A cytochrome P450 CYP87A4 imparts sterol side-chain cleavage in digoxin biosynthesis.

Digoxin extracted from the foxglove plant is a widely prescribed natural product for treating heart failure. It is listed as an essential medicine by the World Health Organization. However, how the foxglove plant synthesizes digoxin is mostly unknown, especially the cytochrome P450 sterol side chain cleaving enzyme (P450), which catalyzes the first and rate-limiting step. Here we identify the long-speculated foxglove P450 through differential transcriptomic analysis. This enzyme converts cholesterol and campesterol to pregnenolone, suggesting that digoxin biosynthesis starts from both sterols, unlike previously reported. Phylogenetic analysis indicates that this enzyme arises from a duplicated cytochrome P450 CYP87A gene and is distinct from the well-characterized mammalian P450. Protein structural analysis reveals two amino acids in the active site critical for the foxglove P450's sterol cleavage ability. Identifying the foxglove P450 is a crucial step toward completely elucidating digoxin biosynthesis and expanding the therapeutic applications of digoxin analogs in future work.