In vitro metabolism of the cardiotonic steroids gomphogenin and calactin

The metabolism of gomphogenin and calactin was studied in vitro using respectively microsomes and the S9 fraction of homogenates from rat liver. These two substrates were previously shown to be in vitro and in vivo metabolites of gomphoside, a cardiotonic steroid belonging to a class of 5 alpha-cardenolide glycosides with doubly-linked hexosulose sugars. Structures of new metabolites were elucidated using 400 MHz 1H-NMR and chemical ionization mass spectrometry, while known compounds were identified by direct comparison. The major metabolite isolated from gomphogenin (2 alpha-hydroxyuzarigenin) metabolism was the oxidation product 2-oxo-uzarigenin which was further oxidized metabolically to 4 alpha-hydroxy-2-oxo-uzarigenin. Other metabolites were 2 alpha-hydroxyuzarigenone and its reduction product 3-epigomphogenin. Calactin was oxidized in vitro to 10-carboxyl-19-norgomphoside, the predominant metabolite, and underwent cleavage of the doubly-linked sugar to yield calotropagenin.