Analysis and clinical significance of pterins

LC-MS/MS methods for direct measurement of sepiapterin and tetrahydrobiopterin in human plasma and clinical applications

Aim: Tetrahydrobiopterin (BH4), a natural cofactor of aromatic amino acid hydroxylases, and sepiapterin, a natural precursor of BH4, are endogenously present in human plasma. This is the first report on methods for direct quantification of sepiapterin and BH4 in human plasma by LC-MS/MS for pharmacokinetic assessment. Materials & methods: The analytes in plasma were harvested from blood that were treated with 10% ascorbic acid (AA) to a final concentration of 1% AA. Results & conclusion: The quantification methods were validated for calibration ranges of 0.75-500 ng/ml and 0.5-500 ng/ml for sepiapterin and BH4, respectively. Quantification of analytes was challenging due to their susceptibility to redox reactions. The validated methods were utilized successfully to support clinical development of sepiapterin.

Impaired systemic tetrahydrobiopterin bioavailability and increased dihydrobiopterin in adult falciparum malaria: association with disease severity, impaired microvascular function and increased endothelial activation

Tetrahydrobiopterin (BH₄) is a co-factor required for catalytic activity of nitric oxide synthase (NOS) and amino acid-monooxygenases, including phenylalanine hydroxylase. BH4 is unstable: during oxidative stress it is non-enzymatically oxidized to dihydrobiopterin (BH₂), which inhibits NOS. Depending on BH₄ availability, NOS oscillates between NO synthase and NADPH oxidase: as the BH₄/BH₂ ratio decreases, NO production falls and is replaced by superoxide. In African children and Asian adults with severe malaria, NO bioavailability decreases and plasma phenylalanine increases, together suggesting possible BH₄ deficiency. The primary three biopterin metabolites (BH₄, BH₂ and B₀ [biopterin]) and their association with disease severity have not been assessed in falciparum malaria. We measured pterin metabolites in urine of adults with severe falciparum malaria (SM; n=12), moderately-severe malaria (MSM, n=17), severe sepsis (SS; n=5) and healthy subjects (HC; n=20) as controls. In SM, urinary BH₄ was decreased (median 0.16 ¼mol/mmol creatinine) compared to MSM (median 0.27), SS (median 0.54), and HC (median 0.34)]; p<0.001. Conversely, BH₂ was increased in SM (median 0.91 ¼mol/mmol creatinine), compared to MSM (median 0.67), SS (median 0.39), and HC (median 0.52); p<0.001, suggesting increased oxidative stress and insufficient recycling of BH2 back to BH4 in severe malaria. Overall, the median BH₄/BH₂ ratio was lowest in SM [0.18 (IQR: 0.04-0.32)] compared to MSM (0.45, IQR 0.27-61), SS (1.03; IQR 0.54-2.38) and controls (0.66; IQR 0.43-1.07); p<0.001. In malaria, a lower BH₄/BH₂ ratio correlated with decreased microvascular reactivity (r=0.41; p=0.03) and increased ICAM-1 (r=-0.52; p=0.005). Decreased BH4 and increased BH₂ in severe malaria (but not in severe sepsis) uncouples NOS, leading to impaired NO bioavailability and potentially increased oxidative stress. Adjunctive therapy to regenerate BH4 may have a role in improving NO bioavailability and microvascular perfusion in severe falciparum malaria.

Impaired systemic tetrahydrobiopterin bioavailability and increased oxidized biopterins in pediatric falciparum malaria: association with disease severity

Decreased bioavailability of nitric oxide (NO) is a major contributor to the pathophysiology of severe falciparum malaria. Tetrahydrobiopterin (BH4) is an enzyme cofactor required for NO synthesis from L-arginine. We hypothesized that systemic levels of BH₄ would be decreased in children with cerebral malaria, contributing to low NO bioavailability. In an observational study in Tanzania, we measured urine levels of biopterin in its various redox states (fully reduced [BH₄] and the oxidized metabolites, dihydrobiopterin [BH₂] and biopterin [B₀]) in children with uncomplicated malaria (UM, n = 55), cerebral malaria (CM, n = 45), non-malaria central nervous system conditions (NMC, n = 48), and in 111 healthy controls (HC). Median urine BH4 concentration in CM (1.10 [IQR:0.55-2.18] μmol/mmol creatinine) was significantly lower compared to each of the other three groups - UM (2.10 [IQR:1.32-3.14];p<0.001), NMC (1.52 [IQR:1.01-2.71];p = 0.002), and HC (1.60 [IQR:1.15-2.23];p = 0.005). Oxidized biopterins were increased, and the BH4:BH2 ratio markedly decreased in CM. In a multivariate logistic regression model, each Log10-unit decrease in urine BH4 was independently associated with a 3.85-fold (95% CI:1.89-7.61) increase in odds of CM (p<0.001). Low systemic BH4 levels and increased oxidized biopterins contribute to the low NO bioavailability observed in CM. Adjunctive therapy to regenerate BH4 may have a role in improving NO bioavailability and microvascular perfusion in severe falciparum malaria.

Determination of pteridines in biological samples with an emphasis on their stability

Pteridines are a group of endogenous heterocyclic compounds whose concentrations in biological fluids may be increased in some disorders, such as infections, autoimmune disorders and cancer. In particular, pteridine concentrations in urine may represent promising noninvasive markers. However, their specificity requires further investigation. Pteridines can occur in three oxidation states with different stability. In order to enable the analysis of the unstable di- and tetra-hydroforms either an oxidation (mainly with iodine) or stabilization by reducing agents is applied. Due to the high polarity of pteridines, many analytical procedures employed ion-pair, ion-exchange or hydrophilic interaction liquid chromatography using mostly fluorescence detection. In the last decade, MS was found to be applicable. The objective of this Review is to show possibilities and different approaches in pteridine analysis in biological samples.

Technical and biochemical factors affecting cerebrospinal fluid 5-MTHF, biopterin and neopterin concentrations

BACKGROUND

The diagnosis of pediatric neurologic disorders with a deficiency in the biosynthesis of either the neurotransmitters serotonin and dopamine, or the co-factor tetrahydrobiopterin or a cerebral 5-methyltetrahydrofolate (5-MTHF) deficiency, strongly relies on a robust analysis of neurotransmitter metabolites, pterins and 5-MTHF in the cerebrospinal fluid (CSF). The aim of this study was to investigate which technical and biochemical factors affect the CSF concentration of 5-MTHF, neopterin and biopterin in a pediatric population.

METHODS

We studied effects of the ventriculo-spinal gradient, total protein concentration, pretreatment with ascorbic acid (in case of 5-MTHF analysis), pretreatment of CSF with trichloro acetic acid (TCA)/dithiotreitol (DTE) and oxidation with either iodine or manganese oxide (in case of pterin analysis), storage time and age of the patients. We included CSF samples from children until the age of 18 years and analysed 5-MTHF, neopterin, biopterin, homovanillic acid (HVA), 5-hydroxy-indoleacetic acid (5-HIAA) and total protein.

RESULTS

The major findings of our study are: (1) CSF 5-MTHF, neopterin and biopterin concentrations are not affected by the ventriculo-spinal gradient; (2) pretreatment of CSF with ascorbic acid has negligible effects on 5-MTHF concentrations; (3) pretreatment of CSF with TCA/DTE and oxidation with iodine results in the most accurate determination of neopterin and biopterin; (4) when adjusted for age and total protein, CSF 5-MTHF correlated with 5-HIAA, but not with HVA; (5) the reference value of 5-MTHF in CSF in childhood is age-dependent (r=-0.634; p0.001); (6) we did not observe an age-dependency for neopterin and biopterin in CSF.

CONCLUSION

5-MTHF, neopterin and biopterin can be analysed in any volume of CSF that is collected. For correct analysis of pterins, CSF will have to be pretreated to stabilize the concentrations and stored properly, whereas such pretreatment is not necessary for 5-MTHF.

Elevated plasma phenylalanine in severe malaria and implications for pathophysiology of neurological complications

Cerebral malaria is associated with decreased production of nitric oxide and decreased levels of its precursor, l-arginine. Abnormal amino acid metabolism may thus be an important factor in malaria pathogenesis. We sought to determine if other amino acid abnormalities are associated with disease severity in falciparum malaria. Subjects were enrolled in Dar es Salaam, Tanzania (children) (n = 126), and Papua, Indonesia (adults) (n = 156), in two separate studies. Plasma samples were collected from subjects with WHO-defined cerebral malaria (children), all forms of severe malaria (adults), and uncomplicated malaria (children and adults). Healthy children and adults without fever or illness served as controls. Plasma amino acids were measured using reverse-phase high-performance liquid chromatography with fluorescence detection. Several plasma amino acids were significantly lower in the clinical malaria groups than in healthy controls. Despite the differences, phenylalanine was the only amino acid with mean levels outside the normal range (40 to 84 microM) and was markedly elevated in children with cerebral malaria (median [95% confidence interval], 163 [134 to 193] microM; P < 0.0001) and adults with all forms of severe malaria (median [95% confidence interval], 129 [111 to 155] microM; P < 0.0001). In adults who survived severe malaria, phenylalanine levels returned to normal, with clinical improvement (P = 0.0002). Maintenance of plasma phenylalanine homeostasis is disrupted in severe malaria, leading to significant hyperphenylalaninemia. This is likely a result of an acquired abnormality in the function of the liver enzyme phenylalanine hydroxylase. Determination of the mechanism of this abnormality may contribute to the understanding of neurological complications in malaria.

Dihydropteridine reductase deficiency in man: from biology to treatment

In 1975, dihydropteridine reductase (DHPR) deficiency was first recognized as a cause of tetrahydrobiopterin (BH(4)) deficiency, leading to hyperphenylalaninemia (HPA) and impaired biogenic amine deficiency. So far, more than 150 patients scattered worldwide have been reported and major progresses have been made in the understanding of physiopathology, screening, diagnosis, treatment, and molecular genetics of this inherited disease. Present knowledge on different aspects of DHPR deficiency, largely derived from authors' personal experience, is traced in this article.

Presentation, diagnosis, and treatment of the disorders of monoamine neurotransmitter metabolism

For many years, all of the described cases of monoamine neurotransmitter deficiency were associated with hyperphenylalaninemia that was generally detected at neonatal screening. It is now clear that inherited deficiency of monoamines often occurs in the absence of hyperphenylalaninemia and that the normal battery of screening tests used to investigate individuals with suspected metabolic disease will not detect these cases. Diagnosis in this situation must rely heavily on clinical suspicion. This article, therefore, describes the presentation and clinical symptoms that result from defective monoamine neurotransmission; outlines therapeutic approaches; and explains how cerebrospinal fluid profiles of monoamine metabolites, their precursors, and the cofactor required for monoamine synthesis can be used to pinpoint the exact site of the metabolic lesion.

Aromatic L-amino acid decarboxylase deficiency: clinical features, diagnosis, and treatment of a second family

Aromatic L-amino acid decarboxylase deficiency is an inborn error of metabolism that leads to combined serotonin and catecholamine deficiency, first described by Hyland et al in 1990. The clinical features, biochemical findings, and treatment of the second family with this condition are reported. Our male patient presented with developmental delay, extreme hypotonia, oculogyric crises, and irritability. The diagnosis of this inborn error of biogenic amine metabolism was accomplished by determining low concentrations of homovanillic, 5-hydroxyindoleacetic acid, and 3-methoxy-4-hydroxyphenyl-ethyleneglycol in cerebrospinal fluid with normal biopterin metabolism and increased L-dopa, in plasma, cerebrospinal fluid, and urine. Greatly reduced activity of aromatic L-amino acid decarboxylase in plasma confirmed this diagnosis. Combined treatment with pyridoxine, tranylcypromine, and bromocriptine produced some clinical improvement.

Methylfolate modulates potassium evoked neuro-secretion: evidence for a role at the pteridine cofactor level of tyrosine 3-hydroxylase

We have previously shown that 5-methyltetrahydrofolate influences neuro-secretion. The present study more precisely characterises the processes involved and considers one probable site of action. Focusing on the tyrosine-noradrenalin axis in cerebellum we showed 5-methyltetrahydrofolate causes a significant reduction in the apparent K+ evoked secretion of noradrenalin to only 12.9% of control release. Evidence supports the idea that this could actually be due to increased synthesis leading to; depletion of reserves, possibly through leakage, exocytotic inhibition via activation of presynaptic receptors or end product inhibition by noradrenalin at the pteridine cofactor level of tyrosine hydroxylase: a) concomitant decreased measurement of perfusate and intracellular tyrosine with released noradrenalin following 5-methyltetrahydrofolate treatment supports the idea of increased transmitter turn over; b) kinetic studies indicate that at saturating concentrations of tyrosine and in the presence of an inhibitor of L-DOPA decarboxylase, 5-methyltetrahydrofolate partially duplicates the rate limiting behaviour of a synthetic pteridine cofactor--DL,2-amino-4-hydroxy-6,7,dimethyltetrahydropteridine. We debate whether, in vivo, CSF 5-methyltetrahydrofolate might interact at the tetrahydrobiopterin cofactor level of tyrosine hydroxylase and other aromatic amino-acid hydroxylases.

Assay of neopterin in serum by means of two-dimensional high-performance liquid chromatography with automated column switching using three retention mechanisms

An automated two-dimensional HPLC method for the determination of neopterin in serum is described. Neopterin is separated from proteins on a short octadecylsilica column by size exclusion and from the majority of the other serum components by adsorption. The fraction containing neopterin is transferred by column switching to a solvent-generated cation-exchange column using dodecylsulfonic acid as surface activator. Parameters influencing the separation performance and sensitivity of the fluorescence detection are discussed. The efficiency of the cleaning of the first column was optimized. The method was validated. It achieves a precision of 1% (R.S.D.) and a detection limit of about 0.3 nmol/l. The accuracy is nearly 100%. The method allows a high sample throughput, requiring 15 min per sample.

Plasma neopterin/C-reactive protein ratio as an adjunct to the assessment of infection and cancer cachexia

Neopterin (NPT), a pteridine intermediate metabolite in the biopterine synthetic pathway, is synthesized and secreted by monocytes/macrophages upon stimulation, mainly by gamma-interferon produced by activated T cells. C-reactive protein (CRP) is one of the major acute-phase reactants and its release is thought to be mediated by interleukin-6. Plasma concentrations of NPT and CRP were synchronously analyzed in 25 determinations of 5 patients with severe infectious complications and 50 determinations of 10 cancer-burden patients representing cachexia. The mean value of NPT (pmol/ml) was 201.6 in the infection group and 16.5 in the cancer cachexia group. The mean value of CRP (mg/dl) was 12.5 in the infection group and 3.4 in the cancer cachexia group. The number of samples in which NPT alone exceeded the cut-off level were 0/25 (0%) in the infection group and 38/50 (76.0%) in the cancer cachexia group. The number of samples in which both NPT and CRP exceeded the cut-off level was 25/25 (100%) in the infection group and 12/50 (24.0%) in the cancer cachexia group. The mean ratio of NPT to CRP was 11.3 in the infection group and 30.7 in the cancer cachexia group, respectively. These results suggest that gamma-interferon could play the principal role in the pathogenesis of cancer cachexia and that interleukin-6 modified the disease status. Interleukin-6 would be the critical mediator of host responses in infectious complications.

Abnormalities of biogenic amine metabolism

The term biogenic amine is an umbrella term that encompasses all amines with an origin in biological processes. This review will be restricted to the biogenic amine abnormalities that affect the metabolism of serotonin and the catecholamines. The synthesis and catabolism of these neurotransmitters are outlined, and a summary is given of the neurological details, biochemical features, and treatment of the inborn errors that primarily affect their metabolism. An idea is also developed that proposes that abnormalities of biogenic amine metabolism are far more common than is currently considered, and that the search for these problems may be appropriate in any neonate or infant who presents with neurological problems of unknown origin.

Folate deficiency, biopterin and monoamine metabolism in depression

Seven (21%) of 34 patients with a severe DSM-III diagnosis of major depression had red-cell folate levels below 150 ng/ml. This subgroup with folate deficiency had significantly lower CSF 5-hydroxyindoleacetic acid (5HIAA) compared to neurological controls. For all depressed patients red-cell folate was significantly correlated with CSF 5HIAA and homovanillic acid (HVA). CSF tetrahydrobiopterin (BH4) was significantly correlated with CSF 5HIAA and HVA and red-cell folate. Our observations provide further evidence of the links between folate, biopterin and monoamine metabolism in depression.

Increased urine neopterin levels in psoriasis

The production of neopterin closely reflects activation of T-lymphocyte-mediated immunity. Oxidized and reduced forms of urine neopterin were measured by reversed-phase ion-pair high-performance liquid chromatography in patients with moderate to severe chronic plaque psoriasis (n = 40), and in a heterogeneous group of patients (n = 14) with cutaneous T-cell malignancies (CTCM). Results were compared with healthy non-psoriatic control subjects (n = 30). Neopterin levels were repeated after a course of ultraviolet B therapy (UVB) plus topical tar or dithranol, or photochemotherapy (PUVA), in 12 psoriatic patients. Fully oxidized urine neopterin levels and neopterin/creatinine ratios were significantly elevated in the psoriatic group compared with controls (P < 0.002, P < 0.05) but not in the CTCM group. Both neopterin and its creatinine ratio were significantly reduced by treatment (P < 0.05, P < 0.01). Psoriasis area and severity index scores (PASI) correlated strongly with urine neopterin levels (P < 0.001). These findings indicate that urine neopterin concentrations may be a marker of psoriatic disease activity, and further support the importance of activated T lymphocytes in the pathogenesis of psoriasis.

Neopterin in tuberculous and neoplastic pleural fluids

Neopterin is derived from guanosine-triphosphate, produced by stimulated macrophages under the influence of gamma interferon of lymphocyte origin. It has been suggested as an excellent marker for activation of the monocyte/macrophage axis in some clinical situations. We evaluated its concentration in the pleural effusions of 25 individuals (10 tuberculous and 15 neoplastic) as well as in the blood of 22 of them (8 tuberculous and 14 neoplastic), comparing these levels with those of a control group in 99 normal individuals. The concentration of neopterin was determined by radioimmunologic assay. This showed a significant increase (p less than 0.001) of neopterin levels in the tuberculous pleural fluid, compared to the neoplastic group (42 +/- 23/17 +/- 9 nmol/L). In the blood, values were nearly identical to the pleural fluid (41.3 +/- 25/15.8 +/- 6.9 nmol/L), although with significant differences between them and in relation to the control group (p less than 0.001), which had a normal serum value (5.11 +/- 1.92 nmol/L). We emphasize the influence of the neopterin levels in the pleural fluid on the diagnosis of causes of pleurisy and its importance as a marker of immunologic cellular activity.

Purification and biochemical characterization of xanthopterin from patients with chronic renal failure. I. Isolation, purification and preliminary characterization

We describe the purification and initial characterization of a hitherto unrecognized fluorescence (excitation/emission maxima at 380/440 nm and 400/460 nm) reported from this laboratory in patients with chronic renal failure (Clin Chem 31: 1988, 1985). Purification was achieved using Sephadex G-10 gel chromatography combined with reverse phase and ion exchange high-performance liquid chromatography (HPLC). Purity of the "blue-green" fluorescent compound was determined to be greater than 99% by HPLC, and two-dimensional thin layer chromatography using an acidic and basic solvent system. The excitation/emission maxima were shown to be 390 nm/456 nm, and ultraviolet scans, at pH 1.0, 7.0, and 13.0, gave absorbance optima at 261 nm/356 nm, 278 nm/390 nm, and 255 nm/394 nm, respectively. The isoelectric point of 4.05 in conjunction with the fluorescent and ultraviolet spectra suggests that the fluorophore belongs to the class of compounds known as pteridines.