Incorporation of tetrakis(o-pivalamido)phenylporphyrinatoiron(II) derivatives with a covalently linked axial imidazole (FeP) into human serum albumin (HSA) provides a new type of artificial hemoprotein (HSA-FeP) that binds and releases dioxygen reversibly under physiological conditions (in aqueous media, pH 7.4, 37 degreesC) and in a fashion similar to hemoglobin and myoglobin. The HSA host adsorbs a maximal eight FeP molecules, and their stepwise equilibrium constants (K1-K8) range from 1.2 x 10(6) to 1.3 x 10(4) M-1. The major binding sites of the synthetic hemes are identical to those of hemin, bilirubin, and long-chain fatty acids. The red-colored solution of HSA-FeP was stored for three months at 4 degreesC and could be kept as a freeze-dried powder for more than six months. The solution properties [[HSA]: 5 wt %, FeP/HSA = 1-8 (mol/mol)] satisfy the physiological requirements for dioxygen infusion for potential clinical use; the specific gravity is 1.013, and the viscosity is 1.1 cP. Mixing the solution with human blood does not induce any coagulation and precipitation. On the basis of the gel permeation chromatography, CD spectroscopy, and IEF measurements, the molecular size, second-order structure, and surface charge distribution of the HSA-FeP conjugate are constant and independent of the binding numbers of heme molecules. Furthermore, the O2-coordination structure of FeP embedded into certain hydrophobic domains of the albumin was confirmed by resonance Raman spectroscopy.