A comparative study of Drosophila phenylalanine hydroxylase with a natural and a synthetic tetrahydropterin as cofactor

1. Phenylalanine hydroxylase activity has been analyzed in Drosophila melanogaster using as cofactors the natural tetrahydropteridine 5,6,7,8-tetrahydrobiopterin (H4Bip) and the synthetic one 5,6-dimethyl-5,6,7,8-tetrahydropterin (H4Dmp). 2. The apparent Vmax and KM for substrate and cofactor showed that the enzyme has two times more affinity for the substrate when H4Bip is the cofactor in the reaction. Similarly to what was found with purified rat liver phenylalanine hydroxylase, H4Bip was the most effective cofactor, leading to 4-5 times more activity than that obtained with H4Dmp. 3. With the natural cofactor H4Bip, no activation of the enzyme with Phe was necessary (in contrast to mammalian phenylalanine hydroxylase), and this tetrahydropteridine inhibits phenylalanine hydroxylase activity when the enzyme is exposed to it before phenylalanine addition. With the synthetic H4Dmp, both types of preincubations led to an increase of phenylalanine hydroxylase activity. 4. The enzyme is highly unstable compared to mammalian phenylalanine hydroxylase, even at -20 degrees C. 5. Thorax and abdomen extracts caused significant inhibition of phenylalanine hydroxylase activity from third instar larvae or newborn adult head extracts, when assayed with the synthetic cofactor H4Dmp. This inhibition did not happen with H4Bip. The presence of the pteridine 7-xanthopterin in adult bodies was not the cause of this inhibition.

Repetitive recycling of guanosine triphosphate cyclohydrolase I for synthesis of dihydroneopterin triphosphate

A procedure for enzymatic production of dihydroneopterin triphosphate is described that allows GTP cyclohydrolase I to be reused repetitively. The reaction takes place in an ultrafiltration cell, and the product is collected in the filtrate, whereas the enzyme remains in the cell to be reused with additional substrate. This is repeated until the enzyme activity drops below a desirable level. The purity of the dihydroneopterin triphosphate is satisfactory for utilization of this compound for studies on enzymes involved in the synthesis of tetrahydrobiopterin and drosopterin. A procedure for purification of dihydroneopterin triphosphate is described that uses C18-silica and silica cartridges.

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.

Cerebrospinal fluid biopterin is decreased in Alzheimer's disease

Tetrahydrobiopterin is the cofactor in the hydroxylation of phenylalanine, tyrosine, and tryptophan leading to the eventual synthesis of the monoaminergic neurotransmitters, dopamine, norepinephrine, and serotonin, respectively. Total biopterin (90% of which is in the tetrahydro form) was measured in cerebrospinal fluid (CSF) and plasma of 30 patients with Alzheimer's disease and of 19 healthy controls. Plasma and CSF biopterin concentrations were not significantly correlated, but the mean CSF biopterin concentration in patients with Alzheimer's disease was significantly less than in age-matched controls, 13.5 pmol/mL as compared with 18.9 pmol/mL. The CSF biopterin concentration was not correlated with ventricular volume, as estimated by quantitative computed tomography, nor with the severity of dementia, as measured by various cognitive tests. The results suggest that a central biopterin deficiency exists in Alzheimer's disease.

An abbreviated synthesis of tetrahydropteridines

5,6,7,8-Tetrahydrobiopterin, the naturally occurring essential cofactor for the enzymatic hydroxylations of phenylalanine, tyrosine and tryptophan, and its synthetic analog 2-amino-6-methyl-5,6,7,8-tetrahydro-4(3H)-pteridinone, have been synthesized in good yield by the direct hydrogenation of 1-(2-amino-1,6-dihydro-5-nitro-6-oxopyrimidin-4-yl-amino)-1,5-dide oxy-L- erythro-pentulose and 2-amino-6-hydroxy-5-phenylazo-4-pyrimidylamino-acetone, respectively. The reactions were carried out at room temperature in trifluoroacetic acid over a platinum catalyst at 2 atm and the products, each containing a mixture of the two possible C-6 isomers, were isolated by precipitation. The simplicity of the preparative method suggests the procedure may be applied generally to the synthesis of all tetrahydropteridines derived from similar pyrimidine precursors.

[Heterocyclics fused to pyrazine-1,4-dioxide. 3. Synthesis and antibacterial effect of substituted pteridine--5,8-dioxides]

The synthesis of substituted pteridine-5,8-dioxides (6) via reaction of furoxanes (5) with enolizing carbonyl compounds is described. Compounds 5 are obtained either through reaction of chloro pyrimidines 1 with sodium azide, or through diazotization of hydrazino pyrimidine 2 and subsequent thermal decomposition of 3 or through nucleophilic displacement of methoxy against amino groups in 5. The antibacterial activity of the compounds 6a and 6c is directed against some gramnegative organisms, i.e. E. coli, Klebsiella spp. and Proteus spp., with MIC values and ED50 values which do not exceed those of the corresponding pyridol [2,3-b]pyrazine-1,4-dioxides.

Folate antagonists. 19. Synthesis and antimalarial effects of 6-(arylthio)-2,4-pteridinediamines

A series of 6-(arylthio)-2,4-pteridinediamines (IIIa) was prepared by allowing 6-chloro-2,4-pteridinediamine to react with the requisite benzenethiols in dimethyl sulfone at 190-200 degrees C. Attempts at oxidation to the corresponding sulfoxide (IIIb) or sulfone (IIIc) were unsuccessful. The compounds exhibited a spectrum of antibacterial activity similar to, but below the potency of, the related quinazolinediamines and pteridinediamines. Unlike these related types, however, they were devoid of antimalarial activity when tested against a normal drug-sensitive strain of Plasmodium berghei in mice by the parenteral route.

Folate antagonists. 18. Synthesis and antimalarial effects of N6-(arylmethyl)-N6-methyl-2,4,6-pteridinetriamines and related N6,N6-disubstituted 2,4,6-pteridinetriamines

N6-(Arylmethyl)-N6-methyl-2,4,6-pteridinetriamines (1-15) and related N6-substituted 2,4,6-pteridinetriamines (16-20) were obtained by the condensation of 6-chloro-2,4-pteridinediamine with N-methylarylmethanamine and other selected secondary amines. The requisite N-methylarylmethanamines (21-32) were prepared by the hydrogenation over Pt/C of the corresponding arylcarboxaldehyde in the presence of methanamine. Several of the N6-(arylmethyl)-N6-methyl-2,4,6-pteridinetriamines exhibited exceptional suppressive antimalarial activity against a drug-sensitive line of Plasmodium berghei in mice. N6-Methyl-N6-(1-naphthalenylmethyl)-2,4,6-pteridinetriamine (9), the most active of these compounds, was also shown to be curative at 3.16 mg/kg in a single oral dose against P. cynomolgi in the rhesus monkey. This compound was also shown to be effective against a chloroquine-resistant line of P. berghei in the mouse but showed cross-resistance to a pyrimethamine-resistant strain. Most of the 2,4,6-pteridinetriamines showed strong antibacterial action against Streptococcus faecalis and Staphylococcus aureus.

A convenient preparation of tetrahydropteridines and tetrahydrofolate analogs. Synthesis of tetrahydro-3',5'-dichloromethotrexate

Tetrahydropteridine enzymatic cofactors and inhibitors may be conveniently prepared by reduction of the aromatic precursors with dimethylamine--borane in glacial acetic acid. A short reaction time at 20-60 degrees C affords tetrahydrofolate analogs in high yield and purity under anhydrous conditions and without protection from air. 3',5'-Dichloromethotrexate is reduced without hydrogenolysis of chlorine.

Synthesis and biological activity of 8-oxadihydropteridines

A series of 6-substituted and 6,7-disubstituted pyrimido[4,5-b][1,4]oxazines (8-oxadihydropteridines) was synthesized through the condensation of an alpha-halo ketone and 2,5-diamino-4,6-pyrimidinediol. The resulting 8-oxadihydropteridines were assayed as potential antifolates in a dihydrofolate reductase enzyme system. The 2-amino-4-hydroxyoxa-dihydropteridines were found to possess greater biological activity than the corresponding 2,4-diamino compounds. The pteroic acid homeostere 2-amino-4-hydroxy-6-phenethyl-8-oxadihydropteridine was the most potent of the compounds tested.

Folate antagonists. 10. Synthesis and antimalarial effects of 6-[[(aryl and aralkyl)amino]methyl]-2,4-pteridinediamines and -pteridinediamine 8-oxides

Various 6-[[(aryl and aralkyl)amino]methyl]-2,4-pteridinediamines and their 8-oxides have been synthesized for antimalarial evaluation. Condensation of 3-amino-6-(bromomethyl)-2-pyrazinecarbonitrile 4-oxide (V) with the appropriately substituted amine afforded a series of 3-amino-6-[[(aryl and aralkyl)amino]methyl]-2-pyrazinecarbonitrile 4-oxides VI. Deoxygenation gave the corresponding pyrazines VII. Cyclization of VI and VII with guanidine then produced the desired 6-(aminomethyl)-2,4-pteridinediamine N-oxides VIII and teridinediamines IX, respectively. Formylation of 6-[[(3,4-dichlorophenyl)amino]methyl]-2,4-pteridinediamine gave N-[(2,4-diamino-6-pteridinyl)-methyl]-N-(3,4-dichlorophenyl)formamide. The N-oxides VIII did not exhibit significant activity against Plasmodium berghei infections in mice. Activity among the 2,4-pteridinediamines IX was generally poor with the exception of the 3,4,5-trimethoxyphenyl and 1-naphthalenyl analogues which showed strong suppressive activity at doses ranging from 80 to 640 mg/kg. Furthermore, several of the 2,4-pteridinediamines exhibited potent prophylactic activity against Plasmodium gallinaceum infections in the chick and also showed strong antibacterial action against Streptococcus faecalis and Staphylococcus aureus.

Folate antagonists. 11. Synthesis and antimalarial effects of 6-[(aryloxy- and arylthio-)methyl]-2,4-pteridinediamines and -pteridinediamine 8-oxides

Condensation of 3-amino-6-(bromomethyl)-2-pyrazinecarbonitrile 4-oxide with 4-chlorophenol gave 3-amino-6-[(4-chlorophenoxy)methyl]-2-pyrazinecarbonitrile 4-oxide (1), which was deoxygenated to obtain the de-N-oxide 4. Cyclization of 4 and 1 produced 6-[(4-chlorophenoxy)methyl]-2,4-pteridinediamine and the 8-oxide, respectively. 6-[(Arylthio)methyl]-2,4-pteridinediamines and their 8-oxides were produced analogously. Controlled oxidation of the former gave the anticipated sulfoxide 12 and sulfone 13. None of these compounds showed significant activity when tested against lethal Plasmodium berghei infections in mice or a select list of bacteria in vitro.

Pteridines. 41. Synthesis and dihydrofolate reductase inhibitory activity of some cycloalka[g]pteridines

A number of homologous 2,4-diaminocycloalka[g]pteridines varying in ring size from 5 to 15 were prepared by (a) condensation of aminomalononitrile tosylate with alpha-oximinocycloalkanones, deoxygenation of the resulting 2-amino-3-cyanocycloalka[b]pyrazine 1-oxides, and guanidine cyclization; (b) guanidine cyclization of the above pyrazine 1-oxides to give 2,4-diaminocycloalka[g]pteridine 8-oxides, followed by deoxygenation; or (c) condensation of 2,4,5,6-tetraaminopyrimidine with a cycloalka-1,2-dione (for the cyclohepta- and cycloocta[g]pteridines only). These compounds were examined for their activity as dihydrofolate reductase inhibitors against Lactobacillus casei, rat liver, L1210, and Trypanosoma cruzi. Activity was found to depend upon ring size, with the greatest activity exhibited by the cyclododeca derivatives 31.