Reactions of Water-Soluble Metalloporphyrins with the Serum Protein, Hemopexin

Article abstract of DOI:10.1021/ja00531a042

Rabbit hemopexin is capable of binding a wide variety of synthetic porphyrins and metalloporphyrins including those of the meso-substituted variety.The protein has a requirement for negatively charged peripheral substituents on the porphyrin suggesting that the binding site(s) have a residual positive charge.The stable porphyrin-protein complexes are 1:1 and involve monomeric porphyrin units regardless of the state of aggregation of the porphyrin in solution.The kinetics of the reactions of tetra(4-sulfonatophenyl)porphinatoferrate(III) (Fe^IIITPPS) with rabbit hemopexin has been studied as a function of pH.The protein is capable of interacting with either monomers or dimers leading to substantial changes in metalloporphyrin absorbance and protein fluorescence.When the bound Fe^IIITPPS is dimeric, a much slower process ensues in which the intermediate complex loses a monomer unit to form the stable product.