Sn-protoporphyrin IX (SnPP), an inhibitor of
heme oxygenase and a potential therapeutic agent for
neonatal hyperbilirubinemia, is bound tightly by
hemopexin. The apparent dissociation constant (Kd) at pH 7.4 is 0.25 +/- 0.15 microM, but estimation of the Kd for the SnPP-
hemopexin complex is hampered by the fact that at physiological pH SnPP exists as monomers and dimers, both of which are bound by
hemopexin. SnPP is readily displaced from
hemopexin by
heme (Kd less than 1 pM). The
hemopexin-SnPP interaction, like that of
heme-
hemopexin, is dependent on the
histidine residues of
hemopexin. However, as expected from the differences in the coordination chemistries of
tin and
iron, the stability of the histidyl-
metalloporphyrin complex is lower for SnPP-
hemopexin than for
mesoheme-
hemopexin. Nevertheless, when SnPP binds to
hemopexin, certain of the
ligand-induced changes in the conformation of
hemopexin which increase the affinity of the
protein for its receptor are produced. Binding of SnPP produces the conformational change in
hemopexin which protects the hinge region of
hemopexin from proteolysis, but SnPP does not produce the characteristic increase in the ellipticity of
hemopexin at 231 nm that
heme does. Competition experiments confirmed that
human serum albumin (apparent Kd = 4 +/- 2 microM) has a significantly lower affinity for SnPP than does
hemopexin. Appreciable amounts of SnPP (up to 35% in adults and 20% in neonates) would be bound by
hemopexin in the circulation, and the remainder of SnPP would be associated with
albumin due to the latter's high concentration in serum. Essentially no non-
protein-bound SnPP is present. Importantly, SnPP-
hemopexin binds to the
hemopexin receptor on mouse
hepatoma cells with an affinity comparable to that of
heme-
hemopexin and treatment of the
hepatoma cells with SnPP-
hemopexin causes a rapid increase in the steady state level of
heme oxygenase messenger RNA. These results show that
hemopexin participates in the transport of SnPP to
heme oxygenase and in its regulation by SnPP.