Giuseppe Pompucci

Neurological Disorders of Purine and Pyrimidine Metabolism

Purines and pyrimidines, regarded for a long time only as building blocks for nucleic acid synthesis and intermediates in the transfer of metabolic energy, gained increasing attention since genetically determined aberrations in their metabolism were associated clinically with various degrees of mental retardation and/or unexpected and often devastating neurological dysfunction. In most instances the molecular mechanisms underlying neurological symptoms remain undefined. This suggests that nucleotides and nucleosides play fundamental but still unknown roles in the development and function of several organs, in particular central nervous system. Alterations of purine and pyrimidine metabolism affecting brain function are spread along both synthesis (PRPS, ADSL, ATIC, HPRT, UMPS, dGK, TK), and breakdown pathways (5NT, ADA, PNP, GCH, DPD, DHPA, TP, UP), sometimes also involving pyridine metabolism. Explanations for the pathogenesis of disorders may include both cellular and mitochondrial damage: e.g. deficiency of the purine salvage enzymes hypoxanthine-guanine phosphoribosyltransferase and deoxyguanosine kinase are associated to the most severe pathologies, the former due to an unexplained adverse effect exerted on the development and/or differentiation of dopaminergic neurons, the latter due to impairment of mitochondrial functions. This review gathers the presently known inborn errors of purine and pyrimidine metabolism that manifest neurological syndromes, reporting and commenting on the available hypothesis on the possible link between specific enzymatic alterations and brain damage. Such connection is often not obvious, and though investigated for many years, the molecular basis of most dysfunctions of central nervous system associated to purine and pyrimidine metabolism disorders are still unexplained.

HPLC determination of oxidized and reduced pyridine coenzymes in human erythrocytes

The nucleotide concentrations in acid and alkaline erythrocyte extracts have been measured by RP-HPLC in healthy controls and in patients bearing different inherited disorders, with altered erythrocyte NAD(P) levels. The objective was the simultaneous determination of the nucleotide profile and of the oxidative state of pyridine coenzymes by the most suitable extraction method. Both alkaline and acid extractions were necessary to obtain the complete pattern, due to defective recovery of the oxidized or reduced coenzymes, respectively, during the extraction procedures. Purine nucleotide quantification seemed to be reliable by all methods. High NADP+ levels were confirmed in two glucose-6-phosphate dehydrogenase deficient patients, coupled with raised NAD levels, lowered NADPH/NADP+ ratio and increased NADH/NAD+ ratio. Higher NAD+ and normal or lower NADH/NAD+ ratios were found in two hypoxanthine-phosphoribosyltransferase deficient patients, while a patient with superactive phosphoribosylpyrophosphate synthetase showed a decreased NADH level in addition to the low NAD+ level previously found.

Simple non-radiochemical HPLC-linked method for screening for purine metabolism disorders using dried blood spot.

BACKGROUND Pathologies associated with rare inherited disorders affecting purine metabolic pathways range from renal failure to neurological dysfunction and immunodeficiency. The disorders are usually diagnosed by measuring enzyme activities in hemolysates. A non-radiochemical HPLC-linked method is described for simultaneous determination of the activities of hypoxanthine-guanine phosphoribosyltransferase (HPRT: E.2.4.2.8.), adenine phosphoribosyltransferase (APRT: E.2.4.2.7.), adenosine deaminase (ADA: E.3.5.4.4.) and purine nucleoside phosphorylase (PNP: E.2.4.2.1.) in dried blood spots. METHOD 7-mm-diameter blood spots stored at 4 degrees C or room temperature were transferred to an Eppendorf tube and eluted with 500-microl 0.1 mol/l Tris-HCl buffer, pH 7.4. The eluate was added to substrate solutions and incubated at 37 degrees C. Reaction products were analysed by HPLC. RESULTS AND CONCLUSIONS The enzyme activities tested in spot eluates were similar to those in erythrocyte lysates from the same subjects. None of the enzymatic activities tested were significantly affected by different storage temperatures. The main advantages of the proposed method are small blood volume required, easy sample collection and transfer, and accurate results. The method is therefore suitable for screening inborn errors of purine metabolism even in newborns.

Thiopurine methyltransferase activity in the erythrocytes of adults and children: an HPLC-linked assay

A non-radioactive method that uses reverse-phase high performance liquid chromatography is described for the determination of thiopurine methyltransferase (E.C. 2.1.1.67) activity in human erythrocytes. The method is based on the direct quantitation of 6-methyl-mercaptopurine produced from 6-mercaptopurine by crude erythrocyte lysates. The method is accurate and reliable and suitable for diagnostic use. Activity values in control adults ranged from 5 to 32 pmol/h/mg haemoglobin. The activity in the erythrocytes of adult males was significantly higher compared to females (21 +/- 5 and 15 +/- 8 pmol/h/mg haemoglobin, respectively). The activity measured in the erythrocytes of children (22 +/- 5 pmol/h/mg haemoglobin) did not show any significant difference compared to adults. Thiopurine methyltransferase activity was measured in a female patient with systemic sclerosis who developed severe bone marrow depression after treatment with azathioprine and allopurinol. Activity (6.3 +/- 0.5 pmol/h/mg haemoglobin) was found in the lowest range of controls thus supporting the hypothesis that it could be responsible for increased azathioprine cytotoxicity.

Metabolic fate of extracellular NAD in human skin fibroblasts

Extracellular NAD is degraded to pyridine and purine metabolites by different types of surface‐located enzymes which are expressed differently on the plasmamembrane of various human cells and tissues. In a previous report, we demonstrated that NAD‐glycohydrolase, nucleotide pyrophosphatase and 5′‐nucleotidase are located on the outer surface of human skin fibroblasts. Nucleotide pyrophosphatase cleaves NAD to nicotinamide mononucleotide and AMP, and 5′‐nucleotidase hydrolyses AMP to adenosine. Cells incubated with NAD, produce nicotinamide, nicotinamide mononucleotide, hypoxanthine and adenine. The absence of ADPribose and adenosine in the extracellular compartment could be due to further catabolism and/or uptake of these products. To clarify the fate of the purine moiety of exogenous NAD, we investigated uptake of the products of NAD hydrolysis using U‐[14C]‐adenine‐NAD. ATP was found to be the main labeled intracellular product of exogenous NAD catabolism; ADP, AMP, inosine and adenosine were also detected but in small quantities. Addition of ADPribose or adenosine to the incubation medium decreased uptake of radioactive purine, which, on the contrary, was unaffected by addition of inosine. ADPribose strongly inhibited the activity of ecto‐NAD‐hydrolyzing enzymes, whereas adenosine did not. Radioactive uptake by purine drastically dropped in fibroblasts incubated with 14C‐NAD and dipyridamole, an inhibitor of adenosine transport. Partial inhibition of [14C]‐NAD uptake observed in fibroblasts depleted of ATP showed that the transport system requires ATP to some extent. All these findings suggest that adenosine is the purine form taken up by cells, and this hypothesis was confirmed incubating cultured fibroblasts with 14C‐adenosine and analyzing nucleoside uptake and intracellular metabolism under different experimental conditions. Fibroblasts incubated with [14C]‐adenosine yield the same radioactive products as with [14C]‐NAD; the absence of inhibition of [14C]‐adenosine uptake by ADPribose in the presence of α‐β methyleneADP, an inhibitor of 5′ nucleotidase, demonstrates that ADPribose coming from NAD via NAD‐glycohydrolase is finally catabolised to adenosine. These results confirm that adenosine is the NAD hydrolysis product incorporated by cells and further metabolized to ATP, and that adenosine transport is partially ATP dependent. J. Cell. Biochem. 80:360–366, 2001.

Poly(ADP-ribose) polymerase activity in systemic lupus erythematosus and systemic sclerosis

The aim of this study is to investigate the role of poly(ADP-ribose) polymerase (PARP), involved in DNA repair and in autoimmune pathologic conditions such as systemic lupus erythematosus (SLE) and both limited systemic sclerosis (lSSc) and diffuse systemic sclerosis (dSSc), to assess its possible implication in the pathogenetic processes. The relationship between PARP activity and the intracellular concentration of its substrate nicotinamide adenine dinucleotide (NAD) is also investigated. Peripheral mononuclear cells (PMC) from controls and patients with SLE, lSSc, and dSSc were irradiated with ultraviolet light (UV) and PARP activity was assayed by a radiochemical method. Pyridine nucleotide concentrations were assayed by a high-performance liquid chromatography-linked method. PARP activity was detectable in nonirradiated cells and showed similar values in all groups. The activity significantly increased after UV irradiation in control, SLE, and lSSc cells, but not in dSSc cells. Irradiated PMC from both SLE and dSSc showed lower enzyme activity with respect to irradiated controls. Higher intracellular NAD content was found in all of the pathologic conditions in comparison to values in the control; this difference was statistically significant in dSSc. Our data demonstrate a lower PARP activity in response to UV damage in PMC from patients affected by the above pathologic conditions compared with controls. An inverse relationship between PARP activity and NAD content was also observed.

An HPLC-linked assay of phosphoribosylpyrophosphate synthetase activity in the erythrocytes of adults and children with neurological disorders

A two-step non-radioactive method that uses reverse-phase high-performance liquid chromatography (RP-HPLC) is described for the determination of phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) activity in human erythrocytes. The method is accurate and easily reproducible in different chromatographic systems; it is based on the quantification of phosphoribosylpyrophosphate by conversion into orotidine monophosphate and uridine monophosphate. Phosphoribosylpyrophosphate synthetase activity was determined in the erythrocytes of healthy adults and children, the latter showing significantly higher activity than the former. The enzyme activity assayed in children with different neurological disorders was significantly lower in patients with Rett syndrome than in control children or in autistic or mentally retarded patients.

Nicotinic acid phosphoribosyltransferase activity in human erythrocytes: studies using a new HPLC method.

A non-radiochemical method linked to reverse-phase high-performance liquid chromatography was developed to determine the activity of nicotinic acid phosphoribosyltransferase (EC 2.4.2.11) in crude lysates of human red blood cells. The method is accurate and easily reproducible in different chromatographic systems. The enzyme activity was determined in erythrocytes of healthy subjects and in patients with different purine disorders showing altered NAD levels. Very low enzyme activity was found in a boy hemizygous for phosphoribosylpyrophosphate synthetase superactivity, consistent with the low erythrocyte NAD concentration.

Enzymatic activities affecting exogenous nicotinamide adenine dinucleotide in human skin fibroblasts

The fate of nicotinamide adenine dinucleotide (NAD), AMP, and ADP‐ribose supplied to intact human skin fibroblasts was monitored, and the concentrations of intra‐ and extracellular pyridine and purine compounds were determined by HPLC analysis. Two enzymatic activities affecting extracellular NAD were detected on the plasma membrane, one hydrolyzing the pyrophosphoric bond and yielding nicotinamide mononucleotide (nucleotide pyrophosphatase) and the other cleaving the glycoside link and releasing nicotinamide (NAD‐glycohydrolase). No AMP or ADP‐ribose was found in the extracellular medium of cells incubated with NAD, the former being completely catabolized to hypoxanthine and the latter degraded to adenine and hypoxanthine.

Pyridine Nucleotide Metabolism: Purine and Pyrimidine Interconnections

The role of the pyridine coenzymes NAD and NADP in oxidation-reduction reactions, leading to energy production and to reductive synthesis, is well documented, and the characteristics of the proteins involved have been extensively studied. Since the balance between the oxidised and reduced form of pyridine coenzymes modulates both catabolic and synthetic pathways, their differing levels in different tissues, organs and cellular compartments indicate metabolic differentiations1. In addition to this role, pyridine coenzymes take part in a number of other cellular processes occurring in prokaryotes or eukaryotes or both2. These processes include the utilization of NAD for: protein ADP-ribosylations, mainly involved in the mechanism of action of bacterial toxins, affecting protein synthesis or some processes mediated by cAMP and G-proteins; poly-ADPribose synthesis, involved in the regulation of DNA repair and replication and in cellular differentiation; DNA ligase reactions, active in prokaryotes. NAD(P) involvement in the production of cytotoxic compounds and in phagocytosis process has also been described. Such findings revealed the versatility of the NAD(P) molecule in cell function, suggesting that its biological role is not yet fully appreciated.