IDENTIFICATION AND CHARACTERIZATION OF HUMAN MITOCHONDRIAL METHENYLTETRAHYDROFOLATE SYNTHETASE-ACTIVITY

We present evidence for the presence of the folate metabolism enzyme m ethenyltetrahydrofolate synthetase (MTHFS) in mitochondria. MTHFS acti vity was identified in the matrix of mitochondria purified from human liver biopsies, Mitochondrial and cytoplasmic MTHFS specific activitie s are similar, 85% of the total cellular MTHFS activity is in the cyto plasm and both native enzymes have similar molecular weights (approxim ately 25 kDa). Studies using purified mitochondrial MTHFS from CA46 hu man Burkitt lymphoma cells reveal that mitochondrial MTHFS behaves kin etically like the cytoplasmic enzyme with K-m values of 4.7, 0.8 and 2 2 mu M respectively for (6R,S)-5-formyltetrahydrofolate monoglutamate, (6S)-5-formyltetrahydrofolate pentaglutamate and ATP. This finding ad ds to previous observations that various folate-dependent enzymes resi de in the mitochondria of eucaryotic cells. Intracellular tetrahydrofo late metabolism is highly compartmentalized and mitochondrial MTHFS ac tivity is necessary for the entry of mitochondrial 5-formyltetrahydrof olate into the mitochondrial folate pool.