An enzymatic method to distinguish tetrahydrobiopterin from oxidized biopterins using UDP-glucose:tetrahydrobiopterin glucosyltransferase

Simultaneous determination of all forms of biopterin and neopterin in cerebrospinal fluid

In humans, genetic defects of the synthesis or regeneration of tetrahydrobiopterin (BH4), an essential cofactor in hydroxylation reactions, are associated with severe neurological disorders. The diagnosis of these conditions relies on the determination of BH4, dihydrobiopterin (BH2), and dihydroneopterin (NH2) in cerebrospinal fluid (CSF). As MS/MS is less sensitive than fluorescence detection (FD) for this purpose, the most widely used method since 1980 involves two HPLC runs including two differential off-line chemical oxidation procedures aiming to transform the reduced pterins into their fully oxidized fluorescent counterparts, biopterin (B) and neopterin (N). However, this tedious and time-consuming two-step indirect method underestimates BH4, BH2, and NH2 concentrations. Direct quantification of BH4 is essential for studying its metabolism and for monitoring the efficacy of BH4 supplementation in patients with genetic defects. Here we describe a single step method to simultaneously measure BH4, BH2, B, NH2, and N in CSF by HPLC coupled to FD after postcolumn coulometric oxidation. All target pterins were quantified in CSF with a small volume (100 μL), and a single filtration step for sample preparation and analysis. As compared to the most widely used method in more than 100 CSF samples, this new assay is the easiest route for accurately determining in a single run BH4, BH2, and NH2 in CSF in deficit situations as well as for monitoring the efficacy of the treatment.

Purification, crystallization and preliminary X-ray diffraction studies of UDP-glucose:tetrahydrobiopterin α-glucosyltransferase (BGluT) from Synechococcus sp. PCC 7942

A UDP-glucose:tetrahydrobiopterin α-glucosyltransferase (BGluT) enzyme was discovered in the cyanobacterium Synechococcus sp. PCC 7942 which transfers a glucose moiety from UDP-glucose to tetrahydrobiopterin (BH4). BGluT protein was overexpressed with selenomethionine labelling for structure determination by the multi-wavelength anomalous dispersion method. The BGluT protein was purified by nickel-affinity and size-exclusion chromatography. It was then crystallized by the hanging-drop vapour-diffusion method using a well solution consisting of 0.1 M bis-tris pH 5.5, 19%(w/v) polyethylene glycol 3350 with 4%(w/v) D(+)-galactose as an additive. X-ray diffraction data were collected to 1.99 Å resolution using a synchrotron-radiation source. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 171.35, b = 77.99, c = 53.77 Å, β = 90.27°.

Guinea pig ascorbate status predicts tetrahydrobiopterin plasma concentration and oxidation ratio in vivo

Tetrahydrobiopterin (BH₄) is an essential co-factor of nitric oxide synthases and is easily oxidized to dihydrobiopterin (BH₂) which promotes endothelial nitric oxide synthase uncoupling and deleterious superoxide production. Vitamin C has been shown to improve endothelial function by different mechanisms, some involving BH₄. The hypothesis of the present study was that vitamin C status, in particular low levels, influences biopterin redox status in vivo. Like humans, the guinea pig lacks the ability to synthesize vitamin C and was therefore used as model. Seven day old animals (n = 10/group) were given a diet containing 100, 250, 500, 750, 1000, or 1500 ppm vitamin C until euthanasia at age 60-64 days. Blood samples were drawn from the heart and analyzed for ascorbate, dehydroascorbic acid (DHA), BH₄ and BH₂ by high-performance liquid chromatography. Plasma BH₄ levels were found to be significantly lower in animals fed 100 ppm vitamin C compared to all other groups (P < .05 or less). BH₂ levels were not significantly different between groups but the BH₂-to-BH₄ ratio was higher in the group fed 100 ppm vitamin C (P < .001 all cases). Significant positive correlations between BH4 and ascorbate and between BH₂-to-BH₄ ratio and DHA were observed (P < .0001 both cases). Likewise, BH₂-to-BH₄ ratio was negatively correlated with ascorbate (P < .0001) as was BH₄ and DHA (P < .005). In conclusion, the redox status of plasma biopterins, essentially involved in vasodilation, depends on the vitamin C status in vivo. Thus, ingestion of insufficient quantities of vitamin C not only leads to vitamin C deficiency but also to increased BH₄ oxidation which may promote endothelial dysfunction.

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.

Altered sugar donor specificity and catalytic activity of pteridine glycosyltransferases by domain swapping or site-directed mutagenesis

CY-007 and CY-049 pteridine glycosyltransferases (PGTs) that differ in sugar donor specificity to catalyze either glucose or xylose transfer to tetrahydrobiopterin were studied here to uncover the structural determinants necessary for the specificity. The importance of the C-terminal domain and its residues 218 and 258 that are different between the two PGTs was assessed via structure-guided domain swapping or single and dual amino acid substitutions. Catalytic activity and selectivity were altered in all the mutants (2 chimeric and 6 substitution) to accept both UDP-glucose and UDP-xylose. In addition, the wild type activities were improved 1.6-4.2 fold in 4 substitution mutants and activity was observed towards another substrate UDP-Nacetylglucosamine in all the substitution mutants from CY-007 PGT. The results strongly support essential role of the C-terminal domain and the two residues for catalysis as well as sugar donor specificity, bringing insight into the structural features of the PGTs. [BMB Reports 2013; 46(1): 37-40]

Simultaneous quantification of tetrahydrobiopterin, dihydrobiopterin, and biopterin by liquid chromatography coupled electrospray tandem mass spectrometry

A simple and rapid liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based method was developed for the quantification of tetrahydrobiopterin (BH4), dihydrobiopterin (BH2), and biopterin (B) in human umbilical vein endothelial cells (HUVECs). Freshly prepared cell samples were treated with a mixture consisting of 0.2M trichloroacetic acid (TCA) and a cocktail of various antioxidants in order to precipitate proteins and other cellular components and to stabilize red/ox conditions in the lysates. Chromatography of the cell lysates was performed on a Poroshell 120 SB-C18 column (2.7μm, 150×2.1mm) using a stepwise gradient elution made from two mobile phases. Quantification was performed on a triple quadrupole mass spectrometer employing electrospray ionization with the operating conditions as multiple reaction monitoring (MRM) at positive ion mode. Total chromatographic run time was 23min. The method was validated for analysis in HUVECs, and the limits of quantification were 1nM for BH4 and BH2 and 2.5nM for B. Standard curves were linear in the concentration ranges of 1 to 100nM for BH4 and BH2 and 2.5 to 100nM for B. The current study reports a novel method for the simultaneous and direct quantification of BH4, BH2, and B in a single injection.

Tetrahydrobiopterin is functionally distinguishable from tetrahydrodictyopterin in Dictyostelium discoideum Ax2

Dictyostelium discoideum Ax2 produces both L-erythro-tetrahydrobiopterin (BH4) and its stereoisomer D-threo-BH4 (DH4). The putative cofactor function of them for phenylalanine hydroxylase (PAH) was investigated through genetic manipulation and quantitative determination of pteridines. In addition to establishing that dihydropteridine reductase (DHPR) and dihydrofolate reductase (DHFR) constitute the regeneration pathway of both BH4 and DH4, the results suggested that BH4 is a preferential cofactor for PAH in vivo, not a secondary product of DH4, which functions mainly as an antioxidant. Our result also demonstrated that PAH may be essential for Dictyostelium growth in nature, and thus it appears that the organism has evolved a strategy to maintain BH4 level via regeneration pathway at the expense of DH4 under oxidative stress conditions.

Molecular cloning of cyanobacterial pteridine glycosyltransferases that catalyze the transfer of either glucose or xylose to tetrahydrobiopterin

Here, we report cloning of cyanobacterial genes encoding pteridine glycosyltransferases that catalyze glucosyl or xylosyl transfer from UDP-sugars to tetrahydrobiopterin. The genes were cloned by PCR amplification from genomic DNA which was isolated from culture and environmental samples and overexpressed in Escherichia coli for an in vitro activity assay.