Erythropoietic protoporphyria (EPP) is an inherited photocutaneous disorder first described in 1961.1 The typical syndrome includes cutaneous photosensitivity, greatly increased erythrocyte and fecal protoporphyrin, and a variable but often marked increase in plasma protoporphyrin. The urine is normal.
Although the photosensitivity is clearly due to protoporphyrin, the site of formation of this porphyrin has been questioned. Initially it was suggested that the photosensitivity was associated with the high red cell porphyrin. However, in keeping with the other cutaneous porphyrias, increased porphyrin in the plasma was subsequently demonstrated.2,3 The possibility that the plasma porphyrin causes the photosensitivity and is synthesized at a nonerythropoietic site was suggested by Redeker and Bryan in 1964.4
The rate-limiting enzyme for porphyrin biosynthesis is aminolevulinic acid synthetase (ALA-S). Hepatic ALA-S is an inducible enzyme in patients with acute intermittent porphyria (AIP) and its rate of synthesis is reciprocal with carbohydrate or protein intake
Redeker AG, Sterling RE. The "Glucose Effect" in Erythropoietic Protoporphyria. Arch Intern Med. 1968;121(5):446-448. doi:10.1001/archinte.1968.03640050056011