Metabolism of 5-Methyltetrahydrofolate by Lactobacittus casei

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