A 60-year-old Japanese man was diagnosed as having hypouricemia at an annua
l health check-up. The routine laboratory data was not remarkable except th
at the patient's hypouricemia and plasma levels of xanthine and hypoxanthin
e were much higher than those of normal subjects. Furthermore, the patient'
s daily urinary excretion of xanthine and hypoxanthine was markedly increas
ed compared with reference values. The xanthine dehyrogenase activity of th
e duodenal mucosa was below the limits of detection. Nevertheless, allopuri
nol was metabolized to oxypurinol in vivo. Based on these findings, a subty
pe of classical xanthinuria (type I) was diagnosed. The xanthine dehyrogena
se protein was detected by Western blotting analysis. Sequencing of the cDN
A of the xanthine dehyrogenase obtained from the duodenal mucosa revealed t
hat a point mutation of C to T had occurred in nucleotide 445. This changed
codon 149 from CGC (Arg) to TGC (Cys), a finding that has not been previou
sly reported in patients with classical xanthinuria type I.