application in practice.

In this study, the wild-type aerobic Xanthomonas maltophilia (X. maltophilia) was used as the original strain, while chenodeoxycholic acid as the substrate for the purpose of catalytic conversion, providing a new way for bioconversion of ursodeoxycholic acid. The reaction involved in the biotransformation is as follows:CDCA+NAD+⟶7α−HSDH7K−LCA+NADPH+H+⟶7β−HSDHUDCA+NADP+

As a type of hydrophilic cage-like molecule, β-cyclodextrin (β-CD) could provide a solution for substrate’s solubility problem and facilitate the reaction’s proceeding (Esmaeilpour et al., 2021, Rapp et al., 2021, Shityakov and Förster, 2013). Herein, considering of low solubility of the substrate and its low utilization rate in the conversion system, chenodeoxycholic acid was encapsulated in β-CD cavities to form chenodeoxycholic acid/β-cyclodextrin (CDCA/β-CD) inclusion complexes, so as to solve the problem of small substrate solubility. Moreover, the biomaterials of chitosan chloride (CHC) and sodium cellulose sulphate (NaCS) (Wu et al., 2019) were used to immobilize the starting strain, which could facilitate the product separation and the conversion system’s reusing. A small-scale continuous reactor was successfully built, and the immobilized microspheres were evaluated for multiple conversion of ursodeoxycholic acid and intermediate 7-ketocholic acid (7K-LCA).