Association between homocysteine and neopterin in healthy subjects measured by a simple HPLC-fluorometric method

Pteridine determination in human serum with special emphasis on HPLC methods with fluorimetric detection

Conjugated and unconjugated pteridines and their derivatives are important cofactors in cellular metabolism. Hence, the amount of unconjugated pteridines in biological fluids has been found to be modified as a result of several disorders. It is necessary to note that while for the control of pteridines in urine samples there are numerous reference data, the literature referred to for the analysis of these analytes in serum/plasma is scarce. In biological fluids, pteridines can exist in different oxidation states, and these compounds can be classified into two groups according to: (a) oxidized or aromatic pteridines and (b) reduced pteridines. Oxidized pteridines yield a strong fluorescence signal, whereas reduced pteridines present a low quantum yield of fluorescence. In order to enable the analysis of the reduced forms, several preoxidation procedures to generate aromatic rings have been established. Also, stabilization of the reduced forms by the addition of reducing agents has been widely reported. The objective of this paper is to show possibilities and different approaches in the analysis of pteridines in serum samples. We have mainly focused on the description of the current situation in the application of high-performance chromatography methods with fluorimetric detection.

HPLC determination of serum pteridine pattern as biomarkers

Pteridinic derivatives are important biomolecules considered as biomarkers for several diseases, especially in cancer and infectious pathologies. A new fluorimetric-HPLC method for the analysis of nine pteridines in human serum has been reported. Two analytical columns composed by C18 porous and fused core particles were assayed and the results compared. Fused core particle column allows us adequate separation, in only one run and in 15 min. Acid precipitation step of the proteins and clean-up process with an Isolute ENV+ (hydroxylated polystyrene-divinylbenzene copolymer) cartridge of the serum samples have been optimized. Analytes were determined by fluorimetric detection, exciting at 272 nm and measuring the fluorescence emission at 410 nm for isoxanthopterin, at 465 nm for xanthopterin, and at 445 nm for the analysis of the other pteridines. Detection limits between 0.07 and 0.61 ng mL(-1) were calculated according to Clayton criterium. Intraday precision varied from 1.2 to 5.3 and interday precision between 1.2 and 7.4, both expressed as RSD (%). External standard and standard addition calibrations were compared in the analysis of serum samples. The pteridine amounts in serum (expressed as ng mL(-1) ± confidence interval) were 3.69 ± 1.78; 1.35 ± 0.24; 0.46 ± 0.14; 0.54 ± 0.24; 0.84 ± 0.55; 2.10 ± 0.51 and 0.23 ± 0.11 for XAN, NEO, MON, ISO, BIO and 6HMPT, respectively, using the external standard method. Comparable results were obtained by the standard addition method. It is noticeable that 7BIO was not detected in the healthy serum samples analyzed.

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.

Macrophage activation and coronary atherosclerosis in systemic lupus erythematosus and rheumatoid arthritis

OBJECTIVE

Activation of macrophages may contribute to increased atherosclerosis and coronary artery disease in systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Neopterin, a pteridine derivative, is a novel marker of monocyte and macrophage activation that is associated with atherosclerosis and cardiovascular risk in the general population. We examined the hypothesis that macrophage activation is associated with accelerated atherosclerosis in SLE and RA.

METHODS

We compared serum neopterin concentrations, adjusted for age, race, sex, and serum creatinine concentration, in patients with SLE (n=148) or RA (n=166) and control subjects (n=177). In patients with SLE or RA, serum neopterin concentrations were then tested for association (adjusted for age, race, sex, serum creatinine, and medication use) with measures of disease activity or damage, inflammatory markers and mediators, and coronary artery calcium measured by electron beam computed tomography.

RESULTS

Neopterin concentrations were significantly higher in patients with SLE (median 8.0, interquartile range [IQR] 6.5-9.8 nmoles/liter) and RA (median 6.7, IQR 5.3-8.9 nmoles/liter) than controls (median 5.7, IQR 4.8-7.1 nmoles/liter), and were higher in SLE patients than in RA patients (all P<0.001). In SLE, neopterin was significantly correlated with higher erythrocyte sedimentation rate (ESR; P=0.001), tumor necrosis factor α (P<0.001), monocyte chemoattractant protein 1 (P=0.005), and homocysteine concentrations (P=0.01), but in RA, it was only associated with ESR (P=0.01). Neopterin was not associated with coronary calcium in either SLE (P=0.65) or RA (P=0.21).

CONCLUSION

Macrophage activation, reflected by increased serum neopterin concentrations, was increased in both SLE and RA. Neopterin was more robustly associated with atherogenic mediators of inflammation and homocysteine in SLE than in RA, but was not associated with coronary atherosclerosis in either disease.

Elevated serum neopterin levels in acetaminophen-induced liver injury

Neopterin is synthesized in macrophage/Kupffer cells by interferon-gamma and other cytokines. This study aimed to evaluate the utility of using neopterin as a biomarker of acetaminophen (APAP)-induced liver injury. Wistar rats, randomly divided into two groups (APAP and normal), received APAP (1.0 g/kg) and distilled water, respectively, by gastric tube. The APAP group had a higher degree of liver necrosis than the control group. The APAP group also had significantly higher serum neopterin levels than the normal group. Serum neopterin levels correlated with serum AST, ALT activities, and degree of necrosis. This study demonstrates the preclinical utility of neopterin as a biomarker for the animal model of APAP-induced liver injury. Further research studies are required to determine the preclinical opportunities of using neopterin as a marker of APAP-induced liver injury.

Alterations in L-arginine and inflammatory markers in type 2 diabetic patients with and without microalbuminuria

Low-grade inflammation is closely involved in the pathogenesis of type 2 diabetes and associated micro- and macrovascular complications. The nitric oxide (NO) precursor L-arginine, is relevant to diverse pathological conditions including type 2 diabetes and its complications. High sensitive-CRP (hs-CRP), neopterin and arginine levels were measured in 46 normoalbuminuric, 45 microalbuminuric type 2 diabetics and in 32 healthy controls in order to assess the relationship between markers of inflammation and L: -arginine. Hs-CRP concentrations were higher in microalbuminuric diabetic patients compared to normoalbuminuric patients and controls. Diabetics had higher serum and urine neopterin levels than controls. Urine neopterin and L-arginine levels differed significantly among diabetic patients with and without microalbuminuria. There were significant positive correlations between hs-CRP and BMI in healthy controls and diabetics with and without microalbuminuria. In microalbuminuric diabetics, hs-CRP correlated with microalbuminuria (MAU). Significant predictors for the development of microalbuminuria were higher postprandial glucose levels, lower creatinine clearance and lower serum L-arginine levels. Assessment of early markers of inflammation and endothelial function, such as neopterin and NO precursor L-arginine, may help to predict incipient nephropathy more accurately in type 2 diabetic patients.

Quantitative profiling of biomarkers related to B-vitamin status, tryptophan metabolism and inflammation in human plasma by liquid chromatography/tandem mass spectrometry

Vitamins B2 and B6 serve as cofactors in enzymatic reactions involved in tryptophan and homocysteine metabolism. Plasma concentrations of these vitamins and amino acids are related to smoking and inflammation, and correlate with other markers of immune activation. Large-scale studies of these relations have been hampered by lack of suitable analytical methods. The assay described includes riboflavin, five vitamin B6 forms (pyridoxal 5'-phosphate, pyridoxal, 4-pyridoxic acid, pyridoxine and pyridoxamine), tryptophan and six tryptophan metabolites (kynurenine, kynurenic acid, anthranilic acid, 3-hydroxykynurenine, xanthurenic acid and 3-hydroxyanthranilic acid), cystathionine, neopterin and cotinine. Trichloroacetic acid containing 13 isotope-labelled internal standards was added to 60 microL of plasma, the mixture was centrifuged, and the resulting supernatant used for analysis. The analytes were separated within 5 min on a stable-bond C8 column by a gradient-type mobile phase containing acetonitrile, heptafluorobutyric acid and high concentration (650 mmol/L) of acetic acid, and detected using electrospray ionization tandem mass spectrometry (ESI-MS/MS). The mobile phase ensured sufficient separation and high ionization efficiency of all analytes. Recoveries were 75-123% and within-day and between-day coefficients of variance (CVs) were 2.5-9.5% and 5.4-16.9%, respectively. Limits of detection ranged from 0.05 to 7 nmol/L. The method enables quantification of endogenous plasma concentrations of 16 analytes related to B-vitamin status and inflammation, and may prove useful in large-scale epidemiological studies.

Plasma total homocysteine: instigator or indicator of cardiovascular disease?

Cardiovascular disease (CVD) is a major cause of morbidity and mortality in developed countries. However, traditional risk factors cannot fully account for this. In the last 20 years, there has been an explosion of interest in plasma total homocysteine (tHcy) as a potential modifiable risk factor for CVD. Recent meta-analyses of epidemiological studies support the concept that increased tHcy concentrations are associated with CVD. This has led to the ‘homocysteine hypothesis’, which states that lowering plasma tHcy using folic acid and other B-vitamins will reduce the risk of CVD. In experimental studies, homocysteine has been shown to cause oxidative stress, endothelial cell dysfunction and promote thrombogenesis. However, data from recent large randomized controlled trials have shown that there is no clinical benefit to lowering plasma tHcy concentrations with folic acid and other B-vitamins. This lack of effect of tHcy lowering strongly suggests that homocysteine is not an instigator but merely an indicator of CVD.

Gamma interferon is dispensable for neopterin production in vivo

Previous studies have indicated that neopterin is synthesized in vitro by human monocyte-derived macrophages and dendritic cells upon stimulation with gamma interferon (IFN-gamma). Neopterin production under specific conditions in vitro has also been obtained upon stimulation with IFN-alpha and/or IFN-beta. However, it is unknown if any IFN-gamma-independent neopterin synthesis is possible in vivo. In the present study we investigated the serum neopterin concentrations in patients affected by the syndrome of Mendelian susceptibility to mycobacterial disease (MSMD). Indeed, this syndrome is characterized by deeply impaired or absent IFN-gamma production or function due to severe mutations in molecules involved in IFN-gamma/interleukin-12 (IL-12)/IL-23-dependent pathway. Serum neopterin levels were measured by an enzyme-linked immunosorbent assay in 27 patients with MSMD. We found that serum neopterin levels are elevated in the complete absence of IFN-gamma activity due either to a complete deficiency of its receptor or to deleterious mutations of IL-12 or its receptor. These data clearly indicate that, as reported from in vitro studies, other stimuli are able to induce neopterin synthesis in vivo. Consequently, neopterin cannot be used as means of diagnosis of MSMD due to IFN-gamma-, IL-12-, and IL-23-dependent pathway defects.

Elevated urine neopterin levels in nonalcoholic steatohepatitis

OBJECTIVES

The aim of this study was to relate urine levels of neopterin, a marker of activation of the cellular immune system, with grading and staging of NASH.

DESIGN AND METHODS

Urine concentrations of neopterin, routine tests, insulin and C-peptide levels were assessed in 50 patients with NASH, 25 patients with chronic viral hepatitis (CVH), and in 26 healthy controls.

RESULTS

Urine neopterin levels were found elevated in the NASH and CVH groups compared with controls. There was no significant correlation between urine neopterin levels and inflammation grade in the liver.

CONCLUSIONS

Urine neopterin levels are a marker of cellular immunity and are higher in patients with NASH. However, neopterin levels were not significantly associated with histopathological grade and stage of disease.

Increasing production of homocysteine and neopterin and degradation of tryptophan with older age

OBJECTIVES

Aging is associated with an increased frequency of abnormal immune system function, which may cause infections, autoimmune diseases, and cardiovascular or neurodegenerative disorders. Th1-type cytokine interferon-gamma (IFN-gamma) induces neopterin production as well as tryptophan degradation via indoleamine (2,3)-dioxygenase (IDO), and quantification of these biochemical alterations allows one to monitor immune system activation. Homocysteine is known to be elevated in the elderly, which is possibly due to an insufficient availability of folate, B6, and/or B12.

DESIGN AND METHODS

Serum concentrations of neopterin, homocysteine, tryptophan and kynurenine, and of vitamins folate and B12 were measured in 43 healthy individuals (21 females, 22 males) aged 34-93 years. The ratio of the concentration of the product of IDO, kynurenine, versus the substrate tryptophan (kyn/trp) was calculated to estimate IDO activity.

RESULTS

Comparing three age groups of similar size (34-60, 61-71, and 72-93 years), neopterin and homocysteine concentrations as well as the kyn/trp ratio were found to increase with older age (all P < 0.01). Folate concentrations were lower in the middle-aged group as compared with the other two subgroups of individuals. Vitamin B12 concentrations did not differ between groups. Positive correlations were found between kyn/trp and neopterin and homocysteine concentrations (all P < 0.01).

CONCLUSIONS

Increasing neopterin concentrations and kyn/trp with older age are in line with the view that aging in healthy people is associated with immune activation especially of the T-cell/macrophage system.

Plasma Homocysteine and Immune Activation in Patients with Malignant Melanoma Undergoing Treatment with IFN-α

Immune activation and cell proliferation may contribute to the development of increased homocysteine concentrations in patients with malignant diseases. In this study, we investigated the effect of interferon-alpha (IFN-alpha) on plasma homocysteine concentrations in patients being treated for malignant melanoma. In parallel, neopterin formation and tryptophan degradation were monitored to assess the capacity of IFN-alpha to activate macrophages. Plasma concentrations of homocysteine, folate, and vitamin B(12) were determined in 15 patients with malignant melanoma during 12 weeks of high-dose IFN-alpha therapy. Concurrently, concentrations of neopterin, tryptophan, and kynurenine were measured, and the kynurenine/tryptophan ratio (kyn/trp) was calculated. Homocysteine and folate concentrations during treatment with IFN-alpha did not differ from baseline. In contrast, significant increases in neopterin formation and tryptophan degradation were apparent during IFN-alpha therapy. Plasma concentrations of vitamin B(12) and cysteine also increased. These results indicate that IFN-alpha directly activates macrophages to release neopterin and to degrade tryptophan, but obviously treatment with INF-alpha did not affect homocysteine metabolism.

Teratogenic alleles and neurodevelopmental disorders

The genetic interaction between mother and fetus during pregnancy is discussed, focusing on teratogenic alleles that act in the mother to alter fetal development and contribute to a neurodevelopmental disorder. For these alleles, the mother is the genetic patient. Teratogenic alleles interact with modifying and specificity alleles that act in the fetus and with environmental factors. Based on examples of the model, two candidate mechanisms emerge as contributors to neurodevelopmental disorders, folate-homocysteine pathways and immune/inflammatory mechanisms. Both, acting in mothers, affect fetal development and contain many polymorphic genes. These two systems interact with each other. Common functional polymorphisms of mild effect in these two systems of interacting genes are good candidates for teratogenic alleles. The presence of teratogenic alleles complicates gene identification for neurodevelopmental disorders. However, using the special methods required to identify teratogenic alleles is important because this could lead to new approaches to prevention and improved therapy of these disorders.