HPRT1

HPRT1 Full Name

hypoxanthine phosphoribosyltransferase 1

HPRT1 Introduction

Hypoxanthine phosphoribosyltransferase 1 (HPRT1) is a key enzyme in the purine salvage pathway that catalyzes the conversion of hypoxanthine to inosine monophosphate (IMP) and guanine to guanosine monophosphate (GMP) using 5-phosphoribosyl-1-pyrophosphate (PRPP) as the phosphoribosyl donor. Encoded by the HPRT1 gene located on the X chromosome (Xq26.2-q26.3), HPRT1 is expressed ubiquitously in all tissues, with particularly high levels in the brain, where purine salvage activity is critical due to the limited capacity for de novo purine synthesis in neurons.

Figure 1. Strcuture of hypoxanthine phosphoribosyltransferase 1.

Enzymatic Mechanism and Purine Metabolism
HPRT1 catalyzes a magnesium-dependent nucleophilic substitution reaction in which the N9 nitrogen of the purine base attacks the C1 carbon of the PRPP ribose moiety, displacing pyrophosphate. The human enzyme functions as a homotetramer, with each 218-amino-acid subunit adopting a PRTase fold characteristic of type I phosphoribosyltransferases. The active site recognizes the purine base with high specificity for hypoxanthine and guanine, discriminating against adenine, which is salvaged by a separate enzyme (APRT). By recycling hypoxanthine and guanine into IMP and GMP, HPRT1 prevents the loss of purine bases from the nucleotide pool and simultaneously reduces the accumulation of their toxic oxidation products, including uric acid. The HPRT1 reaction also consumes PRPP, thereby indirectly limiting the rate of de novo purine synthesis through feedback inhibition mechanisms.

Lesch-Nyhan Syndrome and HPRT-Related Neurological Disorders
Complete or near-complete deficiency of HPRT1 activity causes Lesch-Nyhan syndrome (LNS), an X-linked recessive disorder characterized by hyperuricemia, gouty arthritis, uric acid nephrolithiasis, severe motor disability, and a distinctive behavioral phenotype of compulsive self-injurious biting of lips and fingers. Partial HPRT1 deficiency results in Kelley-Seegmiller syndrome, a milder condition with hyperuricemia and variable neurological involvement. The neurological manifestations of HPRT1 deficiency are thought to arise from dysfunction of the basal ganglia dopaminergic system, with neuropathological studies revealing profound dopamine depletion in the caudate nucleus and putamen. Treatment with allopurinol effectively manages hyperuricemia but does not ameliorate the neurological symptoms. Allogeneic hematopoietic stem cell transplantation and gene therapy strategies are being explored as potential disease-modifying interventions in preclinical models.

Alternate Names for HPRT1

HPRT1; hypoxanthine phosphoribosyltransferase 1; HPRT; HGPRT; hypoxanthine-guanine phosphoribosyltransferase; HGPRTase;