Figure 6.

Intramolecular electron transfer within xanthine oxidase as studied by pH jump. Left, the absorption spectra of oxidized enzyme and enzyme reduced to the level of approximately 50% at pH 10 (solid line) and 6 (dashed line). Center, the kinetic transient seen at 525 nm on jumping the from pH 10 to pH 6. Right, the [low pH]-minus-[high pH] difference spectrum of partially reduced xanthine oxidase, with the extent of the absorbance change observed kinetically at 525 nm indicated by the arrow; ~50% of the total absorbance change occurs in the ~2-ms dead time of the stopped-flow instrument. After Hille and Massey [21].