Enzymatic Activities of Rat Liver Cytosol 10-Formyltetrahydrofolate Dehydrogenase 1,2

Abstract

10-Formyltetrahydrofolate dehydrogenase (10-FTHFDH: EC 1.5.1.6) catalyzes the NADP +-dependent oxidation of 10-formyltetrahydrofolate (10-HCO-H 4PteGlu) to tetrahydrofolate (H 4PteGlu) and CO 2 and the NADP +-independent hydrolytic cleavage of 10-HCO-H 4PteGlu to H 4PteGlu and formate, 10-FTHFDH has a 485 amino acid domain at the C-terminus which is 46% identical to aldehyde dehydrogenase (ALDH: EC 1.2.1.3) and contains a conserved active site cysteine (Cys-707). 10-FTHFDH catalyzed NADP +-dependent oxidation of propanal and the hydrolysis of p-nitrophenyl acetate ( pNPA) in a similar fashion to ALDH. Initial rate studies gave K m values of 46 and 636 μM, respectively, for NADP + and propanal, while pNPA had a K m of 220 μM. Propanal was able to compete with 10-HCO-H 4PteGlu for NADP +-dependent oxidation but had no effect on the NADP +-independent hydrolase reaction. N-Ethylmaleimide inhibited NADP +-dependent 10-HCO-H 4PteGlu oxidation but only partially inhibited (65%) hydrolase activity. Disulfiram, a potent inhibitor of cytosolic ALDH, inhibited NADP +-dependent propanal oxidation by 10-FTHFDH. We propose that the dehydrogenase reaction of 10-FTHFDH has a mechanism which proceeds through thiohemiacetal and thioester intermediates, similar to that described for aldehyde dehydrogenase. 10-FTHFDH hydrolase activity was dependent on a-mercaptoethanol and is probably an artifact of the assay system. The N-terminal domain of 10-FTHFDH shows identity to glycinamide ribonucleotide transformylase (EC 2.1.2.2) and contains a putative 10-HCO-H 4PteGlu binding site but shows no GAR-TF activity. NADP +-dependent oxidation of 10-HCO-H 4PteGlu by 10-FTHFDH was inhibited by the folate anti-metabolite, 5,10-dideazatetrahydrofolate folate, a known GAR-TF inhibitor.