Vol. 14, No. 3, 1964 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS INHIBITION BY DIGITONIN OF BOVINE HEART MUSCLE DPNH-CYTOCHROME 2 REDUCTASE AND ITS SPECIFIC REVERSAL BY THE TOCOPHEROLS* Alvin Nason, Reginald H. Garrett, P. P. Nair, Frank D. Vasington and Thomas C. Detwiler McCollum-Pratt Institute The Johns Hopkins University Baltimore, Maryland Received November 5, 1963 The primary mechanism of action of vitamin E (the tocoph- erols) in the living cell has not yet been determined (see reviews by Boyer, 1960; Vasington et al., 1960, Symposium on Vitamin E, 1962). Among the prominent possibilities for a key metabolic role of vitamin E are (a) a protective action as an intracellular antioxidant, (b) an unknown function in nucleic acid metabolism, and (c) as a component, either directly in electron transport (and/or in phosphorylation reactions) , or indirectly, perhaps as a structural agent of the cytochrome 2 reductase portion of the terminal respiratory chain in mammalian striated muscle. In support of the latter proposal that vitamin E may be an essential component of the cytochrome c reductase of mammalian striated muscle, the present communication reports that the activity of purified DPNH-cytochrome 5 reductase of bovine heart muscle (and of rat skeletal muscle) is markedly inhibited by digitonin, and is specifically and fully restored by the tocoph- erols (a, B, x and 6 -forms). All other substances tried including other fat soluble vitamins, various glycerides, fatty acid esters, synthetic antioxidants, steroids, and coeneyme Q failed to prevent or reverse digitonin inhibition. 'Contribution No. 403 of the McCollum-Pratt Institute. This investigation was supported in part by a research grant (No. AM 3923) from the United States Public Health Service. 220