Figure 1: APRT deficiency causes 2,8-DHA formation. In humans, adenine cannot be converted to adenosine as hypoxanthine to inosine; the only alternative pathway in APRT deficiency is oxidation of adenine to 2,8-DHA by xanthine dehydrogenase (XDH) via the 8-hydroxy-intermediate. Allopurinol acts by inhibiting XDH, thus preventing 2,8-DHA synthesis. AMP: adenosine monophosphate, GMP: guanosine monophosphate, HPRT: hypoxanthine-guanine phosphoribosyltransferase, IMP: inosine monophosphate, PRPP: 5-phosphoribosyl-1-pyrophosphate.