It has been assumed that humans cannot utilize 5,6,7,8-tetrahydrofolates with the unnatural configuration at carbon 6, since these folates are enzymatically and microbiologically inactive. We hypothesized that orally administered unnatural [6R]-5-formyltetrahydrofolate or [6S]-5,10-methenyltetrahydrofolate is bioactive in humans. Subjects were given independent oral doses of these unnatural folates and of a natural [6S]-5 -formyltetrahydrofolate. Plasma, before and after the do se for 4 h, and 2 h urine were collected. Areas under the curve for the change in plasma folate concentrations were measured microbiologically and urinary folates were measured using HPLC. Based on findings of plasma and urinary folates, the unnatural folates were estimated to be 14-50% active as compared to [6S]-5-formyltetrahydrofolate. The major plasma and urinary folate was [6S]-5-methyltetrahydrofolate in all experiments. In urine, a [6S]-5-formyltetrahydrofolate peak was observed only after a [6S]-5-HCO-H(4)folate dose and peaks of unnatural [6S]-10-formyltetrahydrofolate and 5-formyltetrahydrofolate were identified after a [6R]-5-formyltetrahydrofolate dose. A possible pathway that explains our findings is discussed. This pathway includes the oxidation of the unnatural [6S]-10-formyltetrahydrofolate to 10-formyl-7,8-dihydrofolate which can be further metabolized by 5-amino-4-imidazolecarboxamide-ribotide transformylase producing dihydrofolate. Dihydrofolate can then be metabolized to [6S]-5-methyltetrahydrofolate by well-established metabolism. (C) 1999 Elsevier Science B.V. All rights reserved.
