HGPRT is a salvage pathway enzyme for purine metabolism (another is specific for adenine). It catalyzes conversion of hypoxanthine to IMP and guanine to GMP (Figure 22.9).
When HGPRT is present in low levels, the patients usually experience gout, a painful condition arising from precipitation of uric acid in the joints. The relationship of a defect in HGPRT to gout is unclear, but may be related to the fact that when the enzyme is active, it uses PRPP. The enzyme activity is not completely missing in gout patients, just at a low level. Complete absence of HGPRT enzymatic activity is associated with Lesch-Nyhan syndrome.