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 < cobalt-deuteroporphyrin IX < iron-deuteroporphyrin IX (deuteroheme) N 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 pM and 1 PM hemopexin indicates that the dissociation constant (&) for the interaction with deuteroheme 5 lo-* 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 pM. 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 of -2 pM. Hemopexin can be implicated definitely in the metabolism of heme and possibly also in that of its precursors.