Metabolism of 5-methyltetrahydrofolate by Lactobacillus casei

The metabolism of 5-[Me-14 C]methyltetrahydrofolate in Lactobacillus casei proceeded oxidatively with incorporation of label into purine and thymidylate derivatives. No labelled methionine was formed. (l)-5-Methyltetrahydrofolate, the natural isomer, was not a substrate for the L. casei folylpoly-gamma-glutamate synthetase although the unnatural (d)-isomer was slowly metabolized to the diglutamate form.