Warfarin modulates the nitrite reductase activity of ferrous human serum heme–albumin

Abstract

Human serum heme–albumin (HSA–heme–Fe) displays reactivity and spectroscopic properties similar to those of heme proteins. Here, the nitrite reductase activity of ferrous HSA–heme–Fe [HSA–heme–Fe(II)] is reported. The value of the second-order rate constant for the reduction of ${\text{NO}}_{2}^{ - }$ to NO and the concomitant formation of nitrosylated HSA–heme–Fe(II) (i.e., k _on) is 1.3 M⁻¹ s⁻¹ at pH 7.4 and 20 °C. Values of k _on increase by about one order of magnitude for each pH unit decrease between pH 6.5 to 8.2, indicating that the reaction requires one proton. Warfarin inhibits the HSA–heme–Fe(II) reductase activity, highlighting the allosteric linkage between the heme binding site [also named the fatty acid (FA) binding site 1; FA1] and the drug-binding cleft FA2. The dissociation equilibrium constant for warfarin binding to HSA–heme–Fe(II) is (3.1 ± 0.4) × 10⁻⁴ M at pH 7.4 and 20 °C. These results: (1) represent the first evidence for the ${\text{NO}}_{2}^{ - }$ reductase activity of HSA–heme–Fe(II), (2) highlight the role of drugs (e.g., warfarin) in modulating HSA(–heme–Fe) functions, and (3) strongly support the view that HSA acts not only as a heme carrier but also displays transient heme-based reactivity.