Fig. 1.

Oxidation of guanine by xanthine oxidase (XOD). The reactions were performed as described in Materials and methods. (A) The absorption spectrum of 0.3 mM guanine was first determined in 100 mM Tris-HCl pH 8 (black line). XOD was then added to the cuvette, and spectra were recorded at 5, 10, 20, and 40 min after enzyme addition (green, blue, red, and dark-green lines, respectively). A reference cuvette was used containing 100 mM Tris-HCl pH 8 and XOD. (B) Difference between the spectra recorded before and after 40 min of XOD addition.