7-8-dihydropterin and sepiapterin

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems and accumulate in the skin of patients affected by vitiligo, a depigmentation disorder. The study of the emission of 7,8-dihydropterins is difficult because these compounds are more or less unstable in the presence of O(2) and their solutions are contaminated with oxidized pterins which have much higher fluorescence quantum yields (Φ(F)). In this work, the emission properties of six compounds of the dihydropterin family (6-formyl-7,8-dihydropterin (H(2)Fop), sepiapterin (Sep), 7,8-dihydrobiopterin (H(2)Bip), 7,8-dihydroneopterin (H(2)Nep), 6-hydroxymethyl-7,8-dihydropterin (H(2)Hmp), and 6-methyl-7,8-dihydropterin (H(2)Mep)) have been studied in aqueous solution. The fluorescence characteristics (spectra, Φ(F), lifetimes (τ(F))) of the neutral form of these compounds have been investigated using the single-photon-counting technique. Φ(F) and τ(F) values obtained lie in the ranges 3-9 × 10(-3) and 0.18-0.34 ns, respectively. The results are compared to those previously reported for oxidized pterins.

Topics: Biopterins; Neopterin; Oxidation-Reduction; Oxygen; Pterins; Quantum Theory; Solutions; Spectrometry, Fluorescence; Water

The putative gene (ygcM) of Escherichia coli was verified in vitro to encode the ortholog of 6-pyruvoyltetrahydropterin synthase (PTPS). Unexpectedly, the enzyme was found to convert sepiapterin to 7,8-dihydropterin without any cofactors. The enzymatic product 7,8-dihydropterin was identified by HPLC and mass spectrometry analyses, suggesting a novel activity of the enzyme to cleave the C6 side chain of sepiapterin. The optimal activity occurred at pH 6.5-7.0. The reaction rate increased up to 3.2-fold at 60-80 degrees C, reflecting the thermal stability of the enzyme. The reaction required no metal ion and was activated slightly by low concentrations (1-5 mM) of EDTA. The apparent K(m) value for sepiapterin was determined as 0.92 mM and the V(max) value was 151.3 nmol/min/mg. The new catalytic function of E. coli PTPS does not imply any physiological role, because sepiapterin is not an endogenous substrate of the organism. The same activity, however, was also detected in a PTPS ortholog of Synechocystis sp. PCC 6803 but not significant in Drosophila and human enzymes, suggesting that the activity may be prevalent in bacterial PTPS orthologs.

Topics: Amino Acid Sequence; Animals; Catalysis; Drosophila; Enzyme Stability; Escherichia coli; Hot Temperature; Humans; Kinetics; Molecular Sequence Data; Phosphorus-Oxygen Lyases; Pteridines; Pterins; Recombinant Proteins; Substrate Specificity