Synthesis of phosphoribosylpyrophosphate in mammalian cells

A fully integrated new paradigm for lithium's mode of action - lithium utilizes latent cellular fail-safe mechanisms

It is proposed that lithium's therapeutic effects occur indirectly by augmenting a cascade of protective "fail-safe" pathways pre-configured to activate in response to a dangerous low cell [Mg⁺⁺] situation, eg, posttraumatic brain injury, alongside relative cell adenosine triphosphate depletion. Lithium activates cell protection, as it neatly mimics a lowered intracellular [Mg⁺⁺] level.

Phosphoribosyl Diphosphate (PRPP): Biosynthesis, Enzymology, Utilization, and Metabolic Significance

Phosphoribosyl diphosphate (PRPP) is an important intermediate in cellular metabolism. PRPP is synthesized by PRPP synthase, as follows: ribose 5-phosphate + ATP → PRPP + AMP. PRPP is ubiquitously found in living organisms and is used in substitution reactions with the formation of glycosidic bonds. PRPP is utilized in the biosynthesis of purine and pyrimidine nucleotides, the amino acids histidine and tryptophan, the cofactors NAD and tetrahydromethanopterin, arabinosyl monophosphodecaprenol, and certain aminoglycoside antibiotics. The participation of PRPP in each of these metabolic pathways is reviewed. Central to the metabolism of PRPP is PRPP synthase, which has been studied from all kingdoms of life by classical mechanistic procedures. The results of these analyses are unified with recent progress in molecular enzymology and the elucidation of the three-dimensional structures of PRPP synthases from eubacteria, archaea, and humans. The structures and mechanisms of catalysis of the five diphosphoryltransferases are compared, as are those of selected enzymes of diphosphoryl transfer, phosphoryl transfer, and nucleotidyl transfer reactions. PRPP is used as a substrate by a large number phosphoribosyltransferases. The protein structures and reaction mechanisms of these phosphoribosyltransferases vary and demonstrate the versatility of PRPP as an intermediate in cellular physiology. PRPP synthases appear to have originated from a phosphoribosyltransferase during evolution, as demonstrated by phylogenetic analysis. PRPP, furthermore, is an effector molecule of purine and pyrimidine nucleotide biosynthesis, either by binding to PurR or PyrR regulatory proteins or as an allosteric activator of carbamoylphosphate synthetase. Genetic analyses have disclosed a number of mutants altered in the PRPP synthase-specifying genes in humans as well as bacterial species.

Effects of long-term experimental diabetes on adrenal gland growth and phosphoribosyl pyrophosphate formation in growth hormone-deficient dwarf rats

The availability of growth hormone (GH)-deficient dwarf rats with otherwise normal pituitary function provides a powerful tool to examine the relative role of hyperglycaemia and the reordering of hormonal factors in the hypertrophy-hyperfunction of the adrenal gland that is seen in experimental diabetes. Here, we examine the effects of long-term (6 months) experimental diabetes on the growth of the adrenal glands; their content of phosphoribosyl pyrophosphate (PRPP); and the activity of the PRPP synthetase, G6P dehydrogenase and 6PG dehydrogenase enzymes in GH-deficient dwarf rats compared to heterozygous controls. These parameters were selected in view of the known role of PRPP in both de novo and salvage pathways of purine and pyrimidine synthesis and in the formation of NAD, and in view of the role of the oxidative enzymes of the pentose phosphate pathway in both R5P formation and the generation of the NADPH that is required in reductive synthetic reactions. This study shows that GH deficiency prevents the increase in adrenal gland weight, PRPP synthetase, PRPP content and G6P dehydrogenase and 6PG dehydrogenase. This contrasts sharply with the heterozygous group that showed the expected increase in these parameters. The blood glucose levels of the groups of long-term diabetic rats, both GH-deficient and heterozygous, remained at an elevated level throughout the experiment. These results are fully in accord with earlier evidence from studies with somatostatin analogues which showed that the GH-insulin-like growth factor I (IGF-I)-axis plays a key role in the adrenal diabetic hypertrophy-hyperfunction syndrome.

Effects of long-acting somatostatin analogues on adrenal growth and phosphoribosyl pyrophosphate formation in experimental diabetes

Adrenal growth and increased adrenal function occur in experimental diabetes. Previously, we have shown that phosphoribosyl pyrophosphate (PRPP) and PRPP synthetase increased rapidly between 3 and 7 days after induction of diabetes with streptozotocin (STZ), with less marked changes in enzymes of the pentose phosphate pathway. The present study examines the earlier phase of 1-3 days following induction of diabetes, seeking to elucidate whether control of PRPP production is a result of diabetic hyperglycaemia, or to a more general re-ordering of hormonal factors. To investigate this question, the role of insulin and two different long-acting somatostatin analogues, Angiopeptin and Sandostatin, were used in a well-established animal model. PRPP was chosen specifically as a target for these studies in view of its central role in nucleotide formation and nicotinamide mononucleotide synthesis via Nampt which is the rate-limiting step in the synthesis of NAD and which has been shown to have multiple roles in cell signalling in addition to its known function in glycolysis and energy production. Treatment with the somatostatin analogues ab initio effectively abolished the adrenal growth, the increase in PRPP formation and the rise of PRPP synthetase activity in the first 7 days of diabetes, without having any significant effect on blood glucose values. This suggests that elevated glucose per se is not responsible for the diabetic adrenal hypertrophy and implies that growth factors/hormones, regulated by somatostatin analogues, play a significant role in adrenal growth processes.

The genetic and functional basis of purine nucleotide feedback-resistant phosphoribosylpyrophosphate synthetase superactivity

The genetic and functional basis of phosphoribosylpyrophosphate synthetase (PRS) superactivity associated with purine nucleotide inhibitor-resistance was studied in six families with this X chromosome-linked purine metabolic and neurodevelopmental disorder. Cloning and sequencing of PRS1 and PRS2 cDNAs, derived from fibroblast total RNA of affected male patients by reverse transcription and PCR amplification, demonstrated that each PRS1 cDNA contained a distinctive single base substitution predicting a corresponding amino acid substitution in the PRS1 isoform. Overall, the array of substitutions encompassed a substantial portion of the translated sequence of PRS1 cDNA. Plasmid-mediated expression of variant PRS1 cDNAs in Escherichia coli BL21 (DE3/pLysS) yielded recombinant mutant PRS1s, which, in each case, displayed a pattern and magnitude of purine nucleoside diphosphate inhibitor-resistance comparable to that found in cells of the respective patient. Kinetic analysis of recombinant mutant PRS1s showed that widely dispersed point mutations in the X chromosome-linked PRPS1 gene encoding the PRS1 isoform result in alteration of the allosteric mechanisms regulating both enzyme inhibition by purine nucleotides and activation by inorganic phosphate. The functional consequences of these mutations provide a tenable basis for the enhanced production of phosphoribosylpyrophosphate, purine nucleotides, and uric acid that are the biochemical hallmarks of PRS superactivity.

The Saccharomyces cerevisiae mutation elm4-1 facilitates pseudohyphal differentiation and interacts with a deficiency in phosphoribosylpyrophosphate synthase activity to cause constitutive pseudohyphal growth

Saccharomyces cerevisiae mutant E124 was selected in a visual screen based on elongated cell shape. Genetic analysis showed that E124 contains two separate mutations, pps1-1 and elm4-1, each causing a distinct phenotype inherited as a single-gene trait. In rich medium, pps1-1 by itself causes increased doubling time but does not affect cell shape, whereas elm4-1 results in a moderate cell elongation phenotype but does not affect growth rate. Reconstructed elm4-1 pps1-1 double mutants display a synthetic phenotype in rich medium including extreme cell elongation and delayed cell separation, both characteristics of pseudohyphal differentiation. The elm4-1 mutation was shown to act as a dominant factor that potentiates pseudohyphal differentiation in response to general nitrogen starvation in a genetic background in which pseudohyphal growth normally does not occur. Thus, elm4-1 allows recognition of, or response to, a pseudohyphal differentiation signal that results from nitrogen limitation. PPS1 was isolated and shown to be a previously undescribed gene coding for a protein similar in amino acid sequence to phosphoribosylpyrophosphate synthase, a rate-limiting enzyme in the biosynthesis of nucleotides, histidine, and tryptophan. Thus, the pps1-1 mutation may generate a nitrogen limitation signal, which when coupled with elm4-1 results in pseudohyphal growth even in rich medium.

The Saccharomyces cerevisiae mutation elm4-1 facilitates pseudohyphal differentiation and interacts with a deficiency in phosphoribosylpyrophosphate synthase activity to cause constitutive pseudohyphal growth

Saccharomyces cerevisiae mutant E124 was selected in a visual screen based on elongated cell shape. Genetic analysis showed that E124 contains two separate mutations, pps1-1 and elm4-1, each causing a distinct phenotype inherited as a single-gene trait. In rich medium, pps1-1 by itself causes increased doubling time but does not affect cell shape, whereas elm4-1 results in a moderate cell elongation phenotype but does not affect growth rate. Reconstructed elm4-1 pps1-1 double mutants display a synthetic phenotype in rich medium including extreme cell elongation and delayed cell separation, both characteristics of pseudohyphal differentiation. The elm4-1 mutation was shown to act as a dominant factor that potentiates pseudohyphal differentiation in response to general nitrogen starvation in a genetic background in which pseudohyphal growth normally does not occur. Thus, elm4-1 allows recognition of, or response to, a pseudohyphal differentiation signal that results from nitrogen limitation. PPS1 was isolated and shown to be a previously undescribed gene coding for a protein similar in amino acid sequence to phosphoribosylpyrophosphate synthase, a rate-limiting enzyme in the biosynthesis of nucleotides, histidine, and tryptophan. Thus, the pps1-1 mutation may generate a nitrogen limitation signal, which when coupled with elm4-1 results in pseudohyphal growth even in rich medium.

Small elevations of glucose concentration redirect and amplify the synthesis of guanosine 5'-triphosphate in rat islets

Recent studies suggest a permissive requirement for guanosine 5'-triphosphate (GTP) in insulin release, based on the use of GTP synthesis inhibitors (such as myocophenolic acid) acting at inosine monophosphate (IMP) dehydrogenase; herein, we examine the glucose dependency of GTP synthesis. Mycophenolic acid inhibited insulin secretion equally well after islet culture at 7.8 or 11.1 mM glucose (51% inhibition) but its effect was dramatically attenuated when provided at < or = 6.4 mM glucose (13% inhibition; P < 0.001). These observations were explicable by a stimulation of islet GTP synthesis derived from IMP since, at high glucose: (a) total GTP content was augmented; (b) a greater decrement in GTP (1.75 vs. 1.05 pmol/islet) was induced by mycophenolic acid; and (c) a smaller "pool" of residual GTP persisted after drug treatment. Glucose also accelerated GTP synthesis from exogenous guanine ("salvage" pathway) and increased content of a pyrimidine, uridine 5'-triphosphate (UTP), suggesting that glucose augments production of a common regulatory intermediate (probably 5-phosphoribosyl-1-pyrophosphate). Pathway-specific radiolabeling studies confirmed that glucose tripled both salvage and de novo synthesis of nucleotides. We conclude that steep changes in the biosynthesis of cytosolic pools of GTP occur at modest changes in glucose concentrations, a finding which may have relevance to the adaptive (patho) physiologic responses of islets to changes in ambient glucose levels.

Inosine/pyruvate/phosphate medium but not adenosine/pyruvate/phosphate medium introduces millimolar amounts of 5-phosphoribosyl 1-pyrophosphate in human erythrocytes. A 31P-n.m.r. study

Incubation of human erythrocytes in medium containing inosine (10 mM), pyruvate (10 mM), phosphate (50 mM) and NaCl (75 mM) at pH 6.6 leads to a more than 1000-fold increase in the concentration of 5-phosphoribosyl 1-pyrophosphate (PRPP), as identified and quantified by 31P-n.m.r. spectroscopy. The accumulation is highly pH-dependent, with a maximum at extracellular pH 6.60, and the maximum value of 1.3-1.6 mmol/l of erythrocytes is attained within 1 h at 37 degrees C. PRPP was accumulated despite high concentrations of 2,3-bisphosphoglycerate (2,3-BPG), an inhibitor of PRPP synthetase. The concentration of PRPP correlated with the intracellular concentration of inorganic phosphate (Pi). Substitution of either adenosine or adenosine plus inosine for inosine in the medium did not lead to 31P-n.m.r.-detectable accumulation of PRPP. These results show that neither 2,3-BPG nor PRPP itself inhibits the synthesis of PRPP in the human erythrocyte. Adenosine, however, prevents the inosine-stimulated accumulation of PRPP.

Nucleotide sequence analysis of the purEK operon encoding 5'-phosphoribosyl-5-aminoimidazole carboxylase of Escherichia coli K-12

5'-Phosphoribosyl-5-aminoimidazole (AIR) carboxylase (EC 4.1.1.21) catalyzes step 6, the carboxylation of AIR to 5'-phosphoribosyl-5-aminoimidazole-4-carboxylic acid, in the de novo biosynthesis of purine nucleotides. As deduced from the DNA sequence of restriction fragments encoding AIR carboxylase and supported by maxicell analyses, AIR carboxylase was found to be composed of two nonidentical subunits. In agreement with established complementation data, the catalytic subunit (deduced Mr, 17,782) was encoded by the purE gene, while the CO2-binding subunit (deduced Mr, 39,385) was encoded by the purK gene. These two genes formed an operon in which the termination codon of the purE gene overlapped the initiation codon of the purK gene. The 5' end of the purEK mRNA was determined by mung bean nuclease mapping and was located 41 nucleotides upstream of the proposed initiation codon. The purEK operon is regulated by the purR gene product, and a purR regulatory-protein-binding site related to the sequences found in other pur loci was identified in the purEK operon control region.

Enzymes of the pathway of purine synthesis in the rat mammary gland. Changes in the lactation cycle and the effects of diabetes

Measurements were made of the activities of the enzymes of the 'de novo' and salvage pathways of purine synthesis [phosphoribosyl pyrophosphate amidotransferase (EC 2.4.2.14), adenine phosphoribosyltransferase (EC 2.4.2.7) and hypoxanthine phosphoribosyltranferase (EC 2.4.2.8)] at different stages of the lactation cycle, and the effects of diabetes on the activity of these enzymes in lactation were studied. A distinctive pattern of enzyme change was observed, in which the 'de novo' pathway enzyme phosphoribosyl pyrophosphate amidotransferase increased sharply between days 10 and 14 of pregnancy, and then remained sensibly constant until the height of lactation, whereas the enzymes of the salvage pathway increased later in pregnancy and continued to rise during lactation. Diabetes severely depressed the activity of the enzymes of the salvage pathway, but appeared to be without effect on the 'de novo' pathway enzyme. These results are discussed in relation to the provision of purine precursors from tissues outside the mammary gland.

Renal hypertrophy in experimental diabetes. The activity of the 'de novo' and salvage pathways of purine [corrected] synthesis

Measurements were made of the activity of phosphoribosyl pyrophosphate amidotransferase (PPRibP-At, EC 2.4.2.14) and of adenine (APRT, EC 2.4.2.7) and hypoxanthine (HPRT, EC 2.4.2.8) phosphoribosyltransferases, representing the 'de novo' and salvage pathways respectively. PPRibP-At activity increased within 3 days of diabetes, whereas APRT and HPRT increased later. Incorporation of [14C]formate and of [8-14C]adenine into the nucleic acids of kidney slices showed that formate was incorporated earlier, and to a greater extent, than was adenine. These results indicate that, although the 'de novo' pathway for nucleotide synthesis is the main route in early diabetes, the salvage pathway assumes greater importance at later stages.

Hepatic phosphoribosyl pyrophosphate concentration. Regulation by the oxidative pentose phosphate pathway and cellular energy status

Measurements have been made of the tissue content of phosphoribosyl pyrophosphate (PPRibP) and of a range of metabolic intermediates involved in the energy charge of the cell, the glycolytic and pentose phosphate pathways, and of the activity of the enzymes of the pentose phosphate pathway and of PPRibP synthetase (EC 2.7.6.1) in the livers of normal, diabetic, insulin-treated diabetic and starved rats and in livers of rats previously starved and then re-fed with high-fat or high-carbohydrate diets. Diabetes, starvation and high-fat diet all caused a fall in the hepatic PPRibP content, whereas insulin treatment and high-carbohydrate diet raised the tissue content. A positive correlation was shown between the PPRibP content and ATP, energy charge and the cytosolic [NAD+]/[NADH] quotient. A positive association between the PPRibP content and the flux of glucose through the pentose phosphate pathway and the synthesis of ribose 5-phosphate via the oxidative enzymes of that pathway, including ribose-5-phosphate isomerase (EC 5.3.1.6), was also observed. A negative correlation was found between the ADP, AMP and Pi contents, and no correlation existed between PPRibP content and the enzymes of the non-oxidative branch of the pentose phosphate pathway. There was no correlation between hepatic PPRibP content and the activity of PPRibP synthetase measured in vitro. These results are considered in relation to the control of PPRibP synthetase in the liver in vivo.

Concentration of phosphoribosyl pyrophosphate in renal hypertrophy. Contrasting effects of early diabetes and unilateral nephrectomy

Studies were made of the renal phosphoribosyl pyrophosphate (PPRibP) content and PPRibP synthetase (EC 2.7.6.1) activity in rats diabetic for 5, 14 or 20 days, or unilaterally nephrectomized (UN) for 5 days, and in doubly lesioned animals. Approximately equal degrees of renal enlargement were found after 5 days diabetes or 5 days UN. In the doubly lesioned animals the increment of growth was additive. Unilateral nephrectomy of 5 days duration, in contrast with diabetes, had no effect on the PPRibP content of the contralateral kidney, nor did it modify the renal PPRibP content when performed on animals diabetic for 5, 14 or 20 days. The activity of PPRibP synthetase was unaffected by diabetes, UN or diabetes +UN. The results pinpoint a stage of nucleotide synthesis which is differentially affected by the two stimuli, in line with evidence for differences in regulation of nucleic acid turnover in the two conditions.

Phosphoribosyl pyrophosphate and phosphoribosyl pyrophosphate synthetase in rat mammary gland. Changes in the lactation cycle and effects of diabetes, insulin and phenazine methosulphate

Changes in the tissue content of phosphoribosyl pyrophosphate (PPRibP), glucose 6-phosphate, ribose 5-phosphate (Rib5P), RNA and DNA, of the activity of PPRibP synthetase (EC 2.7.6.1) and the conversion of [1-14C]- and [6-14C]-glucose into 14CO2 were measured at mid-lactation in the normal and diabetic rat and in pregnancy, lactation and mammary involution in the normal rat. The PPRibP, glucose 6-phosphate and Rib5P contents increase during pregnancy and early lactation to reach a plateau value at mid-lactation, before falling sharply during weaning. The PPRibP content, PPRibP synthetase activity and flux of glucose through the oxidative pentose phosphate pathway (PPP) all change in parallel during the lactation cycle. Similarly, after 3 and 5 days duration of streptozotocin-induced diabetes, ending on day 10 of lactation, there were parallel declines in PPRibP content, PPRibP synthetase and PPP activity. The effect of streptozotocin was prevented by pretreatment with nicotinamide and partially reversed by insulin administration. Addition of insulin to lactating rat mammary-gland slices incubated in vitro significantly raised the PPRibP content (+47%) and the activity of the PPP (+40%); phenazine methosulphate, which gives a 2-fold increase in PPP activity, raised the PPRibP content of lactating mammary gland slices by approx. 3-fold. It is concluded that Rib5P, generated in the oxidative segment of the PPP, is an important determinant of PPRibP synthesis in the lactating rat mammary gland and that insulin plays a central role in the regulation of the bioavailability of this precursor of nucleotide and nucleic acid synthesis.

Concentration of phosphoribosyl pyrophosphate in the kidney during development and in experimental diabetic hypertrophy

The effect of developmental growth on the kidney content of phosphoribosyl pyrophosphate PPRibP was studied in rats at ages between the foetal animal and up to 100 days of age. In addition, the effect of short-term diabetes (up to 14 days) on the renal content of PPRibP was studied in immature rats and in adults aged approx. 60 days. The developmental pattern of PPRibP is such that the PPRibP content is lowest in the young rat and increases as the rate of kidney growth slows. In the adult rat, the early kidney hypertrophy of diabetes is accompanied by a fall in PPRibP content and, again, the PPRibP content returns to normal as the rate of kidney hypertrophy diminishes. Induction of diabetes in the immature rat causes a lesser degree of kidney hypertrophy and also a smaller depression of renal PPRibP content. The activity of PPRibP synthetase (EC 2.7.6.1) is not significantly affected by age or diabetes. The changes in PPRibP content are discussed in relation to the generation of ribose 5-phosphate by the pentose phosphate pathway and the utilization of PPRibP for nucleotide synthesis via the 'de novo' and salvage pathways.

Selective expression of phosphoribosylpyrophosphate synthetase superactivity in human lymphoblast lines

Phenotypic expression of 5-phosphoribosyl 1-pyrophosphate (PRPP) synthetase superactivity was examined in lymphoblast lines derived from six unrelated male patients. Fibroblasts from these individuals have increased rates of PRPP and purine nucleotide synthesis and express four classes of kinetic derangement underlying enzyme superactivity: increased maximal reaction velocity (catalytic defect); inhibitor resistance (regulatory defect); increased substrate affinity (substrate binding defect); and combined catalytic and regulatory defects. Lymphoblast lines from three patients with catalytic defects and from three normal individuals were indistinguishable with respect to enzyme activities, PRPP concentrations and generation, and rates of purine synthesis. Enzyme in lymphoblasts from a patient with combined defects also showed normal maximal reaction velocity but expressed purine nucleotide inhibitor resistance. A second regulatory defect and a substrate binding defect were also demonstrable in lymphoblasts and were identical to the enzyme defects in fibroblasts from the respective patients. Regulatory and substrate binding defects in lymphoblasts were accompanied by increased rates of PRPP and purine nucleotide synthesis. Among explanations for selective expression of enzyme superactivity, reduced concentrations of catalytically superactive enzymes seemed unlikely: immunoreactive PRPP synthetase was comparable in normal-derived and patient-derived cells. Activation of normal enzyme in transformed lymphocytes was also unlikely because absolute specific activities of lymphoblast PRPP synthetases corresponded to those of normal fibroblast and erythrocyte enzymes. Abnormal electrophoretic mobilities and thermal stabilities, identified in certain catalytically superactive fibroblast PRPP synthetases, were not found in the corresponding lymphoblast enzymes. Thus, lymphoblast PRPP synthetases from patients with catalytic superactivity appeared to differ structurally and functionally from their fibroblast counterparts.