Diastereoisomers of formaldehyde derivatives of tetrahydrofolic acid and tetrahydroaminopterin

Preparation of (6R)-tetrahydrofolic acid and (6R)-5-formyltetrahydrofolic acid of high stereochemical purity

Commercially available 5-formyltetrahydrofolate (5-CHO-H4PteGlu) is chemically prepared in a reaction that introduces an asymmetric center at the 6 carbon, and hence is the mixture of diastereomers differing in chirality about this position. (6R)-5-CHO-H4PteGlu, the diastereomer that is not normally found in vivo, was prepared from folic acid. Folic acid was chemically reduced and (6R)-tetrahydrofolate (H4PteGlu) was obtained from the resultant (6R,S)-H4PteGlu by enzymatic consumption of the natural diastereomer of (6R,S)-5,10-CH2-H4PteGlu (reversibly formed from (6R,S)-H4PteGlu in the presence of formaldehyde) with Lactobacillus casei thymidylate synthase. The 5 position of purified (6R)-H4PteGlu was directly formylated in a carbodiimide-catalyzed reaction. The level of contamination of these preparations with the corresponding 6S diastereomers was estimated using the binding of fluorodeoxyuridylate to thymidylate synthase promoted by folate cofactor (for H4PteGlu) and by the growth of folate requiring bacteria (for 5-CHO-H4PteGlu). Purified preparations of (6R)-H4PteGlu promoted the binding of fluorodeoxyuridylate to L. casei thymidylate synthase (in the presence of formaldehyde) only at concentrations greater than 1000-fold higher than equiactive levels of (6S)-H4PteGlu. Likewise, the (6R)-5-CHO-H4PteGlu made by this method was 600 times less active as a growth factor for Pediococcus cerevisiae than was authentic (6S)-5-CHO-H4PteGlu. Hence, the minimum stereochemical purity of these preparations was 99.9% for (6R)-H4PteGlu and 99.8% for (6R)-5-CHO-H4PteGlu.

The formaldehyde-donating activity of N5,N10-methylene tetrahydrofolic acid in xenobiotic biotransformation

Reaction of reproterol, (7-(3-[2-(3,5-dihydroxyphenyl)-2-hydroxyethylamino]propyl)theophyl line (I), with formaldehyde liberated in a solution of N5,N10-methylene tetrahydrofolic acid at pH 7.3 led to the formation of 7-[3-(4,6,8-trihydroxy-1,2,3,4-tetrahydroisoquinolinyl-2)propyl]th eophylline (II), but no such reaction was observed in a solution of N5-methyl tetrahydrofolic acid. Compound II was also formed by incubation of I with rat-liver subcellular fractions. The highest formaldehyde-donating activity was found in the soluble fraction but some activity was also observed with washed mitochondria and microsomes. Dimedone was transformed to methylene bisdimedone by incubation with the soluble fraction. The amount of formaldehyde liberated in the soluble fraction during a four hour incubation at 22 degrees C was 1-3.6 mumol/g of liver. Metaproterenol and terbutaline, structurally related to I, reacted with formaldehyde to yield the corresponding tetrahydro-isoquinolines. The rate of this chemical reaction for the three drugs correlated with the amount of the tetrahydroquinolines formed by incubation with the soluble fraction and in orally dosed rats.