Protoporphyrin IX is an intermediate in the Heme synthesis pathway and is formed from Protoporphyrinogen III in a reaction catalyzed by the enzyme Protoporphyrin III oxidase.  Iron in the ferrous state is added to Protoporphyrin IX to form heme in a reaction catalyzed by the enzyme ferrochelatase.  Protoporphyrin IX has been found to activate guanylate cyclase¹ and has been found to induce apoptosis in HeLa cells².