The enzyme involved in the cytosolic pathway was
evidenced to be xanthine oxidoreductase (XOR) because
of its susceptibility to inhibitory effects of allopurinol
and by the ability of the process to occur only when the
presence of NAD+
was accompanied by substrates of
the XOR form of the enzyme such as hypoxanthine,
xanthine, caffeine, theobromine, theophylline or 1,7-dimethylxanthine