The P450 Sterol Side Chain Cleaving Enzyme (P450scc) for Digoxin Biosynthesis in the Foxglove Plant Belongs to the CYP87A Family

Abstract

Digoxin from the foxglove plant is a commonly prescribed plant natural product for treating heart failure and atrial fibrillation. Despite its medicinal prominence, how foxglove synthesizes digoxin is largely unknown, especially the cytochrome P450 sterol side chain cleaving enzyme (P450scc), which catalyzes the first and rate-limiting step in this pathway. Here we report the identification of the foxglove P450scc, the gatekeeping enzyme that channels sterols to digoxin. This enzyme converts both cholesterol and campesterol to pregnenolone, thus explaining how pregnenolone is synthesized in plants. Phylogenetic analysis indicates that this enzyme arose from a duplicated CYP87A gene and does not share clear homology with the mammalian P450scc. Identifying this long-speculated plant P450scc enzyme suggests that the digoxin biosynthetic pathway starts from both cholesterol and phytosterols, instead of just cholesterol as previously thought. The identification of this gatekeeping enzyme is a key step towards complete elucidation of digoxin biosynthesis and expanding the therapeutic applications of digoxin analogs in future work.