Liposomal heme as oxygen carrier under semi-physiological conditions

A meso-5,10,15,20-tetra(o-pivalamidophenyl)porphyrinatoiron(II) complex of a 2-methylimidazole, substituted in the 1-position with a hydrophobic group, was incorporated into a lipid bilayer of phosphatidylcholine (liposomal heme). The liposomal heme reversibly bound molecular oxygen in neutral aqueous media at 37 °C, and the half-life of the oxygen adduct was half a day. The oxygen-binding affinity (p_1/2) for the liposomal heme was ca. 50 mmHg, which is similar to that of hemoglobin in blood. The incorporation and structure of the liposomal heme were confirmed by physico-chemical methods, which indicated that the hydrophobic environment of the inner region of liposome protected the oxygen adduct from its proton-driven oxidation.