Figure 2.

The additional methyl group in the novel folate form is not derived from histidine, serine, or glucose. A–C) Representative MALDI-TOF mass spectra of folylpolyglutamates from L. reuteri cultured in LDM containing unlabeled (top) or isotopically labeled (bottom) ¹⁵N₃[¹³C]₆-histidine (A), [¹³C]₃-serine (B), or [¹³C]₆-glucose (C). D) Structures representing the synthesis of folic acid from GTP. Red dots show the 5 glucose [¹³C] atoms that form the ribose part of GTP and are incorporated into folic acid. E–G) MALDI-TOF mass spectra of folylpolyglutamates from L. reuteri cultured in LDM containing increasing amounts of folic acid (E), with or without pABA (F), or isotopically labeled guanine (G). H) Structures representing the synthesis of folic acid from guanine. Colored dots highlight [¹³C] and ¹⁵N atoms in guanine and the synthesized folic acid.