Relationship between DHR oxidation and Fe in transferrin-containing solution, plasmalike medium, and thalassemic serum. Top: A 23-μM human apo-transferrin solution (ϵ₂₇₈ = 93 000 M^-1 cm^-138 ) in HBS containing 10 mM Na₂CO₃ was reacted at room temperature for 20 minutes with Fe (Fe/NTA 1:10) to give final Fe concentrations of 0 to 82.5 μM and tested for formation of Fe-transferrin by absorption at 465 nm (right scale, filled symbols) and for DHR oxidation rate (left scale, fluorescence units/minute, FU × minute^-1, open symbols) as described in “Materials and methods.” The concentration of Fe required to reach maximal A₄₆₅, corresponding to 100% transferrin saturation, was 45 μM (theoretical 46 μM, based on 2 iron-binding sites per transferrin). Bottom: Fe was added as Fe/NTA to a solution of PLM (circles) or a mixture of 5 different TH sera (stars) or normal sera N (squares) supplemented (empty symbols) or not (filled symbols) with 50 μM L1. The estimated transferrin saturation values for the mixed thalassemia sera were 85% and for normal sera 28%. DHR oxidation rates were obtained as described in “Materials and methods.”