Isopoptolidin, originally isolated from a Nocardiopsis sp., is a ring expanded isomer of apoptolidin. In water, an equilibrium mixture of apoptolidin:isoapoptolidin (~1.5:1) is formed within hours. In a rapid F0F1-ATPase assay, isoapoptolidin exhibited an IC50 of 17 μM compared to 0.7 μM for apoptolidin. Isoapoptolidin is an inhibitor of ATP5.
