NADH oxidase activity of mitochondrial apoptosis-inducing factor

Apoptosis-inducing factor (AIF) is a mitochondrial flavoprotein, which translocates to the nucleus during apoptosis and causes chromatin condensation and large scale DNA fragmentation. Here we report the biochemical characterization of AIF's redox activity. Natural AIF purified from mitochondria and recombinant AIF purified from bacteria (AIF Delta1-120) exhibit NADH oxidase activity, whereas superoxide anion (O-2(-)) is formed. AIF Delta1-120 is a monomer of 57 kDa containing 1 mol of noncovalently bound FAD/mol of protein. ApoATF Delta1-120, which lacks FAD, has no NADH oxidase activity. However, native AIF Delta1-120, apoAIF Delta1-120, and the reconstituted (FAD-containing) holoAIF Delta1-120 protein exhibit a similar apoptosis-inducing potential when microinjected into the cytoplasm of intact cells. inhibition of the redox function, by external addition of superoxide dismutase or covalent derivatization of FAD with diphenyleneiodonium, failed to affect the apoptogenic function of AIF Delta1-120 assessed on purified nuclei in a cell-free system. Conversely, blockade of the apoptogenic function of AIF Delta1-120 with the thiol reagent para-chloromercuriphenylsulfonic acid did not affect its NADH oxidase activity. Altogether, these data indicate that AIF has a marked oxidoreductase activity which can be dissociated from its apoptosis-inducing function.