Adenine Phosphoribosyltransferase Activity Research Articles

#2633 Navigating complexity: a case report of chronic kidney disease with renal stone unravelling a rare metabolic disorder

Abstract Clinical Presentation A 48-year-old female of Asian heritage was referred for renal assessment after experiencing recurrent acute kidney injury (AKI) with incomplete recovery between episodes. This was initially attributed to dehydration and interstitial nephritis stemming from long-standing vomiting, a manifestation of gastroscopy-proven erosive gastritis, compounded by the use of high-dose proton pump inhibitors (PPI). Her relevant medical history is of right nephrectomy in 1996 due to recurrent urinary tract infections (UTIs), and previous left renal calculus in 2019 with no visible stones on further CT imaging. Despite living with a single kidney, her baseline eGFR had been normal prior to referral. While unaware of her mother's medical history, her father was recently diagnosed with kidney disease (no clear history) and her sister Wilson's disease. A kidney biopsy was undertaken given the incomplete eGFR recovery between episodes. The biopsy revealed adequate tissue including 24 glomeruli, no significant glomerular pathology was seen. There is 65% tubulointerstitial atrophy and fibrosis, and yellow to red-brown-tinged crystals within tubular lumina and epithelial cells, prompting a histiocytic reaction. These crystals were birefringent under polarized light and did not stain with Perl's and Von Kossa stain (Fig. 1a-d). Immunofluorescence was negative for all immunoglobulins and complement fractions. No immune complex/substructure deposits were seen in the glomeruli on electron microscopy. Blood tests assessing APRT activity indicated values within the carrier range (Table 1). The patient and her family have been referred for a comprehensive genetic testing for APRT genetic mutations. Treatment with Allopurinol 300 mg daily resulted in a sustained eGFR improvement from 19 to 33 ml/min/1.73m2 at 6 months after initiation (Fig. 1). Discussion Adenine phosphoribosyl transferase (APRT) deficiency, also known as Dihydroxyadenineuria, is a rare autosomal recessive inherited condition leading to kidney stones. If left untreated, up to 25% progress to end-stage kidney disease (ESKD). Although inherited, half of patient present in adulthood and may do so with laboratory findings of CKD rather than specifically with stones [1]. This case highlights the significance of precise diagnosis in calculi-related kidney diseases. Early intervention can alter the trajectory of specific conditions, averting progression to ESKD. This case is unusual in being a carrier of an autosomal recessive condition presenting with full manifestations of the disease. Patient Consent The patient has given verbal, documented consent for case publication. Acknowledgment We thank all renal and pathology staff members at Salford Care Organisation who contributed to the diagnosis and management of our case.

Adenine phosphoribosyl transferase (APRT) deficiency and a novel sequence variant in APRT with phenotypic diversity and a literature review.

Adenine phosphoribosyl transferase (APRT) deficiency is an autosomal recessive disorder and a rare cause of urolithiasis due to mutations in APRT (OMIM #102600). APRT deficiency results in increased urinary excretion of 2,8-dihydroxyadenine (DHA) which can cause urolithiasis and kidney failure. However, with prompt diagnosis, patients with APRT deficiency can be treated with xanthine oxidoreductase inhibitors which decrease urinary DHA excretion and improve outcomes. We report a pair of siblings, an 11-year-old brother and his 14-year-old sister with compound heterozygous variants c.270del (p.Lys91Serfs*46) and c.484_486del (p.Leu162del) in APRT with variable clinical presentation of APRT deficiency. The brother presented at 17 months of age with urolithiasis and severe acute kidney injury. His elder sister remained well and asymptomatic with normal kidney function and did not develop renal calculi. Brownish disk or sphere-like crystals with both concentric and radial markings were reported on urine microscopy in the sister on screening. The sister's diagnosis was confirmed with further laboratory evidence of absent red cell lysate APRT activity with corresponding elevated levels of urinary DHA. In conclusion, we identified a novel mutation in the APRT gene in a pair of siblings with greater phenotypic severity in the male.

Characterization of adenine phosphoribosyltransferase (APRT) activity in Trypanosoma brucei brucei: Only one of the two isoforms is kinetically active.

Human African Trypanosomiasis (HAT), also known as sleeping sickness, is a Neglected Tropical Disease endemic to 36 African countries, with approximately 70 million people currently at risk for infection. Current therapeutics are suboptimal due to toxicity, adverse side effects, and emerging resistance. Thus, both effective and affordable treatments are urgently needed. The causative agent of HAT is the protozoan Trypanosoma brucei ssp. Annotation of its genome confirms previous observations that T. brucei is a purine auxotroph. Incapable of de novo purine synthesis, these protozoan parasites rely on purine phosphoribosyltransferases to salvage purines from their hosts for the synthesis of purine monophosphates. Complete and accurate genome annotations in combination with the identification and characterization of the catalytic activity of purine salvage enzymes enables the development of target-specific therapies in addition to providing a deeper understanding of purine metabolism in T. brucei. In trypanosomes, purine phosphoribosyltransferases represent promising drug targets due to their essential and central role in purine salvage. Enzymes involved in adenine and adenosine salvage, such as adenine phosphoribosyltransferases (APRTs, EC 2.4.2.7), are of particular interest for their potential role in the activation of adenine and adenosine-based pro-drugs. Analysis of the T. brucei genome shows two putative aprt genes: APRT1 (Tb927.7.1780) and APRT2 (Tb927.7.1790). Here we report studies of the catalytic activity of each putative APRT, revealing that of the two T. brucei putative APRTs, only APRT1 is kinetically active, thereby signifying a genomic misannotation of Tb927.7.1790 (putative APRT2). Reliable genome annotation is necessary to establish potential drug targets and identify enzymes involved in adenine and adenosine-based pro-drug activation.

Comparison of human erythrocyte purine nucleotide metabolism and blood purine and pyrimidine degradation product concentrations before and after acute exercise in trained and sedentary subjects

This study aimed at evaluating the concentration of erythrocyte purine nucleotides (ATP, ADP, AMP, IMP) in trained and sedentary subjects before and after maximal physical exercise together with measuring the activity of purine metabolism enzymes as well as the concentration of purine (hypoxanthine, xanthine, uric acid) and pyrimidine (uridine) degradation products in blood. The study included 15 male elite rowers [mean age 24.3 ± 2.56 years; maximal oxygen uptake (VO2max) 52.8 ± 4.54 mL/kg/min; endurance and strength training 8.2 ± 0.33 h per week for 6.4 ± 2.52 years] and 15 sedentary control subjects (mean age 23.1 ± 3.41 years; VO2max 43.2 ± 5.20 mL/kg/min). Progressive incremental exercise testing until refusal to continue exercising was conducted on a bicycle ergometer. The concentrations of ATP, ADP, AMP, IMP and the activities of adenine phosphoribosyltransferase (APRT), hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and phosphoribosyl pyrophosphate synthetase (PRPP-S) were determined in erythrocytes. The concentrations of hypoxanthine, xanthine, uric acid and uridine were determined in the whole blood before exercise, after exercise, and 30 min after exercise testing. The study demonstrated a significantly higher concentration of ATP in the erythrocytes of trained subjects which, in part, may be explained by higher metabolic activity on the purine re-synthesis pathway (significantly higher PRPP-S, APRT and HGPRT activities). The ATP concentration, just as the ATP/ADP ratio, as well as an exercise-induced increase in this ratio, correlates with the VO2max level in these subjects which allows them to be considered as the important factors characterising physical capacity and exercise tolerance. Maximal physical exercise in the group of trained subjects results not only in a lower post-exercise increase in the concentration of hypoxanthine, xanthine and uric acid but also in that of uridine. This indicates the possibility of performing high-intensity work with a lower loss of not only purine but also pyrimidine.

Chronic alcohol administration affects purine nucleotide catabolism in vivo

AimsTo investigate the relationship between chronic alcohol administration and purine nucleotide metabolism in vivo. Main methodsRat models of alcohol dependence and withdrawal were used. The concentrations of uric acid (UAC), urea nitrogen (UREA), creatinine (CREA), and beta-2-microglobulin (β2-M) and creatinine clearance rate (CCR) in plasma were measured. The PLC method was used to detect the absolute content of purine nucleotides in different tissues. Enzymatic activities of adenosine deaminase (ADA), xanthine oxidase (XO), ribose 5-phosphate pyrophosphokinase (RPPPK), glutamine phosphoribosylpyrophosphate amidotransferase (GPRPPAT), hypoxanthine-guanine phosphate ribose transferase (HGPRT), and adenine phosphoribosyltransferase (APRT) in the tissues were analyzed. Real-time PCR was used to determine the relative level of ADA and XO. Key findingsThe renal function of rats with alcohol dependence was normal. Further, the content of purine nucleotides (GMP, AMP, GTP, and ATP) in tissues of the rats was decreased, which indicated that the increased uric acid should be derived from the decomposition of nucleotides in vivo. The activity of XO and ADA increased, and their mRNA expression was enhanced in the alcohol dependence group, but there was no significant difference in the activity of RPPPK and GPRPPAT in the liver, small intestine, and muscle; furthermore, no significant difference in the activity of HGPRT and APRT was observed in the brain. SignificanceThese results indicate that chronic alcohol administration might enhance the catabolism of purine nucleotides in tissues by inducing gene expression of ADA and XO, leading to elevation of plasma uric acid levels.

Adenine Phosphoribosyltransferase Deficiency

Complete adenine phosphoribosyltransferase (APRT) deficiency is a rare inherited metabolic disorder that leads to the formation and hyperexcretion of 2,8-dihydroxyadenine (DHA) into urine. The low solubility of DHA results in precipitation of this compound and the formation of urinary crystals and stones. The disease can present as recurrent urolithiasis or nephropathy secondary to crystal precipitation into renal parenchyma (DHA nephropathy). The diagnostic tools available-including stone analysis, crystalluria, and APRT activity measurement-make the diagnosis easy to confirm when APRT deficiency is suspected. However, the disease can present at any age, and the variability of symptoms can present a diagnostic challenge to many physicians. The early recognition and treatment of APRT deficiency are of crucial importance for preventing irreversible loss of renal function, which still occurs in a non-negligible proportion of cases. This review summarizes the genetic and metabolic mechanisms underlying stone formation and renal disease, along with the diagnosis and management of APRT deficiency.

Urate nephropathy associated with impaired kinetic properties of hypoxanthine phosphoribosyl transferase in a 45-day-old infant

We report a 45-day-old male infant who presented with anuric renal failure and fluid overload due to urate nephropathy consequent upon hyperuricemia with hyperuricosuria. His maternal uncle had undergone renal transplantation for chronic renal failure secondary to uric acid nephrolithiasis. The levels of hypoxanthine phosphoribosyl transferase (HPRT) and adenine phosphoribosyl transferase activity in the baby were found to be quantitatively normal. However, when the HPRT activity was measured at low substrate concentrations [phosphoribosyl pyrophosphate (PRPP) and hypoxanthine]and compared with usual assay conditions, the HPRT activity at lower PRPP was less in the propositus, suggesting altered enzyme kinetics. Apparent K (m(PRPP)) and V (max), but not K (m(hypoxanthine)), were then found to be higher in the propositus than the control range. This is the first case of urate nephropathy secondary to altered enzyme kinetics presenting as early as 45days. Uric acid nephropathy should be considered in the differential diagnosis of unexplained acute kidney injury in infants. In such cases, quantitative tests for HPRT enzyme activity may not be sufficient and altered enzyme kinetics should also be investigated.

2,8-Dihydroxyadenine Urolithiasis. Case Report and Literature Review

2,8-Dihydroxyadenine (DHA) urolithiasis is a rare type of urinary stone disease secondary to deficiency of adenine phosphoribosyltransferase (APRT) activity, a rare, inherited autosomal recessive disease with an incidental rate from 0.4 to 1.2%. The prevalence is higher among Japanese than other ethnic groups. APRT normally catalyzes the conversion of adenine to adenosine monophosphate and its deficiency results in 2,8-dihydroxyadenine (2,8-DHA) accumulation. This compound is extremely insoluble and its crystallization can lead to stone formation and renal failure. We report the case of 2,8-dihydroxyadenine (DHA) urolithiasis in a 52-year-old male patient. In December 2008 a 52-year-old Caucasian man was admitted to our hospital with sudden pain in the left lumbar region. Abdominal X-ray did not show any radiopaque urinary stone. I.V. pielography showed a radiolucent left lumbar ureteral (0.6 mm) and renal (1.5 cm) stone. After therapy with tamsulosin, the ureteral stone was excreted. Successful ESWL treatment was performed for renal stone. He presented a clinical history of several episodes of bilateral renal colic and two prior ESWL treatment for radiolucent stones. Chemolitholysis was never successful. RESULTS. Stone analysis by infrared spectroscopy and microscopic examination of urine reveal typical 2,8-DHA crystals. APRT deficiency was detected in the hemolysate of erythrocyte. Partial deficiency of APRT in the patient's relatives showed heterozygosity of the enzyme defect. Allopurinol therapy successfully prevented further stone formation. 20 months later the patient remains stone free. Two types of deficit are commonly distinguished, depending on the level of residual APRT activity. Type I is complete enzyme deficiency. Type II shows residual activity in cell lysates, but enzyme activity is not demonstrable in intact cells. About 78% of the Japanese patients belong to type II. The diagnosis of the disease is based on stone analysis by infrared spectroscopy or microscopic examination of urine, which may reveal typical 2,8-DHA crystals. Molecular approach can identify mutations, which are responsible of this inherited disease. Excessive water intake, restriction of foods with high adenine contents and administration of allopurinol are useful treatments. APRT deficiency is a rare disease but we can consider this pathology in case of recurrent radiolucent stones after chemolitolysis.

Effect of dantrolene on energy metabolism in rats with myocardial ischemia-reperfusion

To investigate the effect of dantrolene on energy metabolism in rats with myocardial ischemia-reperfusion. Forty-eight male rats were randomly divided into 3 groups: control group, ischemia-reperfusion group and dantrolene treatment group. With a Langendorff model system, the hearts were perfused with Krebs buffer for 30 min as pre-ischemia control. For ischemia-reperfusion, the hearts were subjected to global ischemia for 30 min and reperfusion for 60 min. The dantrolene treatment group was perfused in the presence of 5 µmol/L dantrolene before ischemia. Tetrazolium chloride (TTC) staining, lactate dehydrogenase (LDH) release and hemodynamics were used to evaluate the tissue injuries. The effect of dantrolene on energy metabolism was evaluated by measuring the quantity of high-energy phosphates and the activity of 2 enzymes in purine salvage synthesis: hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and adenine phosphoribosyl transferase (APRT) by high-performance liquid chromatography (HPLC). The myocardial infarct size and LDH release in dantrolene treatment group were lower than those of ischemia-reperfusion hearts [22.1% vs 25.3%, (70 ± 6) U/g vs (116 ± 10) U/g, both P < 0.05]. Dantrolene had no effect on the hemodynamics, except for a slight increase in coronary flow. The hearts receiving dantrolene showed a significantly higher levels of high-energy phosphates and a lower activity of HGPRT and APRT than those in the ischemia-reperfusion group [(7.63 ± 0.72) nmol×mg(-1)×min(-1) vs (12.42 ± 1.12) nmol×mg(-1)×min(-1), (4.14 ± 0.22) nmol×mg(-1)min(-1) vs (4.57 ± 0.39) nmol×mg(-1)×min(-1), both P < 0.05]. Dantrolene is cardioprotective for ischemia-reperfusion injury through reducing infarct size and improving energy metabolism.

Adenine phosphoribosyltransferase (APRT) deficiency: a new genetic mutation with early recurrent renal stone disease in kidney transplantation.

Adenine phosphoribosyltransferase (APRT) deficiency, a rare inborn error inherited as an autosomic recessive trait, presents with 2,8-dihydroxyadenine (2,8-DHA) crystal nephropathy. We describe clinical, biochemical and molecular findings in a renal transplant recipient with renal failure, 2,8-DHA stones and no measurable erythrocyte APRT activity. Homozygous C > G substitution at −3 in the splicing site of exon 2 (IVS2 −3 c > g) was found in the APRT gene. The patient’s asymptomatic brother was heterozygous for such mutation, and his APRT activity was 23% of controls. A splicing alteration leading to incorrect gene transcription and virtually absent APRT activity is seemingly associated with the newly identified mutation.

Involvement of rapid nucleotide synthesis in recovery from phosphate starvation of Catharanthus roseus cells

Growth of suspension-cultured Catharanthus roseus cells ceased during phosphate starvation, but the cells grew again upon addition of Pi even after long-term starvation. The metabolic fate of [(33)P]Pi was studied in 1-week-old stationary phase cells in ordinary culture and in 1- or 2-week-old Pi-starved cells. Immediately after administration, the most heavily labelled organic compounds are nucleotides, followed by sugar phosphates. Two weeks Pi starvation slowed down the speed of incorporation of (33)P into nucleotides. The RNA, protein, and free nucleotide content all decreased gradually during Pi starvation; however, these compounds, especially nucleotides, increased markedly in the 24 h after addition of Pi. These responses are found in all cells examined, although the total amounts of these compounds were lower in the long-term Pi-deficient cells. Of the nucleotides, a marked increase was observed in nucleoside triphosphates and UDP-glucose. The transcript level of phosphate transporter and the activities of acid phosphatase, 5'- and 3'-nucleotidase, and adenosine nucleosidase were all reduced by the addition of Pi. In contrast, the activities of adenine phosphoribosyltransferase, nicotinate phosphoribosyltransferase, and nicotinamidase, which are salvage enzymes of purine and pyridine nucleotides, were markedly increased in the Pi-fed cells. Little or no increase was observed in adenosine kinase. In the light of these results, the possible involvement of net nucleotide synthesis in the initial metabolic events of recovery from Pi deficiency are discussed.

Role of adenosine salvage in wound-induced adenylate biosynthesis in potato tuber slices

Levels of ATP and other nucleotides increased in wounded potato tuber slices, maintained on moist paper for 24 h after preparation. The relative expression intensity of genes encoding adenosine kinase ( AK) and adenine phosphoribosyltransferase ( APRT) in wounded slices was greater than the intensity of genes of the de novo pathway, glycineamide ribonucleotide formyltransferase ( GART) and 5-aminoimidazole ribonucleotide synthetase ( AIRS). In vitro activities of adenosine kinase (ATP:adenosine 5′-phosphotransferase; EC 2.7.1.20) and adenine phosphoribosyltransferase (AMP:pyrophosphate phospho- d-ribosyltransferase; EC 2.4.2.7) increased during wounding. Adenosine nucleosidase (adenosine ribohydrolase; EC 3.2.2.7) activity was negligible in freshly prepared slices, but its activity is dramatically enhanced in wounded slices. In situ adenosine salvage activity, estimated from the incorporation of radioactivity from exogenously supplied [8- 14C]adenosine into nucleotides and RNA, increased more than five times in the wounded slices. These results strongly suggest that greater expression of the genes encoding enzymes of adenosine salvage during wounding is closely related to the increased supply of adenine nucleotides in the wounded slices.

Effect of long-term phosphate starvation on the levels and metabolism of purine nucleotides in suspension-cultured Catharanthus roseus cells

The effect of long-term phosphate (Pi) starvation of up to 3 weeks on the levels of purine nucleotides and related compounds was examined using suspension-cultured Catharanthus roseus cells. Levels of adenine and guanine nucleotides, especially ATP and GTP, were markedly reduced during Pi-starvation. There was an increase in the activity of RNase, DNase, 5′- and 3′-nucleotidases and acid phosphatase, which may participate in the hydrolysis of nucleic acids and nucleotides. Accumulation of adenosine, adenine, guanosine and guanine was observed during the long-term Pi starvation. Long-term Pi starvation markedly depressed the flux of transport of exogenously supplied [8- 14C]adenosine and [8- 14C]adenine, but these labelled compounds which were taken up by the cells were readily converted to adenine nucleotides even in Pi-starved cells, in which RNA synthesis from these precursors was significantly reduced. The activities of adenosine kinase, adenine phosphoribosyltransferase and adenosine nucleosidase were maintained at a high level in long-term Pi starved cells.

2,8-dihydroxyadeninuria: Are there no cases in scandinavia?

Homozygosity or mixed heterozygosity for mutations in the adenine phosphoribosyltransferase gene cause enzyme deficiency directing adenine through an alternative metabolic pathway. This results in the production of 2,8-dihydroxyadenine, which is actively secreted into the urine. 2,8-dihydroxyadenine is insoluble at physiological urinary pH but as marked supersaturation is possible the manifestations differ: there may be minimal consequences, there may be infiltration of the tubulointerstitial tissue with acute or chronic damage or there may be stone formation in the urinary tract. Effective treatment can be offered and therefore the prognosis depends upon the renal function at diagnosis. Treatment consists of adequate fluid intake, a low-purine diet and administration of allopurinol. Urinary 2,8-dihydroxyadenine crystals are easily recognized under a microscope. The diagnosis of 2,8-dihydroxyadeninuria can be confirmed by estimation of adenine phosphoribosyltransferase activity in erythrocyte lysates. More than 300 cases of 2,8-dihydroxyadeninuria have been diagnosed worldwide, most of them in Japan, France and Iceland. One case has been reported in Finland but there have been no reports from the Scandinavian peninsula or from Denmark. The relevant mutations may be very rare in these countries but underdiagnosis is also possible.

Purine Salvage Pathways in the Apicomplexan Parasite Toxoplasma gondii

We have exploited a variety of molecular genetic, biochemical, and genomic techniques to investigate the roles of purine salvage enzymes in the protozoan parasite Toxoplasma gondii. The ability to generate defined genetic knockouts and target transgenes to specific loci demonstrates that T. gondii uses two (and only two) pathways for purine salvage, defined by the enzymes hypoxanthine-xanthine-guanine phosphoribosyltransferase (HXGPRT) and adenosine kinase (AK). Both HXGPRT and AK are single-copy genes, and either one can be deleted, indicating that either one of these pathways is sufficient to meet parasite purine requirements. Fitness defects suggest both pathways are important for the parasite, however, and that the salvage of adenosine is more important than salvage of hypoxanthine and other purine nucleobases. HXGPRT and AK cannot be deleted simultaneously unless one of these enzymes is provided in trans, indicating that alternative routes of functionally significant purine salvage are lacking. Despite previous reports to the contrary, we found no evidence of adenine phosphoribosyltransferase (APRT) activity when parasites were propagated in APRT-deficient host cells, and no APRT ortholog is evident in the T. gondii genome. Expression of Leishmania donovani APRT in transgenic T. gondii parasites yielded low levels of activity but did not permit genetic deletion of both HXGPRT and AK. A detailed comparative genomic study of the purine salvage pathway in various apicomplexan species highlights important differences among these parasites.

A new caffeine biosynthetic pathway in tea leaves: utilisation of adenosine released from the S-adenosyl- L-methionine cycle

The four-step caffeine biosynthetic pathway includes three methylation steps that utilise S-adenosyl- L-methionine (SAM) as the methyl donor. In the process SAM is converted to S-adenosyl- L-homocysteine (SAH) which in turn is hydrolysed to L-homocysteine and adenosine. Significant amounts of radioactivity from [methyl- 14C]methionine and [methyl- 14C]SAM were incorporated into theobromine and caffeine in young tea leaf segments, and very high SAH hydrolase activity was found in cell-free extracts from young tea leaves. Substantial amounts of radioactivity from [adenosyl- 14C]SAH were also recovered as theobromine and caffeine in tea leaf segments, indicating that adenosine derived from SAH is utilised for the synthesis of the purine ring of caffeine. From the profiles of activity of related enzymes in tea leaf extracts, it is proposed that the major route from SAM to caffeine is a SAM→SAH→adenosine→adenine→AMP→IMP→XMP→xanthosine→7-methylxanthosine→7-methylxanthine→theobromine→caffeine pathway. In addition, direct adenosine kinase-catalysed formation of AMP from adenosine may participate as an alternative minor route. The activity of two of the three N-methyltransferase activities involved in caffeine biosynthesis and part of the activities of SAH hydrolase, adenosine nucleosidase, adenine phosphoribosyltransferase and adenosine kinase were located in tea chloroplasts. In contrast, no detectable activity of SAM synthetase was associated with the purified chloroplast fraction. This is a first demonstration that the purine skeleton of caffeine is synthesised from adenosine released from the SAM cycle.

Ascorbic acid changes the pattern of purine metabolism during germination of white spruce somatic embryos

It has previously been shown that exogenous applications of ascorbic acid (AA) increase the conversion frequency of somatic embryos of white spruce (Picea glauca (Moench) Voss). To determine whether ascorbic acid alters purine metabolism during the early phases of embryo germination, the relative rates of purine salvage and degradation were investigated by following the metabolic fates of exogenously applied [8-14C]adenine, [8-14C]adenosine, and [8-14C]inosine, and the activities of several key enzymes. We demonstrated that both the salvage and the degradation pathways operate during germination. Specifically, adenine and adenosine were mainly salvaged to nucleotides and nucleic acids, whereas an appreciable amount of inosine was degraded to CO2 and ureides. Comparisons of purine metabolism between control and AA-treated embryos showed that exogenous applications of ascorbic acid enhanced the ability of the embryos to take up adenine and adenosine throughout the germination period. Furthermore, the higher enzymatic activities of adenosine kinase and adenine phosphoribosyltransferase were responsible for the larger proportion of adenine and adenosine being salvaged in AA-treated embryos compared with control embryos. Thus, there was a positive correlation between the ability to anabolize purine precursors and successful embryo conversion.

Purine metabolism during white spruce somatic embryo development: salvage of adenine, adenosine, and inosine

Contribution of the adenine, adenosine and inosine salvage to the purine nucleotide and nucleic acid biosynthesis during white spruce ( Picea glauca) somatic embryo maturation was estimated by in situ assays using [8- 14C]adenine, [8- 14C]adenosine and [8- 14C]inosine. The salvage of adenine and adenosine was high during the initial stages of embryo maturation, characterized by rapid cell proliferation, but it declined upon further embryo development. Inosine salvage activity was always much lower than that observed for adenine and adenosine. Consistent with these results, activities of adenine phosphoribosyltransferase (APRT) and adenosine kinase (AK) measured in the embryo extracts in vitro were much higher than the activity of inosine kinase (IK) during all stages of embryo development. Utilization of adenosine and inosine for nucleotide and nucleic acid synthesis was found to be regulated by the enzymes AK and IK, as the pattern of their activities was very similar to the activity of adenosine and inosine salvage, estimated with exogenously supplied precursors. However, little correlation between salvage of adenine and activity of APRT was found throughout somatic embryo maturation. As no adenosine nucleosidase activity was found in white spruce embryos, adenosine, but not adenine, seems to be the major end product of adenylate catabolism and becomes the predominant substrate for purine salvage in vivo. Thus, adenosine salvage appeared to have the most important role in white spruce embryos. Studies on the metabolic fate of [8- 14C]adenine and [8- 14C]adenosine suggest that turnover of adenine nucleotides is rapid, as some of them are utilized for nucleic acid synthesis. In contrast, most of [8- 14C]inosine taken up by the embryos seems to be directly catabolized by the conventional purine catabolic pathway via ureides in all stages of embryo maturation.

2,8-Dihydroxyadenine Urolithiasis in a Patient with Considerable Residual Adenine Phosphoribosyltransferase Activity in Cell Extracts but with Mutations in Both Copies of APRT

We have examined the mutational basis of adenine phosphoribosyltransferase (APRT, EC 2.4.2.7) deficiency (MIM 102600) in a patient of Polish origin who has been passing 2,8-dihydroxyadenine (DHA) stones since birth, but has considerable residual enzyme activity in lymphocyte extracts. The five exons and flanking regions of APRT were amplified by PCR and then sequenced. A single T insertion was identified at the intron 4 splice donor site (TGgtaa to TGgttaa:IVS4+2insT) in one allele from the proband, his mother, and brother. A G-to-T transversion in exon 5 (GTC-to-TTC:c.448G>T, V150F) was identified in the other allele, and this mutation was also present in one allele from the father and the paternal grandmother. Tru91 and AvaII digestions of PCR products spanning exons 4 and 5, respectively, confirmed the mutations. The mother was heterozygous for an intragenic TaqI site, but all other family members were homozygous for the presence of this site. IVS4+2insT, located on the allele containing the TaqI site, has been identified previously in several families from Europe, suggesting a founder effect, but the substitution in exon 5 is a novel mutation. IVS4+2insT is known to result in complete loss of enzyme activity, and our results suggest that V150F produces an enzyme that is nonfunctional in vivo but has considerable residual activity in vitro.

Induced activity of adenine phosphoribosyltransferase (APRT) in iron‐deficient barley roots: a possible role for phytosiderophore production

To isolate the genes involved in the response of graminaceous plants to Fe-deficient stress, a protein induced by Fe-deficiency treatment was isolated from barley (Hordeum vulgare L.) roots. Based on the partial amino acid sequence of this protein, a cDNA (HvAPT1) encoding adenine phosphoribosyltransferase (APRT: EC 2.4.2.7) was cloned from a cDNA library prepared from Fe-deficient barley roots. Southern analysis suggested that there were at least two genes encoding APRT in barley. Fe deficiency increased HvAPT1 expression in barley roots and resupplying Fe to the Fe-deficient plants rapidly negated the increase in HvAPT1 mRNA. Analysis of localization of HvAPT1-sGFP fusion proteins in tobacco BY-2 cells indicated that the protein from HvAPT1 was localized in the cytoplasm of cells. Consistent with the results of Northern analysis, the enzymatic activity of APRT in barley roots was remarkably increased by Fe deficiency. This induction of APRT activity by Fe deficiency was also observed in roots of other graminaceous plants such as rye, maize, and rice. In contrast, the induction was not observed to occur in the roots of a non-graminaceous plant, tobacco. Graminaceous plants generally synthesize the mugineic acid family phytosiderophores (MAs) in roots under Fe-deficient conditions. In this paper, a possible role of HvAPT1 in the biosynthesis of MAs related to adenine salvage in the methionine cycle is discussed.