Binding of Protoporphyrin to hemoglobin in red blood cells of patients with erythropoietic protoporphyria.

Virtually all protoporphyrin in erythrocytes of patients with erythropoietic protoporphyria is bound to hemoglobin. The maximum of the fluorescence excitation spectrum of this protoporphyrin-hemoglobin complex shifted, with increasing concentration, from 405 nm to 389 nm. A similar shift was observed when titrating a solution of free protoporphyrin with hemoglobin. The Soret maximum of free protoporphyrin itself, on the other hand, was not concentration-dependent. These observations indicate that spectrofluorometric measurements do not allow conclusions concerning the mode of protoporphyrin binding to hemoglobin. Experiments on protoporphyrin exchange between the hemoglobins A, F and S reinforced the previously drawn conclusion that protoporphyrin is bound to hemoglobin at the heme-binding sites.