Adsorption of Lithocholic Acid to Fusarium equiseti M41 as an Essential Process in Its Conversion to Ursodeoxycholic Acid.

Fusarium equiseti M41 converts lithocholic acid to ursodeoxycholic acid. Adsorption of lithocholic acid particles to mycelia of F. equiseti M41 is essential in the conversion of lithocholic acid to ursodeoxycholic acid. Production of ursodeoxycholic acid was negligible when particles of lithocholic acid were absent. As the concentration of lithocholic acid particles increased, both the amount of mycelium-bound lithocholic acid and the production of ursodeoxycholic acid increased hyperbolically (K(1/2) = 1.9 g/liter and K(m(apparent) ) = 1.9 g/liter. A fluorescent lithocholic acid derivative was used to confirm that insoluble particles of lithocholic acid attached to the surface of the mycelia. The hydrophobic nature of this binding was estimated from the close relationship observed between the hydrophobicity of bile acids and their binding capacity to the mycelia. By repeated washing with 30% dimethyl sulfoxide, two binding modes of lithocholic acid were distinguished, i.e., surface binding (59% of bound lithocholic acid) and tight binding (41% of bound lithocholic acid). From the amount of tightly bound lithocholic acid, the intracellular concentration of lithocholic acid was calculated to be 1,433-fold higher than its saturating concentration in the reaction mixture, thus promoting effective conversion to ursodeoxycholic acid in the mycelia. Several lines of evidence indicated that glycoproteins of the cell wall participated in the binding of lithocholic acid.