HGPRT, partial deficiency

HGPRT is responsible for the conversion of guanine to guanylic acid and hypoxanthine to inosinic acid. These two conversions require PRPP as the cosubstrate and are important reutilization reactions involved in the synthesis of nucleic acids. A deficiency in the HGPRT enzyme leads to increased metabolism of guanine and hypoxanthine to uric acid, and more PRPP to interact with glutamine in the first step of the purine pathway." Complete absence of HGPRT results in the childhood Lesch-Nyhan syndrome, characterized by choreoathetosis, spasticity, mental retardation, and markedly excessive production of uric acid. A partial deficiency of the enzyme may be responsible for marked hyperuricemia in otherwise normal, healthy individuals. 

In summary, the presentation of patients with partial deficiency of HGPRT may be more varied than is usually described, and before absence or near absence of HGPRT activity is accepted, intact erythrocytes and/or nucleated cells must be examined. We suggest that intact erythrocyte studies, together with red cell GTP levels (see Simmonds et al, this volume) may provide a better guide to prognosis in affected subjects. 

Increased intracellular levels of PP-ribose-P have been implicated in the cause of certain hyperuricemic states associated with uric acid overproduction. Fibroblasts from two patients with the Lesch-Nyhan syndrome were found previously to have an elevated intracellular concentration of PP-ribose-P with a normal rate of PP-ribose-P production (Rosenbloom, et al., 1968). Green and Seegmiller (1969) subsequently reported a mean PP-ribose-P value of 47.1 in erythrocytes from seven patients with HGPRT deficiency. We have confirmed these elevated PP-ribose-P levels in three additional patients with the Lesch-Nyhan syndrome with values of 20.5, 39.4 and 49.5 juM (Table 1). The mothers of these patients are obligate heterozygotes and have normal PP-ribose-P levels. Two diseases associated with a deficiency of other PRT enzymes are not associated with altered erythrocyte PP-ribose-P levels (Table 1). PP-ribose-P levels were in the normal range in one patient with a partial deficiency of adenine phosphoribosyltransferase (APRT) and in one patient with orotic aciduria, which is due to a deficiency... 

In primary gout increased erythrocyte PP-ribose P levels have been reported in patients with a partial deficiency of HGPRT (Greene and Seegmiller, 1969). Several studies have provided evidence for an increased capacity to synthesize PP-ribose-P in other patients with gout not related to a deficiency of HGPRT. We have determined the intracellular PP-ribose-P concentration in erythrocytes from 34 patients with hyperuricemia and normal HGPRT and in erythrocytes from 33 normouricemic control subjects. In these studies there was no apparent relationship between intracellular erythrocyte PP-ri-bose-P levels and serum urate concentration or urinary uric acid excretion (Fig. 3). In addition, there was no relationship between capacity for PP-ribose-P synthesis in erythrocytes and serum urate concentration in 47 patients 22 are illustrated in Fig. 4. 

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... 

Partial deficiency of HGPRT, a salvage enzyme of purine metabolism, has been demonstrated to be the primary abnormality causing purine overproduction in a small proportion of patients with gout (l-4). The quantitative deviation in the activity of this enzyme has been shown by Kelley et al. to be associated with decreased stability to thermal inactivation (2). These authors suggest that in the affected subjects HGPRT is structurally altered. Furthermore, in some of these patients erythrocyte adenine phosphoribosyltransferase (APRT) activity was found to be increased and relatively thermostable (2). 

Adenine phosphoribosyltransferase activity assayed by an analogous method using 0.6 mM adenine-8-l C (specific activity 4 mCI/mmole) was not significantly altered in any of the HGPRT-deficient clones (Table II). Starch gel electrophoresis using a method only slightly modified from that of Watson et. al. (19) showed no difference in electrophoretic mobility of mutants partially deficient in HGPRT (Fig. 3) but could not be used to examine the clones with severely deficient enzyme activity. 

Recent advances in the understanding of human purine metabolism have been stimulated by the discovery of specific inborn errors of this pathway in man. In particular, the demonstration of the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) in the Lesch-Nyhan syndrome and in some patients with gout has contributed essential information on the regulation of purine biosynthesis novo and on the critical role of this reutilization pathway in central nervous system function in man. The search for other disorders led to the description of a partial deficiency of adenine phosphoribosyltransferase (APRT) in four members in three generations of one family. Each of the subjects partially deficient in APRT exhibited a normal serum urate concentration and the propositus had a normal excretion of uric acid (Kelley, et al., 1968). We have investigated a second family partially deficient in APRT (Fox and Kelley, in press). 

The per cent of control values in partial HGPRT deficiency is the same as well as with hypoxanthine and guanine or with alio and thiopurinol as substrat. In the particular enzymatic deficiency LUG, who has nearly normal PRT activity with hypoxanthine as substrat, rate of synthesis of alio and thiopurinol are about 30 % of normal value. 

In the present study on a mutant HGPRT in two gouty relatives with partial HGPRT deficiency (4), the enzyme was found to exhibit increased sensivitivy to inhibition by various purine nucleosides and nucleotides. On the other hand, the APRT in these patients erythrocytes, except for increased activity and thermostability, did not display any additional abnormality. 

Regarding the APRT of these patients with partial HGPRT deficiency, all properties examined - Km for substrates, electrophoretic mobility on starch gel, pH profile and sensitivity to product and feedback inhibition, were found normal. In agreement to the findings of other investigators (2), the APRT exhibited increased activity and relative thermostability and PRPP stabilized the enzyme against thermal inabtivation. 

Fig. 5. Heat inactivation curves for crude extracts of WI-L2 lymphoblasts and four azaguanine resistant clones with partial HGPRT deficiency. 100% activity was 327 nmoles/mg protein/hr (WI-L2),...

Starch gel electrophoresis shows no evidence of altered mobility in clones with partial HGPRT deficiency but heat inactivation studies in crude extracts suggest the possibility of more than one structural gene mutation. 

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