Synthesis and biological evaluation of 2-amino-4-hydroxy-6-hydroxymethylpteridine pyrophosphate

Syntheses of labeled vitamers of folic acid to be used as internal standards in stable isotope dilution assays

[2H4]Folic acid was synthesized by deuterating p-aminobenzoic acid, which was then coupled to glutamic acid and 6-formylpterin. Using [2H4]folic acid as starting component enabled the preparation of labeled vitamers tetrahydrofolate, 5-formyltetrahydrofolate, 5-methyltetrahydrofolate, and 10-formylfolate which were characterized by electrospray mass spectrometry and collision-induced dissociation. The mass spectrometric studies confirmed that the compounds could be used as internal standards in stable isotope dilution assays.

Biosynthesis of methanopterin

The biosynthetic pathway for the generation of the methylated pterin in methanopterins was determined for the methanogenic bacteria Methanococcus volta and Methanobacterium formicicum. Extracts of M. volta were found to readily cleave L-7,8-dihydroneopterin to 7,8-dihydro-6-(hydroxymethyl)pterin, which was confirmed to be a precursor of the pterin portion of the methanopterin. [methylene-2H]-6-(Hydroxymethyl)pterin was incorporated into methanopterin by growing cells of M. volta to an extent of 30%. Both the C-11 and C-12 methyl groups of methanopterin originate from [methyl-2H3]methionine, as confirmed by the incorporation of two C2H3 groups into 6-ethyl-7-methylpterin, a pterin-containing fragment derived from methanopterin. Cells grown in the presence of [methylene-2H]-6-(hydroxymethyl)pterin, [ethyl-2H4]-6-[1 (RS)-hydroxyethyl]pterin, [methyl-2H3]-6- (hydroxymethyl)-7-methylpterin, [ethyl-2H4, methyl-2H3]-6-[1 (RS)-hydroxyethyl]-7-methylpterin, and [1-ethyl-3H]-6-[1 (RS)-hydroxyethyl]-7-methylpterin showed that only the non-7-methylated pterins were incorporated into methanopterin. Cells extracts of M. formicicum readily condensed synthetic [methylene-3H]-7,8-H2-6-(hydroxymethyl)pterin-PP with methaniline to generate demethylated methanopterin, which is then methylated to methanopterin by the cell extract in the presence of S-adenosylmethionine. These observations indicate that the pterin portion of methanopterin is biosynthetically derived from 7,8-H2-6-(hydroxymethyl)pterin, which is coupled to methaniline by a pathway analogous to the biosynthesis of folic acid. This pathway for the biosynthesis of methanopterin represents the first example of the modification of the specificity of a coenzyme through a methylation reaction.