Hb β-chain contains the putative CD163 binding site. Human HbA₀ was cleaved by CNBr, and the 5 resultant fragments (A) were purified and analyzed for their ability to compete with Hb-Hpfl (50 nM) endocytosis by CD163. When added at a 100-fold molar-excess concentration, the peptide corresponding to the C-terminal Hb β-chain (β-2) had a quantitatively similar potency to antagonize Hb-Hpfl uptake, as does complete HbA₀ (B). Unlike native HbA₀, the β-2 peptide does not form complexes with Hp, as evidenced by its inability to support the uptake of fluorescent Hp. CD163-HEK293 cells were incubated for 30 minutes with Alexa 488-Hp (Hpfl; 5μg/mL) with or without (control) HbA₀ (10 μg/mL) or β-2 peptide (10 μg/mL), respectively (C).