Erythropoietic protoporphyria: two populations of reticulocytes, with and without protoporphyrin

Erythrocytes from patients with erythropoietic protoporphyria contain large amounts of protoporphyrin. The photosensitivity experienced by these patients is assumed to be due to a leakage of protoporphyrin from the erythrocytes and transfer to the skin, where protoporphyrin acts as a photosensitizer. The leakage of protoporphyrin from the erythrocytes has been offered as an explanation for the great variety in protoporphyrin content observed among erythrocytes in this disease. Based on density gradient separation of red cells, it has been concluded that all reticulocytes and young erythrocytes contain large amounts of protoporphyrin. From our results, density gradient centrifugation is not suitable for age separation of red cells from patients with erythropoietic protoporphyria. By developing a new method for isolation of reticulocytes and applying flow cytometry to determine protoporphyrin content in individual cells, it was observed that two populations of reticulocytes were present in patients with erythropoietic protoporphyria, one with and the other without protoporphyrin. The half-life of protoporphyrin in red cells was found to be 12-14 days, in contrast to 1-2 days described previously, suggesting a slower release of protoporphyrin from the red cells than previously anticipated.