Synthesis of 6β-N(5)-Methyl-5,6,7,8-tetrahydro-L-biopterin

Summary 6β-N(5)-methyl-5.6J,8-tetrahydro-L-biopterin·2 HC1. a new derivate of biological interest, was synthesized from 6β-5,6,7,8-tetrahydro-L-biopteril·2 HC1 and characterized. The latter compound has been used therapeutically in atypical phenylketonuria for many years. The catalytic reductive methylation with PtO2 /H2 and formaldehyde allowed selective methylation at the N(5) position. The chemical properties and stnlctures of the new compound arc compared with N(5)-methyl-tetrahydropterin and N(5)-methyl-tetrahydrofolate.

Effects of sepiapterin and 6-acetyldihydrohomopterin on the guanosine triphosphate cyclohydrolase I of mouse, rat and the fruit-fly Drosophila

The regulation of GTP cyclohydrolase I would lead to the regulation of tetrahydrobiopterin, an important cofactor for synthesis of neurotransmitters. In an attempt to extend a previous finding [Bellahsene, Dhondt, & Farriaux (1984) Biochem. J. 217, 59-65] that GTP cyclohydrolase I of rat liver is inhibited by subnanomolar concentrations of reduced biopterin and sepiapterin, we found that this could not be verified with the enzyme from mouse liver, fruit-fly (Drosophila) heads or, indeed, from rat liver. It was shown, however, that 12 microM-sepiapterin inhibited mouse liver GTP cyclohydrolase I. Another compound, namely 6-acetyldihydrohomopterin, was also employed in the present study to explore its effect on enzymes that lead to its synthesis in Drosophila and for effects on mammalian systems; at 2-5 microM this compound was shown to stimulate one form of mouse liver GTP cyclohydrolase I and then to inhibit at higher concentrations (40 microM). Neither sepiapterin nor 6-acetyldihydrohomopterin caused any effect on the Drosophila head enzyme. On the other hand, the sigmoid GTP concentration curve for the Drosophila enzyme may indicate a regulatory characteristic of this enzyme. Another report, on the lower level of GTP cyclohydrolase I in mutant mouse liver [McDonald, Cotton, Jennings, Ledley, Woo & Bode (1988) J. Neurochem. 50, 655-657], was confirmed and extended. Instead of having 10% activity, we find that the hph-1 mouse mutant has less than 2% activity in the liver. These studies demonstrate that micromolar levels of reduced pterins may have regulatory effects on GTP cyclohydrolase I and that a mouse mutant is available that has low enough activity to be considered as a model for human atypical phenylketonuria.

Study on the Pteridines Metabolism in Children Affected by Hyperphenylalaninaemia and Phenylketonuria

Summary Atypical phenylketonuria (PKU) is caused by tetrahydrobiopterin (BH4) deficiency. In Italy a systematic screening service for BH4 deficiency is not currently performed, therefore its real frequency is not known yet. We determined urinary excretion of biopterin (B) and neopterin (N) by HPLC in 74 phenylketonuric and hyperphenylalaninemic subjects, including all newborns with positive Guthrie's test for PKU since 1984. We found two patients with N and B urinary values above the classical PKU range. In these subjects we also performed a BH4 loading test that restored a normal plasma phenylalanine level in one case, while it had no effect in the other one. In both of them, the enzymatic activity of dihydropteridine reductase on dried blood spots was absent. We conclude that the determination of urinary pteridines together with the BH4 loading test are useful and informative tools in the screening of BH4 deficiencies, but that a conclusive diagnosis can only be achieved by the enzymatic assays.