5-Formyltetrahydropteroylpolyglutamates are the major folate derivatives in Neurospora crassa conidiospores.

5-Formyltetrahydropteroylpolyglutamate (5-CHO-H4PteGlun) is the only reduced folate derivative that is stable to oxidation and alkaline pH. However, no metabolic role has been assigned to this folate derivative, and evidence for its existence in cells has been questioned. Recently, serine hydroxymethyltransferase was shown to catalyze the formation of 5-CHO-H4PteGlun from 5,10-methenyl-H4PteGlun (Stover, P., and Schirch, V. (1990) J. Biol. Chem. 265, 14227-14233). We have proposed that 5-CHO-H4PTeGlun may serve as a storage form of reduced folates and one-carbon groups in cells that are in a dormant stage. This hypothesis was tested by determining the levels of H4PteGlun derivatives in the mycelia and conidiospores of Neurospora crassa and a mutant strain that lacks cytosolic serine hydroxymethyltransferase. N. crassa serine hydroxymethyltransferase was purified to homogeneity and characterized with respect to kinetic constants, quaternary structure, stability, and reaction specificity. A new assay for determining the concentration of the polyglutamate forms of H4PteGlun derivatives was also developed. Using this assay, it was shown that 85% of the tetrahydropteroylpolyglutamates in conidiospores was 5-CHO-H4PteGlun. After adding the spores to growth media, the 5-CHO-H4PteGlun was reduced to less than 10% of the folate pool in 20 min. Mycelia had no detectable 5-CHO-H4PteGlun. Only 10-20% of the folate pool in conidiospores from the mutant strain lacking cytosolic serine hydroxymethyltransferase was in the form of 5-CHO-H4PteGlun.