Intracellular PRPP

Lesch-Nyhan syndrome, an overproduction hyperuricemia characterized by frequent episodes of uric acid hthiasis and a bizarre syndrome of self-mutilation, reflects a defect in hypoxanthme-guanine phosphoribo-syl transferase, an enzyme of purine salvage (Figure 34—4). The accompanying rise in intracellular PRPP results in purine overproduction. Mutations that decrease or abohsh hypoxanthine-guanine phosphoribosyltrans-ferase activity include deletions, frameshift mutations, base substitutions, and aberrant mRNA splicing. 

Increased levels of intracellular PRPP enhance de novo purine biosynthesis. For example, in patients with HPRT deficiency, the fibroblasts show accelerated rates of purine formation. Several mutations of PRPP synthetase, which exhibit increased catalytic activity with increased production of PRPP, have been described in gouty subjects. 

Despite structural diversity in the superactive enzymes of individual families, studies of PRPP and purine metabolism carried out both vivo and in cells cultured from affected hemizygous males support the idea that a common mechanism accounts for the association of PRPP synthetase superactivity with uric acid overproduction. Increased intracellular PRPP concentrations and rates of PRPP generation as well as increased rates of all PRPP-dependent purine nucleotide synthetic processes are constant accompaniments of enzyme superactivity. These findings suggest a scheme to explain the association of the enzyme defect with uric acid overproduction PRPP synthetase superactivity -> increased intracellular PRPP generation and concentration > increased rate of purine nucleotide synthesis excessive uric acid synthesis. 

This is in agreement with our results obtained in human cultured lymphoblasts. Oxipurinol inhibits orotidyl decarboxylase(ODC), Purines might inhibit the conversion of orotic acid to orotidine monophosphate by lowering the intracellular PRPP-concentration (Crandall et al. 1978). 

After a lag time which (like the intracellular PRPP concentration ) increased with increasing phosphate concentration and the time of preincubation in phosphate buffered saline, radioactive material, identified as hypoxanthine by paper chromatography, was released from erythrocytes in the medium (fig. 1). The lag phase was... 

Oral administration of 5o gm xylitol (within 30 minutes) leads to a significant and rapid increase in serum uric acid concentrations with a peak one to two hours after the beginning, followed by a gradual decrease. Seven hours after the start of the experiment, uric acid levels still are markedly elevated. This rise in serum uric acid levels is accompanied by a varying but marked decrease of intracellular PRPP concentrations without reaching the initial value within 7 hours. In contrast to the PRPP content the "generation" of PEPP within the hemolysates increases significantly, in one case the initial value was exceeded fourfold. 

Fig. 1, Effect of Pi concentration on PRPP formation in intact erythrocytes and in stroma-free hemolysates. Packed washed human erythrocytes were suspended in an equal volume of a mixture consisting of 0.9% NaCl solution and O.IM potassium phosphate buffer (pH 7.4) in suitable proportions to give the various concentrations of Pi, as indicated. The cell suspensions were incubated in the presence of lOmM glucose for 60 min at 37°C with continous shaking. Intracellular PRPP and PRPP synthetase in the cell-free hemolysates were assayed as previously describedl.

Intracellular PRPP and rates of de novo purine biosynthesis (FGAR) correlate well with HGPRT deficiency lending support to the hypothesis that elevated PRPP levels are responsible for accelera-ted de novo purine biosynthesis in HGPRT deficient cells. 

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