Human serum albumin-bound synthetic hemes as an oxygen carrier: Determination of equilibrium constants for heme binding to host albumin

Human serum albumin (HSA) incorporating synthetic tetraphenylporphinato-iron(II) derivatives (FeP1 or FeP2) can bind and release oxygen reversibly under physiological conditions (in aqueous media, pH 7.4, 37°C). The maximal binding ratio of FeP1/HSA was estimated to be eight, and the stepwise equilibrium constants for FeP1 binding to HSA (K₁-K₈) ranged from 1.2×10⁶ to 1.3×10⁴ M^-1. The major binding sites of FeP1 are presumably identical to those of hemin, bilirubin and long-chain fatty acids. The O₂-binding ability of the HSA-FeP can be regulated by changing the molecular structure of the incorporated hemes. The half-lifetime of the O₂-coordinated FeP2 in HSA was significantly longer than that of HSA-FeP1.