Gas chromatography—mass spectrometry of trimethylsilyl pteridines

A Shared Prebiotic Formation of Neopterins and Guanine Nucleosides from Pyrimidine Bases

The prebiotic origins of biopolymers and metabolic co-factors are key questions in Origins of Life studies. In a simple warm-little-pond model, using a drying phase to produce a urea-enriched solution, we present a prebiotic synthetic path for the simultaneous formation of neopterins and tetrahydroneopterins, along with purine nucleosides. We show that, in the presence of ribose and in a formylating environment consisting of urea, ammonium formate, and water (UAFW), the formation of neopterins from pyrimidine precursors is robust, while the simultaneous formation of guanosine requires a significantly higher ribose concentration. Furthermore, these reactions provide a tetrahydropterin-pterin redox pair. This model suggests a prebiotic link in the origin of purine nucleosides and pterin cofactors that provides a possible deep prebiotic temporal connection for the emergence of nucleic acids and metabolic cofactors.

MASS SPECTROMETRIC FRAGMENTATION OF TRIMETHYLSILYL AND RELATED ALKYLSILYL DERIVATIVES

This review describes the mass spectral fragmentation of trimethylsilyl (TMS) and related alkylsilyl derivatives used for preparing samples for analysis, mainly by combined gas chromatography and mass spectrometry (GC/MS). The review is divided into three sections. The first section is concerned with the TMS derivatives themselves and describes fragmentation of derivatized alcohols, thiols, amines, ketones, carboxylic acids and bifunctional compounds such as hydroxy- and amino-acids, halo acids and hydroxy ethers. More complex compounds such as glycerides, sphingolipids, carbohydrates, organic phosphates, phosphonates, steroids, vitamin D, cannabinoids, and prostaglandins are discussed next. The second section describes intermolecular reactions of siliconium ions such as the TMS cation and the third section discusses other alkylsilyl derivatives. Among these latter compounds are di- and trialkyl-silyl derivatives, various substituted-alkyldimethylsilyl derivatives such as the tert-butyldimethylsilyl ethers, cyclic silyl derivatives, alkoxysilyl derivatives, and 3-pyridylmethyldimethylsilyl esters used for double bond location in fatty acid spectra. © 2019 Wiley Periodicals, Inc. Mass Spec Rev 0000:1-107, 2019.

Determination of six pterins in urine by LC-MS/MS

BACKGROUND

The present work describes an analytical method for urinary pterins by LC-MS/MS, with emphasis on the separation of 6- and 7-positional isomers of bio- and neopterins.

RESULTS

Urine sample preparation consisted of oxidation by MnO(2), filtration and direct dilution in the mobile phase. The method was validated in urine spiked at five concentration levels with true triplicates of each level. Separation of the pterins, including the positional isomers, was achieved by employing a LUNA amino column. Six pterins were quantified (pterin, isoxanthopterin, 6-biopterin, 7-biopterin, 6-neopterin, 7-neopterin) and a linear behavior was observed; LOD varied from 7 to 360 pg/ml and correlation coefficients above 0.98 were obtained for all pterins. In addition, pterin levels were evaluated in 41 urine samples of healthy subjects, in ten urine samples of patients with classical phenylketonuria (PKU) and in one with atypical PKU.

CONCLUSION

The proposed method allowed to identify, separate and quantify six pterins in urine, using a simple and rapid sample preparation. The atypical PKU was unequivocally differentiated from the classical form, demonstrating that this method could be very useful for characterization and follow-up of diseases.

7-substituted pterins in humans with suspected pterin-4a-carbinolamine dehydratase deficiency. Mechanism of formation via non-enzymatic transformation from 6-substituted pterins

A recently described new form of hyperphenylalaninemia is characterized by the excretion of 7-substituted isomers of biopterin and neopterin and 7-oxo-biopterin in the urine of patients. It has been shown that the 7-substituted isomers of biopterin and neopterin derive from L-tetrahydrobiopterin and D-tetrahydroneopterin and are formed during hydroxylation of phenylalanine to tyrosine with rat liver dehydratase-free phenylalanine hydroxylase. We have now obtained identical results using human phenylalanine hydroxylase. The identity of the pterin formed in vitro and derived from L-tetrahydrobiopterin as 7-(1',2'-dihydroxypropyl)pterin was proven by gas-chromatography mass spectrometry. Tetrahydroneopterin and 6-hydroxymethyltetrahydropterin also are converted to their corresponding 7-substituted isomers and serve as cofactors in the phenylalanine hydroxylase reaction. Dihydroneopterin is converted by dihydrofolate reductase to the tetrahydro form which is biologically active as a cofactor for the aromatic amino acid monooxygenases. The 6-substituted pterin to 7-substituted pterin conversion occurs in the absence of pterin-4a-carbinolamine dehydratase and is shown to be a nonenzymatic process. 7-Tetrahydrobiopterin is both a substrate (cofactor) and a competitive inhibitor with 6-tetrahydrobiopterin (Ki approximately 8 microM) in the phenylalanine hydroxylase reaction. For the first time, the formation of 7-substituted pterins from their 6-substituted isomers has been demonstrated with tyrosine hydroxylase, another important mammalian enzyme which functions in the hydroxylation of phenylalanine and tyrosine.

Purification and biochemical characterization of xanthopterin from patients with chronic renal failure. II. Biochemical elucidation and structural analysis

We have identified the primary endogenous fluorescent substance, which has characteristic excitation/emission maxima at 380/440 nm and 400/460 nm, found in the sera of patients with chronic renal failure (Clin Chem 32: 1276, 1988). Preliminary studies, using thin layer chromatography (with cellulose) in conjunction with pteridine standards, indicated that the compound is an unconjugated pteridine. Characterization by gas chromatography-mass spectrometry (electron impact), direct probe-mass spectrometry (electron impact/chemical ionization), and Fourier Transform Infrared analysis showed this compound to be xanthopterin (2-amino 4,6 pteridinedione), an unconjugated pteridine known to be present in man in trace quantities. An authentic sample of this compound had a retention time with high-performance liquid chromatography (HPLC) identical to that of the purified fluorophore. The physiological role of xanthopterin in the pathogenesis of uremia has yet to be elucidated.

Analysis and clinical significance of pterins

This review briefly describes the biochemistry of pterins, their involvement in pathological processes and the use of pterin measurement in diagnosis and monitoring of disease. Chromatographic and other methods of pterin analysis are detailed with particular emphasis being placed on the need for correct sample collection and handling.

Primapterin, anapterin, and 6-oxo-primapterin, three new 7-substituted pterins identified in a patient with hyperphenylalaninemia

Three unknown compounds present in the urine of a patient with mild hyperphenylalaninemia were identified to be L-erythro-7-iso-biopterin, D-erythro-7-iso-neopterin, and L-erythro-6-oxo-7-iso-biopterin. The newly identified pterins were named primapterin, anapterin, and 6-oxo-primapterin, respectively. Primapterin and anapterin are present in very low concentrations in every human urine, as well as in the liver of man and mouse, whereas 6-oxo-primapterin was detected in the patient's urine only. Substantial amounts of primapterin were excreted in the patient described. The metabolic origin of primapterin and anapterin is still obscure.

Synthesis of enzymatically active D-7,8-dihydroneopterin-3'-triphosphate

The first chemical synthesis of D-neopterin-3'-triphosphate and D-7,8-dihydroneopterin-3'-triphosphate is described. D-neopterin-3'-monophosphate was first 1'-2'-0-formylated with anhydrous formic acid, then activated with 1,1'-carbonyldiimidazole and phosphorylated with n-tributyl-ammonium pyrophosphate. The yield of 3'-NTP was 24%. D-7,8-dihydroneopterin-3'-triphosphate was obtained by chemical (hyposulfite) or catalytic (Pd:H2) reduction of 3'-NTP. Preparations from both reductions were fully active in two different enzymatic systems: synthesis of L-5,6,7,8-tetrahydrobiopterin and in the C-2'-epimerization reaction to L-7,8-dihydromonapterin-3'-triphosphate.

Tetrahydrobiopterin biosynthesis. Studies with specifically labeled (2H)NAD(P)H and 2H2O and of the enzymes involved

The biosynthesis of tetrahydrobiopterin from either dihydroneopterin triphosphate, sepiapterin, dihydrosepiapterin or dihydrobiopterin was investigated using extracts from human liver, dihydrofolate reductase and purified sepiapterin reductase from human liver and rat erythrocytes. The incorporation of hydrogen in tetrahydrobiopterin was studied in either 2H2O or in H2O using unlabeled NAD(P)H or (R)-(4-2H)NAD(P)H or (S)-(4-2H)NAD(P)H. Dihydrofolate reductase catalyzed the transfer of the pro-R hydrogen of NAD(P)H during the reduction of 7,8-dihydrobiopterin to tetrahydrobiopterin. Sepiapterin reductase catalyzed the transfer of the pro-S hydrogen of NADPH during the reduction of sepiapterin to 7,8-dihydrobiopterin. In the presence of partially purified human liver extracts one hydrogen from the solvent is introduced at position C(6) and the 4-pro-S hydrogen from NADPH is incorporated at each of the C(1') and C(2') position of BH4. Label from the solvent is also introduced into position C(3'). These results suggest that dihydrofolate reductase is not involved in the biosynthesis of tetrahydrobiopterin from dihydroneopterin triphosphate. They are consistent with the assumption of the occurrence of a 6-pyruvoyl-tetrahydropterin intermediate, which is proposed to be formed upon triphosphate elimination from dihyroneopterin triphosphate, and via an intramolecular redox reaction. Our results suggest that the reduction of 6-pyruvoyl-tetrahydropterin might be catalyzed by sepiapterin reductase.

Application of gas chromatography-mass spectrometry to the study of biopterin metabolism in man. Detection of biolumazine and 2'-deoxysepialumazine

The separation characteristics of the trimethylsilyl ether derivatives of various naturally occurring and synthetic pteridines on a apolar glass capillary column, together with their mass spectra, permit their identification and quantitation in biological samples. Examples are given of the determination of the ratio of monapterin to neopterin in urine, of monitoring excreted pterin metabolites after a loading test with 6-methyltetrahydropterin in urine and of structure elucidation of lumazines , previously unknown in man. 6- Methylisoxanthopterin was shown to be the main metabolite in urine after administration of 6-methyl-5,6,7,8-tetrahydropterin. Biolumazine and 2'- deoxysepialumazine were found in human faeces after administration of ( 6R ,S)-5,6,7,8-tetrahydro-L-erythro-biopterin.