Metabolism of diosgenin-derived saponins: implications for hepatogenous photosensitization diseases in ruminants

Disposition of sapogenins in the gastrointestinal tract of a lamb was determined after daily dosing of a diosgenin-derived saponin extract during late spring. The saponin (26.1 g) was obtained by ethanolic extraction of powdered Costus speciosus rhizomes followed by butanol–water partitioning. Upon completion of dosing, samples of gut contents and internal organs were subjected to chemical analysis and histological examination. No clinical symptoms, changes in serum chemistry, or microscopic changes were observed. Dosing of a saponin extract revealed that ruminal metabolism resulted in conversion of diosgenin to epismilagenin, smilagenone, smilagenin and tigogenin. Electrospray mass spectrometry of selected ethanolic ruminal extracts showed the absence of diosgenin-derived saponins 1 h after dosing, indicating that rapid hydrolysis of saponins by ruminal micro-flora occurred. Epismilagenin was the dominant conjugated sapogenin detected in the bile sample, no free sapogenins were identified. Three distinct zones of metabolic activity were identified in the gastrointestinal tract of the dosed lamb. The rumen appeared to be a region where the ingested diosgenin-derived saponins underwent hydrolysis and reduction and were converted to tigogenin, smilagenin, smilagenone and epismilagenin. The duodenum, jejunum and ileum formed an absorption and secretion zone where there appeared to be variation in both the concentrations of free and conjugated sapogenins. Sapogenins appeared to undergo continued epimerization in the caecum and colon. The plant saponins administered in this study were readily metabolized, but did not appear to induce crystal-associated hepatogenous photosensitization in the dosed lamb.