Phosphodiesterase enzymes (PDE) catalyze the degradation of cGMP, a molecular regulator of ion channels, cellular apoptosis (programmed cell death) and smooth muscle tissue. In particular, drugs that inhibit PDE-5 have been successful in the treatment of erectile dysfunction. This figure shows a model vardenafil (Levitra) bonded to the active site of PDE-5, which includes a magnesium(2+) ion (pink) and a zinc(2+) ion (grey). For vardenafil the atoms are represented as spheres and are colour-coded: carbon (black), hydrogen (white), nitrogen (blue), oxygen (red) and sulphur (yellow).