Fluorimetric study of the binding of protoporphyrin to haemopexin and albumin

Abstract

Fluorescence spectra of protoporphyrin bound to its most affinitive site on human serum albumin, bound to human hemopexin and dissolved in human plasma reveal that, when present in plasma, at least 90% of this porphyrin is bound to albumin. Human serum albumin binds protoporphyrin with an affinity KA = 3 .times. 109 M-1 in phosphate-buffered saline. The affinity of hemopexin for protoporphyrin is 4 times smaller. Apparently less than 1% of plasma protoporphyrin is bound to hemopexin. Implications of the data for protoporphyrin transport and clearance are discussed.