Binding of Porphyrins to Rabbit Hemopexin and Albumin

The binding of porphyrins by two serum proteins, hemopexin and albumin, has been compared using a fluorescence method in order to clarify the biological roles of the proteins in the disposal of these pigments. The affinity of hemopexin for metalloporphyrins increases in the order: nickel-deuteroporphyrin IX l cobalt-deuteroporphyrin IX l iron-deuteroporphyrin IX (deuteroheme) ≃ iron-protoporphyrin IX (heme); and the binding of nickel-deuteroporphyrin IX is approximately equal to that of deuteroporphyrin IX and protoporphyrin IX. The data are consistent with a single binding site on the hemopexin molecule for all porphyrins except cobalt-deuteroporphyrin. The titration behavior of 0.5 µm and 1 µm hemopexin indicates that the dissociation constant (Kd) for the interaction with deuteroheme ≤ 10-8 m. The results of both dialysis equilibrium and fluorescence quenching experiments demonstrate that the Kd for the binding of deuteroporphyrin IX by hemopexin is 1 to 2 µm. Like hemopexin, albumin binds deuteroheme more strongly than protoporphyrin. The avidity of hemopexin for deuteroheme, an analogue whose structure is closely related to heme, is at least 60-fold higher than that of albumin. The affinity of both proteins for protoporphyrin is similar and characterized by a Kd ∼2 µm. Hemopexin can be implicated definitely in the metabolism of heme and possibly also in that of its precursors.