A heme-containing flavoprotein (FAD). Other purine and pyrimidine nucleosides (as well as 2'-deoxyribonucleosides and arabinosides) are substrates, but ribose and nucleotides are not substrates. The overall reaction takes place in two separate steps, with the 5'-dehydro nucleoside being released from the enzyme to serve as substrate for the second reaction. This enzyme differs from EC 1.1.3.28, nucleoside oxidase, as it produces hydrogen peroxide rather than water.