Excretion of hypoxanthine and xanthine in a genetic disease of purine metabolism

Paper-Based Enzyme Biosensor for One-Step Detection of Hypoxanthine in Fresh and Degraded Fish

Food freshness monitoring, which can reflect the quality of the product at the time of use, remains a great challenge for consumers and the food industry. Herein, we report the development of a cost-effective enzyme-based paper biosensor, which can monitor fish freshness and predict spoilage. The biosensor measures the release of hypoxanthine (HX), an indicator of meat and fish degradation, using the enzymatic conversion of HX by xanthine oxidase (XOD). We demonstrate that the entrapment of XOD and an organic dye, nitro blue tetrazolium chloride (NBT), within a sol-gel biohybrid enables their stabilization on paper and promotes the enzymatic reaction with further retention of the reaction products within the cellulosic network . Linearity in the micromolar concentration range with a detection limit of 3.7 μM for HX is obtained. The biosensor has high selectivity toward HX and is manufactured in few steps from inexpensive widely available materials. The applicability of the biosensor is demonstrated by following fish degradation over time and measuring HX concentrations ranging from 117 (±9) to 198 (±5) μM within 24 h of degradation, at levels that are comparable with those measured by a commercial enzymatic kit for HX detection. As compared to the commercial kit, our biosensors are more cost-effective, do not require addition of exogenous reagents and are portable, having all of the reagents needed for analysis embedded within the sensing platform. This proof-of-concept work demonstrates that the paper-based HX biosensor has potential as a robust reagentless device for real-time monitoring of food freshness and for other applications in which HX plays an important role.

Genotypic and phenotypic spectrum in attenuated variants of Lesch-Nyhan disease

Lesch-Nyhan disease and its attenuated variants are caused by deficiency of the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt). All patients exhibit excessive production of uric acid, which increases the risk for nephrolithiasis, renal failure, gouty arthritis and tophi. The mildest phenotype includes only problems related to overproduction of uric acid. The most severe clinical phenotype includes prominent neurological abnormalities and the universal feature is self-injurious behavior. In between the mildest and most severe syndromes is a broad spectrum of phenotypes with varying degrees of neurological, neurocognitive and behavioral abnormalities. The effect of HPRT1 gene mutations on residual HGprt enzyme activity is the most relevant factor contributing to disease phenotype. Attenuated clinical phenotypes are associated with residual enzyme function, whereas the most severe phenotype is usually associated with null activity. In cases of gouty arthritis with urate overproduction, a careful evaluation for motor impairments or neurocognitive abnormalities may help to identify attenuated variants of Lesch-Nyhan disease for better management.

Update on the phenotypic spectrum of Lesch-Nyhan disease and its attenuated variants

Congenital deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a spectrum of clinical phenotypes. All of these phenotypes are associated with marked overproduction of uric acid and related problems such as hyperuricemia, urate nephrolithiasis, tophi, and gout. The mildest phenotypes include only problems related to overproduction of uric acid. The most severe phenotype is known as Lesch-Nyhan disease, in which the phenotype also includes severe motor handicap, intellectual disability, and self-injurious behavior. In between these two extremes is a continuous spectrum of phenotypes with varying degrees of motor and cognitive handicap but no self-injurious behavior. The pathogenesis of overproduction of uric acid in HPRT deficiency is well-understood, and treatments are available to control it. The pathogenesis of the neurobehavioral problems is less well-understood, and effective treatments for them are lacking.

Efficacy and safety of allopurinol in patients with the Lesch-Nyhan syndrome and partial hypoxanthine- phosphoribosyltransferase deficiency: a follow-up study of 18 Spanish patients

Allopurinol is used widely for the treatment of purine disorders such as gout, but efficacy and safety of allopurinol has not been analyzed systematically in an extensive series of patients with HPRT deficiency. From 1984 to 2004 we have diagnosed 30 patients with HPRT deficiency. Eighteen patients (12 with Lesch-Nyhan syndrome or complete HPRT deficiency, and 6 with partial HPRT deficiency) were treated with allopurinol (mean dose, 6.44 mg/Kg of weight per day) and followed-up for at least 12 months (mean follow-up 7,6 years per patient). Mean age at diagnosis was 7 years (range, 5 months to 35 years). Treatment with allopurinol was associated to a mean reduction of serum urate concentration of 50%, and was normalized in all patients. Mean urinary uric acid excretion was reduced by 75% from baseline values, and uric acid to creatinine ratio was close or under 1.0 in all patients. In contrast, hypoxanthine and xanthine urinary excretion rates increased by a mean of 6 and 10 times, respectively, compared to baseline levels. These modifications were similar in patients with complete or partial HPRT deficiency. In 2 patients xanthine stones were documented despite allopurinol dose adjustments to prevent markedly increased oxypurine excretion rates. Neurological manifestations did not appear to be influenced by allopurinol therapy. Allopurinol is a very efficacy and fairly safety drug for the treatment of uric acid overproduction in patients with complete and partial HPRT deficiency. Allopurinol was associated with xanthine lithiasis.

Activation of methionine synthase by insulin-like growth factor-1 and dopamine: a target for neurodevelopmental toxins and thimerosal

Methylation events play a critical role in the ability of growth factors to promote normal development. Neurodevelopmental toxins, such as ethanol and heavy metals, interrupt growth factor signaling, raising the possibility that they might exert adverse effects on methylation. We found that insulin-like growth factor-1 (IGF-1)- and dopamine-stimulated methionine synthase (MS) activity and folate-dependent methylation of phospholipids in SH-SY5Y human neuroblastoma cells, via a PI3-kinase- and MAP-kinase-dependent mechanism. The stimulation of this pathway increased DNA methylation, while its inhibition increased methylation-sensitive gene expression. Ethanol potently interfered with IGF-1 activation of MS and blocked its effect on DNA methylation, whereas it did not inhibit the effects of dopamine. Metal ions potently affected IGF-1 and dopamine-stimulated MS activity, as well as folate-dependent phospholipid methylation: Cu(2+) promoted enzyme activity and methylation, while Cu(+), Pb(2+), Hg(2+) and Al(3+) were inhibitory. The ethylmercury-containing preservative thimerosal inhibited both IGF-1- and dopamine-stimulated methylation with an IC(50) of 1 nM and eliminated MS activity. Our findings outline a novel growth factor signaling pathway that regulates MS activity and thereby modulates methylation reactions, including DNA methylation. The potent inhibition of this pathway by ethanol, lead, mercury, aluminum and thimerosal suggests that it may be an important target of neurodevelopmental toxins.

A review of behavioral treatments used for Lesch-Nyhan syndrome

Lesch-Nyhan syndrome is a genetic disorder resulting in hyperuricemia, choreoathetosis, mental retardation, and self-mutilation. The most salient feature of this disorder is the self-injurious behavior (SIB). Although the utility of behavioral interventions with SIB has been well documented, behavioral interventions with Lesch-Nyhan syndrome have been limited in number and long-term success. This article reviews the behavioral treatments that have been used in treating individuals with Lesch-Nyhan syndrome and discusses the strengths and weaknesses of these methods. Suggestions for future directions in the use of behavioral interventions for controlling SIB in Lesch-Nyhan syndrome are provided.

Positional cloning and gene identification

After genetic mapping and physical representation of a particular genomic region containing the gene underlying a particular Mendelian trait, a successful positional cloning strategy depends on the efficient detection and analysis of genes in the critical interval. Several gene detection strategies are presently available to compile an inventory of genes from large genomic regions. Here, the principle of these methods is briefly reviewed and their relative value for positional cloning projects compared.

The orofacial manifestations of the Lesch-Nyhan syndrome

The Lesch-Nyhan syndrome is a rare sex-linked inborn error of purine metabolism characterised by hyperuricaemia, mental handicap, neurologic disorders, and self-destructive behaviour. The literature is reviewed and a 12-year-old patient exhibiting mutilation of the lip and tongue is reported.

Behavior in the Lesch--Nyhan syndrome

The Lesch--Nyhan syndrome is a heritable disorder of the metabolism of uric acid in which behavioral manifestations are prominent and among the most provocative. The mutated or variant gene that determines this disorder is carried on the X chromosome. The disease is expressed exclusively in males. The molecular expression of the abnormal gene is in the completely defective activity of the enzyme hypoxanthine guanine phosphoribosyl transferase. As a result these patients overproduce uric acid and may develop early in life many of the clinical findings we associate with gout. They have in addition a variety of neurological abnormalities including mental retardation, spastic cerebral palsy, and involuntary, choreoathetoid movements. Involved patients have unusual, compulsive, aggressive behavior. Its most prominent but by no means exclusive feature is self-mutilation. The central feature in the management of this behavior is physical restraint. A number of practical procedures have been learned which facilitate the care and feeding of these patients. Promising new findings suggest that behavioral modification using extinction techniques and pharmacologic methods utilizing agents designed to increase the effective cerebral content of serotonin may each have a place in the management of behavior in this syndrome.

Hypoxanthine-guanine phosphoribosyltransferase. Characterization of a mutant in a patient with gout

The mutation in a young gouty male with a partial deficiency of hypoxanthine-guanine phosphoribosyltransferase has been evaluated. The serum uric acid was 11.8 mg/100 ml, and the urinary uric acid excretion was 1,279 mg/24 h. Erythrocyte hypoxanthine-guanine phosphoribosyltransferase was 34.2 nmol/h/mg, adenine phosphoribosyltransferase was 36.5 nmol/h/mg and phosphoribosylpyrophosphate was 2.6 muM. Hypoxanthine-guanine phosphoribosyltransferase from peripheral leukocytes and cultured diploid skin fibroblasts was within the normal range, but enzyme activity in rectal mucosa was below the normal range. Initial velocity studies of the normal enzyme and the mutant enzyme from erythrocytes with the substrates hypoxanthine, guanine, or phosphoribosylpyrophosphate showed that the Michaelis constants were similar. Product inhibition studies distinguished the mutant enzyme from the normal enzyme. Hyperbolic kinetics with increasing phosphoribosylpyrophosphate were converted to sigmoid kinetics by 0.2 mM GMP with the mutant enzyme but not with the normal enzyme. The mutant erythrocyte hypoxanthine-guanine phosphoribosyltransferase was inactivated normally at 80 degrees C and had a normal half-life in the peripheral circulation. The mol wt of 48,000 was similar to the normal enzyme mol wt of 47,000. With isoelectric focusing, the mutant erythrocyte enzyme had two major peaks with isoelectric pH's of 5.50 and 5.70, in contrast to the isoelectric pH's of 5.76, 5.82, and 6.02 of the normal isozymes. Isoelectric focusing of leukocyte extracts from the patient revealed the presence of the mutant enzyme. Cultured diploid fibroblasts from the propositus appeared to function normally, as shown by the inability to grow in 50-100 muM azaguanine and by the normal incorporation of [14C]hypoxanthine into nucleic acid. In contrast, erythrocytes from the patient displayed abnormal properties, including the increased synthesis of phosphoribosylphyrophosphate and elevated functional activity of orotate phosphoribosyltransferase and orotidylic decarboxylase. These unique kinetic, physical, and functional properties provide support for heterogeneous structural gene mutations in partial deficiencies of hypoxanthine-guanine phosphoribosyltransferase.

Mechanism of excessive purine biosynthesis in hypoxanthine-guanine phosphoribosyltransferase deficiency

Certain gouty subjects with excessive de novo purine synthesis are deficient in hypoxanthineguanine phosphoribosyltransferase (HG-PRTase [EC 2.4.2.8]). The mechanism of accelerated uric acid formation in these patients was explored by measuring the incorporation of glycine-(14)C into various urinary purine bases of normal and enzyme-deficient subjects during treatment with the xanthine oxidase inhibitor, allopurinol. In the presence of normal HG-PRTase activity, allopurinol reduced purine biosynthesis as demonstrated by diminished excretion of total urinary purine or by reduction of glycine-(14)C incorporation into hypoxanthine, xanthine, and uric acid to less than one-half of control values. A boy with the Lesch-Nyhan syndrome was resistant to this effect of allopurinol while a patient with 12.5% of normal enzyme activity had an equivocal response. Three patients with normal HG-PRTase activity had a mean molar ratio of hypoxanthine to xanthine in the urine of 0.28, whereas two subjects who were deficient in HG-PRTase had reversal of this ratio (1.01 and 1.04). The patterns of (14)C-labeling observed in HG-PRTase deficiency reflected the role of hypoxanthine as precursor of xanthine. The data indicate that excessive uric acid in HG-PRTase deficiency is derived from hypoxanthine which is insufficiently reutilized and, as a consequence thereof, catabolized inordinately to uric acid. The data provide evidence for cyclic interconversion of adenine and hypoxanthine derivatives. Cleavage of inosinic acid to hypoxanthine via inosine does not contribute significantly to the formation of uric acid in either normal man or in patients with HG-PRTase deficiency.HG-PRTase was not completely absent in red blood cells from a boy with the Lesch-Nyhan syndrome; with hypoxanthine as substrate, the activity in erythrocyte hemolysates was 0.64% of normal values.