Association of erythrocyte glycophorin with protoporphyrin-IX derivatives: a fluorimetric study

Based on demonstrations that protoporphyrin-IX and its metabolic derivatives bilirubin and hemin bind to the red cell membrane, their association with glycophorin A, the main transmembrane sialoglycoprotein, was assessed. No interaction between bilirubin and glycophorin could be demonstrated but both protoporphyrin-IX and hemin were found to bind to the protein. Interaction of protoporphyrin-IX and glycophorin was demonstrated by changes in both ligand and protein fluorescence characteristics in the presence of the other reactant. Binding of hemin, (Fe+3-protoporphyrin-IX) with glycophorin was revealed by quenching of the proteins' intrinsic fluorescence intensity by hemin and a shifted Soret absorption of hemin in the presence of glycophorin. The association constants of protoporphyrin-IX and hemin with glycophorin at 25 degrees C were calculated as 2.5 +/- 0.5 x 10(6)M-1 and 1.4 +/- 0.4 x 10(6)M-1 respectively.