Activities of amidophosphoribosyltransferase (EC2.4.2.14) and the purine phosphoribosyltransferases (EC2.4.2.7 and 2.4.2.8), and the phosphoribosylpyrophosphate content of rat central nervous system at different stages of development--their possible relationship to the neurological dysfunction in the Lesch-Nyhan syndrome

The mammalian purine salvage pathway as an exploitable route for cerebral bioenergetic support after brain injury

Purine-based molecules play ancient, fundamental, and evolutionarily-conserved roles across life on Earth, ranging from DNA and RNA, to the universal energy currency, ATP. In mammals, the two primary routes for the synthesis of the adenine nucleotides ATP, ADP and AMP, and, as a consequence, the major bioactive metabolite adenosine, are the de novo purine biosynthesis (DNPB) pathway, and the purine salvage pathway (PSP). Of the two, the PSP dominates in both the mammalian brain and heart. This is because the PSP utilizes the breakdown products of ATP, occasioned by the high energy demands of these organs, to rapidly regenerate adenine nucleotides. This resynthesis route, while efficient and energetically favourable, leaves these organs vulnerable to loss of salvageable metabolites, with the potential for protracted depletion of the means to synthesize ATP, and the ability to deploy neuro- and cardioprotective adenosine. Having previously shown that hippocampal cellular ATP and adenosine release can be increased by supplying substrates for the PSP (d-ribose and adenine), we now explore the expression of DNPB and PSP enzymes in hippocampal neurons and astrocytes based on available transcriptomic data. We find that key enzymes of the PSP are expressed at higher levels than those in the DNPB pathway, and that PSP enzymes are expressed at higher levels in neurons than in astrocytes. These data reflect the importance of the PSP in the mammalian brain and imply that pharmacological targeting of the PSP may be particularly beneficial to neurons at times of metabolic stress. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Metabolomics Based Identification of SIRT5 and Protein Kinase C Epsilon Regulated Pathways in Brain

The role of Sirtuins in brain function is emerging, yet little is known about SIRT5 in this domain. Our previous work demonstrates that protein kinase C epsilon (PKCε)-induced protection from focal ischemia is lost in SIRT5-/- mice. Thus, metabolic regulation by SIRT5 contributes significantly to ischemic tolerance. The aim of this study was to identify the SIRT5-regulated metabolic pathways in the brain and determine which of those pathways are linked to PKCε. Our results show SIRT5 is primarily expressed in neurons and endothelial cells in the brain, with mitochondrial and extra-mitochondrial localization. Pathway and enrichment analysis of non-targeted primary metabolite profiles from Sirt5-/- cortex revealed alterations in several pathways including purine metabolism (urea, adenosine, adenine, xanthine), nitrogen metabolism (glutamic acid, glycine), and malate-aspartate shuttle (malic acid, glutamic acid). Additionally, perturbations in β-oxidation and carnitine transferase (pentadecanoic acid, heptadecanoic acid) and glutamate transport and glutamine synthetase (urea, xylitol, adenine, adenosine, glycine, glutamic acid) were predicted. Metabolite changes in SIRT5-/- coincided with alterations in expression of amino acid (SLC7A5, SLC7A7) and glutamate (EAAT2) transport proteins as well as key enzymes in purine (PRPS1, PPAT), fatty acid (ACADS, HADHB), glutamine-glutamate (GAD1, GLUD1), and malate-aspartate shuttle (MDH1) metabolic pathways. Moreover, PKCε activation induced alternations in purine metabolites (urea, glutamine) that overlapped with putative SIRT5 pathways in WT but not in SIRT5-/- mice. Finally, we found that purine metabolism is a common metabolic pathway regulated by SIRT5, PKCε and ischemic preconditioning. These results implicate Sirt5 in the regulation of pathways central to brain metabolism, with links to ischemic tolerance.

Multifocal atrophy of cerebellar internal granular neurons in lesch-nyhan disease: case reports and review

The neuropathologic findings in 31 cases (aged 6 months to 33 years) of Lesch-Nyhan disease (hypoxanthine-guanine phosphoribosyltransferase deficiency) have been previously reported. Herein 2 additional cases, a 10-year-old boy and a 21-year-old man, are described. Both cases had unusual cerebellar abnormalities comprising multifocal internal granular layer atrophy with sparing of the Purkinje layer, one had a slightly small brain, and neither had striatal abnormalities. Careful review of the literature indicates that the most prevalent neuropathologic abnormalities are small cerebrum (13 of 33 cases) and multifocal cerebellar lesions (9 of 33 cases), although these could be underreported. Other authors have disregarded these abnormalities, focusing on the apparently normal basal nuclei, and they have suggested that the clinical neurologic abnormalities are based solely on changes in neurotransmitters. We discuss potential mechanisms of cerebellar damage, suggest that the cerebellar abnormality could in part explain the clinical syndrome, and recommend that cerebellar structure and function should be more carefully studied in Lesch-Nyhan disease.

Adenosine transport in peripheral blood lymphocytes from Lesch-Nyhan patients

We postulated that adenosine function could be related to some of the neurological features of Lesch-Nyhan syndrome and therefore characterized adenosine transport in PBLs (peripheral blood lymphocytes) obtained from Lesch-Nyhan patients (PBL(LN)) and from controls (PBL(C)). Adenosine transport was significantly lower in PBL(LN) when compared with that in PBL(C) and a significantly lower number of high affinity sites for [(3)H]nitrobenzylthioinosine binding were quantified per cell ( B (max)) in PBL(LN) when compared with that in PBL(C). After incubation with 25 microM hypoxanthine, adenosine transport was significantly decreased in PBL(LN) with respect to PBL(C). Hypoxanthine incubation lowers [(3)H]nitrobenzylthioinosine binding in PBL(C), with respect to basal conditions, but does not affect it in PBL(LN). This indicates that hypoxanthine affects adenosine transport in control and hypoxanthine-guanine phosphoribosyltransferase-deficient cells by different mechanisms.

Development of a new reporter gene system--dsRed/xanthine phosphoribosyltransferase-xanthine for molecular imaging of processes behind the intact blood-brain barrier

We report the development of a novel dual-modality fusion reporter gene system consisting of Escherichia coli xanthine phosphoribosyltransferase (XPRT) for nuclear imaging with radiolabeled xanthine and Discosoma red fluorescent protein for optical fluorescent imaging applications. The dsRed/XPRT fusion gene was successfully created and stably transduced into RG2 glioma cells, and both reporters were shown to be functional. The level of dsRed fluorescence directly correlated with XPRT enzymatic activity as measured by ribophosphorylation of [14C]-xanthine was in vitro (Ki = 0.124 +/- 0.008 vs. 0.00031 +/- 0.00005 mL/min/g in parental cell line), and [*]-xanthine octanol/water partition coefficient was 0.20 at pH = 7.4 (logP = -0.69), meeting requirements for the blood-brain barrier (BBB) penetrating tracer. In the in vivo experiment, the concentration of [14C]-xanthine in the normal brain varied from 0.20 to 0.16 + 0.05% dose/g under 0.87 + 0.24% dose/g plasma radiotracer concentration. The accumulation in vivo in the transfected flank tumor was to 2.4 +/- 0.3% dose/g, compared to 0.78 +/- 0.02% dose/g and 0.64 +/- 0.05% dose/g in the control flank tumors and intact muscle, respectively. [14C]-Xanthine appeared to be capable of specific accumulation in the transfected infiltrative brain tumor (RG2-dsRed/XPRT), which corresponded to the 585 nm fluorescent signal obtained from the adjacent cryosections. The images of endogenous gene expression with the "sensory system" have to be normalized for the transfection efficiency based on the "beacon system" image data. Such an approach requires two different "reporter genes" and two different "reporter substrates." Therefore, the novel dsRed/XPRT fusion gene can be used as a multimodality reporter system in the biological applications requiring two independent reporter genes, including the cells located behind the BBB.

Lesch-Nyhan disease and the basal ganglia

The purpose of this review is to summarize emerging evidence that the neurobehavioral features of Lesch-Nyhan disease (LND), a developmental disorder caused by congenital deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT), may be attributable to dysfunction of the basal ganglia. Affected individuals have severe motor disability described by prominent extrapyramidal features that are characteristic of dysfunction of the motor circuits of the basal ganglia. They also display disturbances of ocular motility, cognition, and behavioral control that may reflect disruption of other circuits of the basal ganglia. Though neuropathologic studies of autopsy specimens have revealed no obvious neuroanatomical abnormalities in LND, neurochemical studies have demonstrated 60-90% reductions in the dopamine content of the basal ganglia. In addition, recent PET studies have documented significant reductions in dopamine transporters and [18F]fluorodopa uptake in the basal ganglia. These findings support the proposal that many of the neurobehavioral features of LND might be related to dysfunction of the basal ganglia.

Abnormal purine and pyrimidine nucleotide content in primary astroglia cultures from hypoxanthine-guanine phosphoribosyltransferase-deficient transgenic mice

Lesch-Nyhan syndrome is a pediatric metabolic-neurological syndrome caused by the X-linked deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). The cause of the metabolic consequences of HGPRT deficiency has been clarified, but the connection between the enzyme deficiency and the neurological manifestations is still unknown. In search for this connection, in the present study, we characterized purine nucleotide metabolism in primary astroglia cultures from HGPRT-deficient transgenic mice. The HGPRT-deficient astroglia exhibited the basic abnormalities in purine metabolism reported before in neurons and various other HGPRT-deficient cells. The following abnormalities were found: absence of detectable uptake of guanine and of hypoxanthine into intact cell nucleotides; 27.8% increase in the availability of 5-phosphoribosyl-1-pyrophosphate; 9.4-fold acceleration of the rate of de novo nucleotide synthesis; manyfold increase in the excretion into the culture media of hypoxanthine (but normal excretion of xanthine); enhanced loss of label from prelabeled adenine nucleotides (loss of 71% in 24 h, in comparison with 52.7% in the normal cells), due to 4.2-fold greater excretion into the media of labeled hypoxanthine. In addition, the HGPRT-deficient astroglia were shown to contain lower cellular levels of ADP, ATP, and GTP, indicating that the accelerated de novo purine synthesis does not compensate adequately for the deficiency of salvage nucleotide synthesis, and higher level of UTP, probably due to enhanced de novo synthesis of pyrimidine nucleotides. Altered nucleotide content in the brain may have a role in the pathogenesis of the neurological deficit in Lesch-Nyhan syndrome.

A new stable isotope method enables the simultaneous measurement of nucleic acid and protein synthesis in vivo in mice

We developed a method based on the incorporation of 13C2-units derived from [U-13C]glycine that allows the simultaneous quantification of tissue protein and RNA synthesis in vivo. Two groups of 26 mice were fed diets containing a high (HF diet) or a low quantity of fiber (LF diet). After 6 d, [U13C]glycine was added to the diet and groups of four mice were killed after 2, 4, 6, 8, 12 and 24 h. Hepatic and intestinal mucosal free and RNA-bound purine nucleosides were extracted and enzymically degraded to allantoin. Allantoin was degraded to glyoxylate, which was then reductively aminated to glycine, which contains the two 13C-atoms incorporated via de novo synthesis. Ingestion of the HF diet was associated with significantly (P < 0.05) higher rates of total RNA synthesis in both the liver ( HF diet, 29%/d; LF diet, 21%/d) and mucosa (HF diet, 27%/d; LF diet, 17 %/d). The mean rates of RNA synthesis in each tissue were significantly (P < 0.01) lower than the respective rates of protein synthesis (liver, 67%/d; mucosa, 74%/d). The isotopic enrichment of the free purine nucleotide pool increased rapidly and exponentially, but the steady-state value was substantially (P < 0. 001) lower than that of the RNA-bound purines. The results suggest that the free nucleotide pool consists of two kinetically distinct compartments, one of which is small and has a rapid rate of turnover. This, we propose, acts as the RNA precursor pool. The other is large, has a low rate of turnover and, we believe, is the pool of adenosine triphosphate involved in cellular energetics.

Developmental changes in purine nucleotide metabolism in cultured rat astroglia

The present study was conducted in order to clarify the role of the glia in brain purine metabolism. This, in connection with the clarification of the etiology of the neurological manifestations associated with some of the inborn errors of purine metabolism in man. Purine nucleotide content, the capacity for de novo and salvage purine synthesis and the activity of several enzymes of purine nucleotide degradation, were assayed in primary cultures of rat astroglia in relation to culture age. The capacity of the intact cells to produce purine nucleotides de novo exhibited a marked decrease with the culture age, but the activity of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), catalyzing salvage nucleotide synthesis, increased. Aging was also associated with a marked increase in the activity of the degradation enzymes AMP deaminase, purine nucleoside phosphorylase (PNP) and guanine deaminase (guanase). The activity of adenosine deaminase and of AMP-5'-nucleotidase, increased markedly during the first 17 days in culture, but decreased thereafter. The results indicate that purine nucleotide metabolism in the cultured astroglia is changing with aging to allow the cells to maintain their nucleotide pool by reutilization of preformed hypoxanthine, rather than by de-novo production of new purines. Aging is also associated with increased capacity for operation of the adenine nucleotide cycle, contributing to the homeostasis of adenine nucleotides and to the energy charge of the cells. In principle, the age-related alterations in purine metabolism in the astroglia resemble those occurring in the maturating neurons, except for the capacity to produce purines de novo, which exhibited inverse trends in the two tissues. However, in comparison to the neurons, the cultured astroglia possess the capacity for a more intensive metabolism of purine nucleotides.

Characterization of the alterations in purine nucleotide metabolism in hypoxanthine-guanine phosphoribosyltransferase-deficient rat neuroma cell line

A rat neuroma cell line (B103 4C), deficient of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), was utilized as a model tissue in search for the biochemical basis of the Lesch-Nyhan syndrome (LNS). The HGPRT-deficient neurons exhibited the following properties: an almost complete absence of uptake of guanine and of hypoxanthine into intact cell nucleotides (0.92% and 0.69% of normal, respectively); a significant increase in the availability of 5'-phosphoribosyl-1-pyrophosphate; a three- to fourfold acceleration of the rate of de novo nucleotide synthesis; a normal excretion of xanthine, but 15-fold increase in the excretion of hypoxanthine into the culture media; a normal cellular purine nucleotide content, including the absence of 5-amino-4-imidazole carboxamide nucleotides (Z-nucleotides), but enhanced turnover of adenine nucleotides (loss of 86% of the radioactivity of the prelabeled pool in 24 h, in comparison to 73% in the normal line), and an elevated UTP content. The results suggest that, under physiological conditions, guanine salvage does not occur in the normal neurons, but that hypoxanthine salvage is of great importance in the homeostasis of the adenine nucleotide pool. The finding of the normal profile of purine nucleotides in the HGPRT-deficient neurons indicates that the lack of hypoxanthine salvage is adequately compensated by the enhanced de novo nucleotide synthesis. These results did not furnish evidence in support of the possibility that GTP or ATP depletion, or Z-nucleotide accumulation, occurs in HGPRT-deficient neurons and that these are etiological factors causing the neurological abnormalities in LNS.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of guanine and guanine nucleotides in primary rat neuronal cultures

The metabolic fate of guanine and of guanine ribonucleotides (GuRNs) in cultured rat neurons was studied using labeled guanine. 8-Aminoguanosine (8-AGuo), an inhibitor of purine nucleoside phosphorylase, was used to clarify the pathways of GMP degradation, and mycophenolic acid, an inhibitor of IMP dehydrogenase, was used to assess the flux from IMP to GMP and, indirectly, the activity of the guanine nucleotide cycle (GMP----IMP----XMP----GMP). The main metabolic fate of guanine in the neurons was deamination to xanthine, but significant incorporation of guanine into GuRNs, at a rate of approximately 8.5-13.1% of that of the deamination, was also demonstrated. The turnover rate of GuRNs was fast (loss of 80% of the radioactivity of the prelabeled pool in 22 h), reflecting synthesis of nucleic acids (32.8% of the loss in radioactivity) and degradation to xanthine, guanine, hypoxanthine, guanosine, and inosine (49.3, 4.3, 4.1, 1.1, and 0.5% of the loss, respectively). Of the radioactivity in GuRNs, 7.9% was shifted to adenine nucleotides. The accumulation of label in xanthine indicates (in the absence of xanthine oxidase) that the main degradative pathway from GMP is that to xanthine through guanosine and guanine. The use of 8-AGuo confirmed this pathway but indicated the operation of an additional, relatively slower degradative pathway, that from GMP through IMP to inosine and hypoxanthine. Hypoxanthine was incorporated mainly into adenine nucleotide (91.5%), but a significant proportion (6%) was found in GuRNs.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in the activity of enzymes of purine metabolism in rat neuronal cells in culture and in whole brain

The activities (Vmax) of several enzymes of purine nucleotide metabolism were assayed in premature and mature primary rat neuronal cultures and in whole rat brains. In the neuronal cultures, representing 90% pure neurons, maturation (up to 14 days in culture) resulted in an increase in the activities of guanine deaminase (guanase), purine-nucleoside phosphorylase (PNP), IMP 5'-nucleotidase, adenine phosphoribosyltransferase (APRT), and AMP deaminase, but in no change in the activities of hypoxanthine-guanine phosphoribosyltransferase (HGPRT), adenosine deaminase, adenosine kinase, and AMP 5'-nucleotidase. In whole brains in vivo, maturation (from 18 days of gestation to 14 days post partum) was associated with an increase in the activities of guanase, PNP, IMP 5'-nucleotidase, AMP deaminase, and HGPRT, a decrease in the activities of adenosine deaminase and IMP dehydrogenase, and no change in the activities of APRT, AMP 5'-nucleotidase, and adenosine kinase. The profound changes in purine metabolism, which occur with maturation of the neuronal cells in primary cultures in vitro and in whole brains in vivo, create an advantage for AMP degradation by deamination, rather than by dephosphorylation, and for guanine degradation to xanthine over its reutilization for synthesis of GMP. The physiological meaning of the maturational increase in these two ammonia-producing enzymes in the brain is not yet clear. The striking similarity in the alterations of enzyme activities in the two systems indicates that the primary culture system may serve as an appropriate model for the study of purine metabolism in brain.

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.

Ontogenesis of adenosine deaminase activity in rat brain

The activity of adenosine deaminase (ADA) was determined in whole brain of rats at the embryonic age of 15 days through to adulthood and in nine brain regions in rats 1 day old through to adulthood. In 1-day-old rats, the highest activity was seen in olfactory bulbs (550 +/- 15 nmol/mg protein/30 min) and this was 4.5-fold higher than that in the pons, which was the lowest. In adult animals, olfactory bulb still contained the greatest activity, which was about eightfold higher than hippocampus, which had the lowest. Except for hypothalamus, where ADA activity increased nearly twofold in rats between the ages of 1 and 50 days, significant decreases of as much as fivefold were found in whole brain, superior colliculus, cortex, hippocampus, cerebellum, olfactory bulbs, and olfactory nucleus. In contrast, ADA activity in pons and subcortex remained relatively constant throughout the developmental period. The Km values for ADA in whole brain at 18 days gestation (48 +/- 5 microM) were not significantly different from that observed in adult rats (38 +/- 7 microM), whereas the Vmax values decreased significantly from 339 +/- 9 to 108 +/- 8 nmol/mg protein/30 min. Taken together, the developmental patterns observed in the various brain regions appear not to correspond to any one particular process such as periods of rapid cell proliferation, cell death, synaptogenesis, or myelination. Nor do they correspond to known developmental profiles of transmitters, their receptors, or their metabolic enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Purine phosphoribosyltransferase (EC 2.4.2.7 and 2.4.2.8) and purine de novo synthesis activity in rat testicular tissue at different stages of development, and their correlation with the circulating levels of gonadotrophins and testosterone, and with structural changes

The overall activity of the purine de novo synthesis pathway and the activities of purine phosphoribosyltransferase in the rat testis were measured at different ages and were correlated with histological observations. Similar studies of the concentration of circulating gonadotrophins and testosterone were performed. The purine phosphoribosyltransferase activities were between two and three orders of magnitude greater than purine de novo synthesis. The peak activity of the purine de novo synthesis pathway coincided with the first appearance of meiosis in the spermatocytes immediately before the luteinising hormone (LH) level rose to its peak. The highest activity of the hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8) - catalysed purine salvage pathway coincided with the first appearance of mature spermatozoa in the tubules just after the occurrence of peak levels of follicle-stimulating hormone (FSH). These findings are linked to the development of testicular atrophy in cases of severe HPRT deficiency in man.

Purine nucleotide synthesis in adrenal chromaffin cells

The synthesis of purine nucleotides from the salvage precursors adenine and adenosine, and from the de novo precursors formate and glycine, was studied in isolated adrenal chromaffin cells. Both [8-14C]adenine and [8-14C]adenosine from extracellular medium are effectively incorporated into intracellular nucleotides. [14C]Formate and [U-14C]glycine are also incorporated, but de novo synthesis is clearly lower than synthesis from salvage precursors, although similar to de novo synthesis in liver. The enzymes responsible for adenine and adenosine salvage, adenine phosphoribosyltransferase and adenosine kinase, were purified about 1,500-fold. Both enzymes are mainly cytosolic and exhibit a similar molecular weight of around 42,000. The results suggest that chromaffin cells can replenish their intracellular nucleotides lost during the secretory event by an active synthesis from salvage and de novo precursors.

The biochemical basis of the behavioral disorder in the Lesch-Nyhan syndrome

An inherited complete deficiency of hypoxanthine-guanine phosphoribosyltransferase in male children is associated with a severe neurological disorder characterized by chloroform and athetoid movements, hypertonicity, mental retardation, and self-injurious behavior. In the review that follows several possible mechanisms by which the enzyme defect may cause the CNS disorder are discussed. Current evidence suggests that the primary neural deficit in the Lesch-Nyhan syndrome is a deficiency of dopamine in the basal ganglia. It is argued that this neurochemical lesion results from a deficiency of purine nucleotides which impairs arborization of nigrostriatal neurons during perinatal development. Differences in the ontogenetic timing of the neurochemical lesion may be partly responsible for the different neurological symptoms displayed by persons afflicted with the Lesch-Nyhan and Parkinson's syndromes.

Defects of tetrahydrobiopterin synthesis and their possible relationship to a disorder of purine metabolism (the Lesch-Nyhan syndrome)

The metabolic pathways of pterin de novo synthesis, interconversion and salvage which lead to the tetrahydrobiopterin cofactor of phenylalanine 4-monooxygenase, tyrosine 2-monooxygenase and tryptophan 5-monooxygenase are reviewed and data on the enzymes which catalyze the individual steps are presented. Analogies drawn between the inborn errors of tetrahydrobiopterin production and the Lesch-Nyhan syndrome, in which purine salvage is deficient, are used as a basis for the hypothesis that the neurological manifestations of the Lesch-Nyhan syndrome are due to neurotransmitter imbalance which stems from an imbalance of the aromatic amino acid monooxygenase activities which are themselves due to impaired pterin biosynthesis. The latter arises because, in the absence of the hypoxanthine phosphoribosyltransferase catalyzed purine salvage pathway, the supply of GTP for the GTP cyclohydrolase reaction, which is the first reaction on the pterin de novo synthesis pathway, is reduced. It is proposed that the different aromatic amino acid monooxygenases are differentially affected by this constrained pterin production. The activities of those most directly related to the quantal production of the cerebral neurotransmitters dopamine, norepinephrine and 5-hydroxytryptamine are affected whereas liver phenylalanine 4-monooxygenase activity is not overtly impaired. The results of different lines of research which support this concept are cited, as is direct evidence for a selective reduction of dopamine production in the basal ganglia of patients with the Lesch-Nyhan syndrome. It is proposed that lack of GMP for functions, other than its role in pterin de novo synthesis, accounts for the features of the Lesch-Nyhan syndrome which do not occur when only tetrahydrobiopterin production is deficient as in the inborn errors of tetrahydrobiopterin synthesis.

Some regulatory and integrative aspects of purine nucleotide biosynthesis and its control: an overview

The regulation and integration of purine nucleotide biosynthesis is considered from the viewpoint of the main groups of reaction sequences involved and with respect to some specific organs and tissues. Inhibiting either IMP dehydrogenase or adenylosuccinate synthetase in rat liver in vitro reduced the rate of purine do novo synthesis with respect to the purine remaining in the tissue and did not materially affect the rate with respect to the purines extruded into the incubation medium. These results are considered in contrast to the results of previous studies in cultured lymphoblasts. The relative activities of purine de novo synthesis and of purine salvage have been assessed in different tissues by the activities of amidophosphoribosyltransferase and hypoxanthine phosphoribosyltransferase (HPRT), respectively. Changes in purine de novo synthesis as measured by [14C]formate incorporation into cellular purines were reflected in the amidophosphoribosyltransferase activities. The capacity of different tissues to synthesize purines de novo is widespread and the role of the liver as the main site of purine de novo synthesis in vivo and exporting purines to other tissues appears questionable. Regulatory mechanisms may well be tissue specific. The age-related changes in the activity of the purine de novo synthesis and purine salvage pathways, respectively, in the brain suggest that it is physiological or neuropharmacological functions of the developed brain rather than cell division and organogenesis which require a high level of purine salvage relative to purine de novo synthesis. This is compatible with the observation that purine de novo synthesis alone can meet the needs for additional purine nucleotides which lectin induced lymphocyte transformation involves. The mechanism whereby purine de novo synthesis is initiated during lectin induced lymphoblast transformation remains obscure.

Developmental changes in the folate-dependent enzymes of de novo purine biosynthesis in rat brain

The activities of the two folate-dependent enzymes in the de novo purine biosynthetic pathway (e.g., glycinamide ribonucleotide transformylase and aminoimidazolecarboxamide ribonucleotide transformylase), have been evaluated as a function of age in crude extracts from rat brain, liver, kidney, and spleen. The activities of the enzymes in brain are similar to those found in liver and kidney. In all tissues the activity of both enzymes was higher during early development, more than nine times above adult levels. In the CNS the enzymatic activities are apparently related to the periods of increased nucleic acid synthesis, with different activities being found in different regions during development. Our findings lend strong support to the suggestion that folic acid-dependent metabolism plays an important role during early development of the brain.