Overproduction of PRPP

The mechanism of the hyperuricemia in most individuals who have gout is unknown. Following is a discussion of biochemical lesions that lead to hyperuricemia and may eventually lead to gout. Enhanced PRPP synthesis results from X-chromosome-linked mutants of PRPP synthetase. Several variants show increased Umax, resistance to feedback inhibition, or a low for ribose 5-phosphate. 

PRPP levels can also be increased as a result of underutilization in purine salvage pathways. Thus, HPRT deficiency (partial or complete) causes hyperuricemia as an X-linked recessive trait. In situations in which ATP is consumed more rapidly than it is synthesized or in which 

A variety of drugs are used in the management of gout in three clinical situations  

Leads to excess PRPP production enhanced ATP turnover 

Decreased purine salvage conversion of bases to uric acid enzyme is completely absent in Lesch-Nyhan syndrome 

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Since the end products of pyrimidine catabolism are highly water-soluble, pyrimidine overproduction results in few clinical signs or symptoms. In hypemricemia associated with severe overproduction of PRPP, there is overproduction of pyrimidine nucleotides and increased excretion of p-alanine. Since A, A -methyl-ene-tetrahydrofolate is required for thymidylate synthesis, disorders of folate and vitamin Bjj metabofism result in deficiencies of TMP. 

Primary gout can be caused by overproduction of purine catabolites due to X-linked mutations of PRPP synthetases that render the enzyme insensitive to allosteric inhibitors. 

Two enzyme abnormalities resulting in an overproduction of uric acid have been well described (Fig. 91-1). The first is an increase in the activity of phosphoribosyl pyrophosphate (PRPP) synthetase, which leads to an increased concentration of PRPP. PRPP is a key determinant of purine synthesis and thus uric acid production. The second is a deficiency of hypoxanthine guanine phosphoribosyl transferase (HGPRT). 

PRPP amidotransferase - Defects in PRPP amidotransferase may render it insensitive to feedback inhibition by purine nucleotides, too, leading to the overproduction of purine nucleotides, excessive uric acid synthesis, and gout. 

A metabolic alteration leading to overproduction of either PRPP or glutamine would cause an overproduction of 5 -phosphoribosylamine. Excess PRPP could result from overproduction of its precursor glucose, or from reduced use in other pathways, such as pvrimidine biosynthesis. Increased incorporation of " Qglucose and accelerated turnover of PRPP have been reported. A case of gout with low orotidylic acid pyrophosphorylase and decarboxylase activity has also been described. Whether these metabolic changes constitute the primary injury or are several steps removed from it remains to be established. 

Since the initial report by Drs. Sperling and de Vries and their colleagues, inherited superactivity of PRPP synthetase has become established as an unusual cause of purine overproduction, hyperuricemia, and gout in man. To date, detailed investigations of 7 families with superactive PRPP synthetase have been published, and I am aware of 4 additional families currently under study. In each family, the index cases have been males, and where studied, the patterns of inheritance of the enzyme aberrations have been consistent with X-linked transmission, reflecting the apparent structural basis of superactivity in each defective enzyme. 

Evidence of purine overproduction in childhood has led to detection of superactive PRPP synthetases in 2 families which are of special interest for several reasons, First, the hemizygous affected males in these families show severe sensorineural deafness in addition to uric acid overproduction. Second, the mothers of these boys share both the metabolic and hearing abnormalities with their sons, and one of these women has had both acute gouty arthritis and uric acid urolithiasis. Finally, as discussed below, the functional derangement in the enzyme of one of the families is unusually marked with more severe metabolic consequences of PRPP synthetase superactivity which might explain the childhood clinical onset, the development of gout in the mother, and even the associated deafness. 

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. 

For practical reasons, screening for abnormalities of PRPP synthetase will amost certainly continue to utilize hemolysates, with the use of fibroblast or lymphoblast samples remaining the province of a relatively few laboratories. From the above discussion, it is clear that enzyme activities should be measured under a variety of conditions. In our laboratory, screening of PRPP synthetase activity is carried out in dialyzed hemolysates from patients with uric acid overproduction as follows 1) as a function of Pi concentration (range, 0.2-32 mM) at saturating siabstrate and Mg " " concentrations ... 

M. A. Becker. Abnormalities of PRPP metabolism leading to an overproduction of uric acid. in "Uric Acid". W. N. Kelley and I. M. Weiner eds., Springer-Verlag, Berlin (1978). 

The important effect on the oxypurine excretion (with high ratio of hypoxanthine/xanthine)during allopurinol therapy and the inefficacy of thiopurinol, in both patients LUG., and MON., (m) are quite the same as in patient with complete deficiency. They have 50 % of control values of rate of synthesis of alio and thiopurinol nucleotide in vitro. (PRPP content of erythrocytes is normal.) The response of allopurinol therapy on uric acid and oxypurine excretion in the mother of LESCH-NYHAN BRE., (m) is the same as other gouty patient with normal HGPRT and normal rate of synthesis of alio and thiopurinol nucleotide. The gouty patient GIR., with partial deficiency of APRT (38 of normal) has an important overproduction of uric acid. This was confirmed by incorporation of labeled glycine into uric acid (DELBARRE et al 1969). Allopurinol and thiopurinol has an explosive effect on uric acid biosynthesis de novo (-U0 ). With allopurinol therapy there... 

PRPP is a substrate for the enzyme glutamine-PRPP amidotransferase catalyzing the first rate-limiting step of purine nucleotide synthesis de novo (l). Evidence has been obtained that PRPP is an important regulator of this pathway (2-4). A possible role of increased PRPP availability in the enhancement of de novo purine synthesis in primary metabolic gout, a purine overproduction disease, has been suggested by the demonstration of an increased PRPP turnover in gouty subjects (5) and of an increased rate of PRPP formation in erythrocytes and cultured fibroblasts from such patients (6-8). 

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. 

No evidence for the presence of a known enzyme abnormality causing purine overproduction could be obtained. The erythrocyte activity of hypoxanthine-guanine phos-phoribosyltransferase (HGPRT), of adenine phosphoribosyltransferase (APRT), and of phosphoribosylpyrophosphate (PRPP) synthetase were all in the normal range. Erythrocyte PRPP generation, as well as the acitivity of the pentose phosphate pathway was also normal (Table 1). In addition, the rate of de novo synthesis of purine nucleotides in cultured skin fibroblasts from the patient was found to be normi. 

A final category of functional abnormality underlying PRPP synthetase superactivity is represented by the enzyme characterized from the fibroblasts of the child with purine overproduction and deafness referred to earlier. Both increased maximal reaction velocity and... 

It thus appears that the mutant PRPP synthetase enzyme is structurally altered in such a way that only its regulatory properties but not its catalytic properties are affected. This selective alteration proves that these two properties are located at different sites, the enzyme being allosteric. Examples are known of mutations in bacteria (18,19) and Ehrlich ascites cells (20) which altered the susceptibility of regulatory enzymes to effector mulecules. An indication for such a mutation in man has been obtained by Henderson et al in studies on fibroblasts from two patients with purine overproduction and gout, showing reduced effectiv-ness of product inhibition of purine biosynthesis (2l). 

To our knowledge the present identification of an erythrocyte PRPP synthetase mutant in the gouty family studied by us provides the first demonstration in man of excess activity of a regulatory enzyme causing an overproduction disease as a direct effect of mutation. 

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