In the introduction, the sources and use of diosgenin and other steroid raw materials in the Pharmaceutical Industry are reviewed. The biotransformations of steroids, and in particular diosgenin, are described. Microorganisms, selected from natural sources and from culture collections, were screened for activity against diosgenin. Species were found which had not been reported to attack the steroid. Biotransformation products of diosgenin were isolated and their structures elucidated. The major products were diosgenone and 1-dehydrodiosgenone; androstenes were also formed. The formation of diosgenone from diosgenin was shown to be mediated by a cholesterol oxidase enzyme which was inducible in growing cells. Methods were investigated to prevent the total degradation of the diosgenin steroid nucleus by Mycobacterium phlei. Addition of the chelating agent, a, a -bipyridyl, resulted in a 7% yield of androstenes. However, mutagenesis of M. phlei with N-methyl-N-nitro-N-nitrosoguanidine was shown to be of more promise. To optimise the biotransformation process, the effects of the physical form of the diosgenin and the physiological condition of the microorganisms (including their immobilisation in calcium alginate) were studied. Addition of an ethanolic solution of diosgenin to stationary phase cells in a rich medium, was found to give the best rate of biotransformation. Methods were investigated to increase the solubility of diosgenin in the fermentation medium. Water-immiscible organic solvents allowed an increased initial biotransformation activity with Nocardia rhodochrous and M. phlei. The use of crude water-soluble glycosidic precursors of diosgenin resulted in a different pattern of biotransformations. Fusarium solani produced diosgenin (84% efficiency), thus avoiding an acid hydrolysis step. Silastic resin was used in microbial fermentations. It provided a novel and convenient method for the collection of hydrophobic products from biotransformations. Diosgenin, produced from its glycosidic precursors by F. solani, was collected by the resin and transferred in it to a culture of M. phlei which further transformed the diosgenin to androstenes. These remained within the resin and could be readily recovered from it with solvents. Silastic resin offers considerable advantages in its retention of intermediate and then final product, in a sequence of biotransformations with successive organisms.
|Date of Award||1982|