Ibuprofen and warfarin modulate allosterically ferrous human serum heme-albumin nitrosylation

Ferrous human serum heme-albumin (HSA-heme-Fe(II)) displays globin-like properties. Here, the effect of ibuprofen and warfarin on kinetics of HSA-heme-Fe(II) nitrosylation is reported. Values of the second-order rate constant for HSA-heme-Fe(II) nitrosylation (k_on) decrease from 6.3×10⁶M^-1s^-1 in the absence of drugs, to 4.1×10⁵M^-1s^-1 and 4.8×10⁵M^-1s^-1, in the presence of saturating amounts of ibuprofen and warfarin, respectively, at pH 7.0 and 20.0°C. From the dependence of k_on on the drug concentration, values of the dissociation equilibrium constant for ibuprofen and warfarin binding to HSA-heme-Fe(II) (i.e., K=3.2×10^-3M and 2.6×10^-4M, respectively) were determined. The observed allosteric effects could indeed reflect ibuprofen and warfarin binding to the regulatory fatty acid binding site FA2, which brings about an alteration of heme coordination, slowing down HSA-heme-Fe(II) nitrosylation. Present data highlight the allosteric modulation of HSA-heme-Fe(II) reactivity by heterotropic effectors.