Protoporphyrins are tetrapyrroles containing 4 methyl, 2 propionic, and 2 vinyl side chains. Protoporphyrin is produced by oxidation of the methylene bridge of protoporphyrinogen. Protoporphyrin IX is the only naturally occurring isomer; it is an intermediate in heme biosynthesis, combining with ferrous iron to form protoheme IX, the heme prosthetic group of hemoglobin. Protoporphyrin IX is created by the enzyme protoporphyrinogen oxidase. The enzyme ferrochelatase converts it into heme. Protoporphyrin IX naturally occurs in small amounts in feces. Protoporphyrin IX is also responsible for the brown pigment (ooporphyrin) of birds' eggs. Protoporphyrin IX is used as a branch point in the biosynthetic pathway leading to heme (by insertion of iron) and chlorophylls (by insertion of Mg and further side-chain transformation). Protoporphyrin IX can be used to treat liver disorders, mainly as the sodium salt. Under certain conditions, protoporphyrin IX can act as a neurotoxin, a phototoxin, and a metabotoxin. A neurotoxin causes damage to nerve cells and nerve tissues. A phototoxin causes cell damage upon exposure to light. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Chronically high levels of porphyrins are associated with porphyrias such as porphyria variegate, acute intermittent porphyria, and hereditary coproporphyria (HCP). In particular, it is accumulated and excreted excessively in the feces in acute intermittent porphyria, protoporphyria, and variegate porphyria. There are several types of porphyrias (most are inherited). Hepatic porphyrias are characterized by acute neurological attacks (seizures, psychosis, extreme back and abdominal pain, and an acute polyneuropathy), while the erythropoietic forms present with skin problems (usually a light-sensitive blistering rash and increased hair growth). The neurotoxicity of porphyrins may be due to their selective interactions with tubulin, which disrupt microtubule formation and cause neural malformations (PMID: 3441503).