Molecular aspects of the inherited porphyrias

The porphyrias are diseases due to marked deficiencies of enzymes of the ha em biosynthetic pathway (Fig. 1). Except for the first enzyme of the pathwa y, delta-aminolevulinate synthase (ALAS), deficiencies in seven other enzym es are associated with the various forms of porphyria (Fig. 2). Porphyrias can be classified as either hepatic or erythroid, depending on the major si te of production of porphyrins or their precursors. The pathogenesis of all inherited porphyrias has now been defined at the molecular level, and it i s clear that there is a great deal of genetic heterogeneity in each porphyr ia [1].