B. T. Emmerson

A review of the molecular basis of hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency

Hypoxanthine-guanine phosphoribosyltransferase (HPRT, EC 2.4.2.8) is a purine salvage enzyme that catalyses the conversion of hypoxanthine and guanine to their respective mononucleotides. Partial deficiency of this enzyme can result in the overproduction of uric acid leading to a severe form of gout, whilst a virtual absence of HPRT activity causes the Lesch-Nyhan syndrome which is characterised by hyperuricaemia, mental retardation, choreoathetosis and compulsive self-mutilation. The HPRT-encoding gene is located on the X chromosome in the region q26–q27 and consists of nine exons and eight introns totalling 57 kb. This gene is transcribed to produce an mRNA of 1.6 kb, which contains a protein encoding region of 654 nucleotides. With the advent of increasingly refined techniques of molecular biology, it has been possible to study the HPRT gene of individuals with a deficiency in HPRT activity to determine the genetic basis of the enzyme deficiency. Many different mutations throughout the coding region have been described, but in the absence of precise information on the three-dimensional structure of the HPRT protein, it remains difficult to determine any consistent correlation between the structure and function of the enzyme.

Hyperlipidaemia in hyperuricaemia and gout

The finding of hyperlipidaemia in patients with hyperuricaemia and gout is common. The usual abnormality is hypertriglyceridaemia (type IV hyperlipoproteinaemia),1 2 being reported in between 25% and 60% of patients with gout.3 4 This finding has been related to reports of an increased frequency of coronary artery disease in some patients with gout and has contributed to the suggestion that the urate concentration might be an indicator of coronary risk. Now that gouty arthritis itself is so treatable, an associated disorder that might reduce the life span becomes even more important in the management of a patient with gout. However, our understanding of the mechanism of any association between hypertriglyceridaemia and hyperuricaemia is far from complete and this becomes increasingly important when determining the factors that contribute to the development of hyperuricaemia and gout in a particular person. No longer can it be assumed that there is a common aetiology for the hyperuricaemia in patients who present with an acute urate crystal arthropathy and refer to it as “primary gout”. Similarly, it is no longer expected that each patient with gout would have inherited a “gouty diathesis”, unless you regard this concept as meaning a relatively poor renal clearance of urate in the presence of otherwise normal renal function.5 Hyperuricaemia in most patients is found to have multiple causes, some genetic and others environmental, with many of the latter being able to be modified, potentially correcting the contribution of that cause to the hyperuricaemia.6 7 In such an assessment, the …

Genetic control of the renal clearance of urate: a study of twins

Although a genetic predisposition to gout has been recognised for centuries, its mechanism has never been defined. This study was designed to determine whether this factor might be the renal clearance of urate, which is an important determinant of the concentration of urate in serum. In this study the renal clearance of urate was examined in 37 pairs of normouricaemic twins to determine whether this resemblance was genetically mediated. Monozygotic twins had more similar values of urate clearance and fractional excretion of urate than dizygotic twins. The heritability of the renal clearance of urate was estimated as about 60% (95% confidence limits 40 to 100%), whereas the heritability of the fractional excretion of urate was 87% (confidence limits 45 to 100%). This study supports the hypothesis that genetic factors exert an important control on the renal clearance of urate, which determines some of the familiarity of hyperuricaemia and gout.

Inhibition of purified rat liver glutathione S-transferase isozymes by diuretic drugs

Seven soluble rat liver glutathione S-transferase isozymes were isolated and the inhibition of these isozymes by selected diuretics was investigated using 1-chloro-2,4-dinitrobenzene as substrate. All isozymes were inhibited to some extent under the experimental conditions used, but there was significant isozyme dependent selectivity of inhibition. The greatest inhibitory effect (over 80%) was found when the phenoxyacetic acid diuretics and indacrynic acid were incubated with glutathione S-transferase 3-3, 3-4 and 4-4. The sulphamoylbenzoic acid diuretics, furosemide and bumetanide, were found to have a lesser effect on the isozymes studies. As glutathione S-transferase are thought to play an important protective role in the various tissues of animals and man, by catalysing the glutathione conjugation of electrophilic drugs and drug metabolites, their inhibition may be toxicologically important.

Adenine phosphoribosyltransferase: a simple spectrophotometric assay and the incidence of mutation in the normal population.

The significance of partial deficiency of erythrocyte adenine phosphoribosyltransferase (APRT), reported in a number of subjects with gout, has been investigated by studying its incidence in 700 normal blood donors. Three clearly deficient subjects were found, an incidence not significantly different from that in patients with abnormalities of urate metabolism. A new assay method for APRT is described in which an erythrocyte lysate is incubated with adenine and phosphoribosylpyrophosphate (PRPP) for a given time; both hemoglobin and adenine nucleotide (AMP) are then precipitated with lanthanum phosphate; the change in absorbance of adenine at 260 nm reflects the extent of its conversion to AMP by APRT.

Inhibition of soluble glutathione S-transferase by diuretic drugs

Glutathione transferases are believed to play an important protective role in the various tissues of animals and man by catalysing the glutathione conjugation of electrophilic drugs and electrophilic drug metabolites. Many of these compounds have the potential to react with vital cellular macromolecules in the absence of this enzyme system. We have investigated the interaction of a number of high ceiling diuretics with the glutathione transferases contained in the cytosolic fraction of the rat liver. Of bumetanide, ethacrynic acid, furosemide, indacrynic acid and tienilic acid, only ethacrynic acid was conjugated with glutathione. Further experiments revealed that ethacrynic, indacrynic and tienilic acids are all potent inhibitors of glutathione S- aryltransferase . Glutathione S- alkyltransferase and glutathione S-epoxide transferase were also inhibited by the diuretics, but to a lesser extent than glutathione S- aryltransferase . The diuretics giving the greatest inhibition of these reactions were chemically related to ethacrynic acid. The concept where inhibition of glutathione-S-transferase by a drug may enhance its own toxicity is considered. This mechanism has also the potential of enhancing the toxicity of other concurrently administered drugs which normally require glutathione S-transferase for detoxication.

Detection of the carrier state for an X-linked disorder, the Lesch-Nyhan syndrome, by the use of lymphocyte cloning

SummaryUsing a limiting dilution technique, we found that the frequency of thioguanine resistant (TGR) lymphocyte clones was less than 5.0x10-5 in 14 normal individuals, between 9.0x10-3 and 8.9x10-2 in seven heterozygotes for Lesch-Nyhan syndrome, and 0.88 and 0.87 in two hemizygotes. TGR clones from heterozygotes were expanded and had the hemizygote phenotype as evidenced by low hypoxanthine incorporation and severely deficient hypoxanthine-guanine-phosphoribosyl-transferase activity. Enumeration of TGR lymphocyte clones provides a simple technique for detection of heterozygosity for Lesch-Nyan syndrome. A similar approach using lymphocyte cloning may be suitable for detection of the carrier state for other X-linked disorders.

Xanthine-containing calculi during allopurinol therapy.

We report a case of urate overproduction owing to the Lesch-Nyhan syndrome (deficiency of hypoxanthine-guanine phosphoribosyltransferase). Urate crystalluria was controlled by allopurinol therapy but renal calculi developed, which contained a variety of purines, particularly the relatively insoluble xanthine, as well as oxypurinol and hypoxanthine. The potential hazard from the increased amounts of xanthine that are produced during allopurinol therapy for urate overproduction is stressed, as well as the importance of maintaining a high urine flow rate even during such therapy.

Inhibition of Plasmodium falciparum hypoxanthine-guanine phosphoribosyltransferase mRNA by antisense oligodeoxynucleotide sequence.

Comparative biochemical studies between the malaria parasite, Plasmodium falciparum, and its host have revealed differences in a number of metabolic pathways. For example, the parasite has enzymes to carry out de novo pyrimidine synthesis but is not capable of de novo synthesis of purine nucleotides. The parasite must salvage purines from its host to supply the requirement for purine nucleotides. The enzyme required for this salvage is hypoxanthine-guanine phosphoribosyltransferase This enzyme has been suggested as a target for chemotherapy by the design of analogue substrate inhibitors of the parasite enzyme. An alternate strategy, and one which we have been exploring, is the inhibition of expression of the parasite HPRT using antisense oligodeoxynucleotides.

Urate Deposits in the Renal Medulla

Urate deposits within microtophi were found in 8% of unselected autopsies in Brisbane, Australia. Significant associations were demonstrated with (a) a history of gouty arthritis and (b) the existence