Dimethylglycine dehydrogenase (EC 18.104.22.168) carries out the oxidative demethylation of dimethylglycine to sarcosine in liver mitochondria. In vivo, the enzyme uses tightly bound tetrahydropteroyl pentaglutamate (H4PteGlu5) as an acceptor of the one-carbon group generated during the reaction. The purified enzyme can use, but does not require, H4PteGlu5 and under these conditions formaldehyde is the one-carbon unit produced. It is reported that folic acid may be covalently linked to dimethylglycine dehydrogenase in a specific and saturable manner so that only 1 mole of folic acid is bound per mole of enzyme. Covalently bound folic acid blocks the subsequent binding of H4PteGlu, and does not inhibit the rate of dimethylglycine dehydrogenase activity in vitro.