L(-)-10-Formyltetrahydrofolate is the cofactor for glycinamide ribonucleotide transformylase from chicken liver.

It is shown that L(-)-10-formyltetrahydrofolate serves as the cofactor for glycinamide ribonucleotide transformylase from chicken liver. The utilization of L(-)-10-formyl-H4folate was not previously recognized, because L-(+)-10-formyl-H4folate is an excellent competitive inhibitor of the enzyme, Ki = 0.75 +/- 0.07 microM, and historically the transformylase assay was carried out with a mixture of diastereomers. The results are discussed in relation to the utilization of L(+)-5,10-methenyl-H4folate.