Glycosyltransferase activity of PNPase. An assay mixture containing 100 mM Tris (pH 8), 0.15 mM thymine, 0.25 mM inosine, and 500 mU/mL of PNPase was prepared, added to sample and reference cuvettes to record a baseline in the 250–450 nm wavelength interval. 500 mU/mL of XOD were then added to the sample cuvette, and difference spectra were recorded every 30 min after enzyme addition.