Figure 3. Hb oxidation by the xanthine/xanthine oxidase system.

The upper panel shows a typical spectral change observed when 0.1 mM Hb in 50 mM potassium phosphate, 0.5 mM xanthine, 50 mM KCN and 1 mM EDTA (pH 7.3) is exposed to 0.5 units of xanthine oxidase at 37 °C. Spectra were obtained every 5 s. The lower panel shows the absorbance changes recorded at λ=576 nm to measure the maximal oxidation rate. The Figure also shows the decay of pO₂ (right axis) when the mixture is reacted in the absence of Hb. The maximal oxidation rate of Hb corresponds to the linear portion of the fall in pO₂.