Enzyme defects in hereditary porphyria.

Heme is an important prosthetic group for proteins concerned with energy metabolism. All cells in the body probably make heme, but nucleated erythroid and hepatic cells have been studied the most. Feedback control of heme formation differs in the red cells and in the liver. About eight enzymes have a place in the formation of heme. Defects in the enzyme pathways may be the result of genetic abnormalities and phenotypically occur as hereditary porphyrias. If the major defect occurs in the red cell line, erythropoietic porphyrias occur; if the liver has the major defect, than hepatic porphyrias are present. There are probably three erythropoietic porphyrias and four hepatic porphyrias which are genetically determined. However, some are not clearly classified,--with erythropoietic protoporphyria involving hepatic and erythroid cells and porphyria cutanea tarda not being a clear cut genetic abnormality, at least some of the time. Elucidation of the genetic enzymatic defects introduces new diagnostic tools and also has led to at least one revolutionary new treatment for some hepatic porphyrias.