In this paper we wish to describe what we consider to be improved techniques that permit the isolation of mitochondrla and cell nuclei from the same liver homogenate. In the past it has been necessary to use different types of homogenates in isolating mltochondria and nuclei of acceptable quality. We also wish to demonstrate the importance of maintaining mitochondrial integrity during the isolation of nuclei (1, 2) and we offer evidence indicating that if the mitochondria are disrupted, an enzyme contained within them escapes and enters the nuclei, altering certain of their properties.The mitochondrla produced by the new technique not only are well preserved from the standpoint of morphology, but also fulfill two criteria that have been established for biochemically intact mitochondria (3), namely, that they have latent ATP-ase activity and that they require the presence of a phosphate acceptor for optimal oxidase 1 activity in the presence of those substrates, whose oxidation is coupled to phosphorylation. Previously, mitochondria which satisfied these two criteria have been altered morphologically (4,5). In this paper a detailed comparison is made of oxidase activities of mitochondrla isolated by the new procedure with the corresponding activities of those isolated in 0.25 M sucrose which are biochemically intact but morphologically altered. The ATP-ase activity of mitochondrla isolated by the new procedure is described in another publication (6).The new method has been applied mainly to rat liver, but it would probably work with other soft tissues, although in certain cases special steps to remove fiber would be necessary. The method fails when applied to Walker carcinoma

Recent studies of the ATP-asO of mitochondria isolated from rat liver have shown that this enzyme is latent (1-5). Thus it does not exhibit full activity unless dinitrophenol (DNP) is present (1,3,5,6), or the preparation is aged at 37°C. (1-3), or frozen and thawed (4, 7). The mitochondrla used in most studies have been isolated in 0.25 M sucrose solutions (8). Such preparations differ in morphology from the mitochondria in situ. The former are spherical and appear to be swollen, whereas the latter are rod-or oval-shaped (9). Rod-shaped mitochondrla can be isolated in 0.88 M sucrose (9) and have been found to possess much higher ATP-ase activity (10, 11) than the round type (1-5).In view of these differences in enzymatic activity between the rod-and the round-shaped mitochondrla, the possibility arises that the observed properties of mitochondria isolated in 0.25 x¢ sucrose may be artifacts of preparation which may be correlated with changes in morphology upon isolation. This point becomes even more significant when it is realized that the round swollen mitochondria have been successfully used in the study of oxidative phosphorylation because of the very fact that their ATP-ase is latent thus permitting the measurement of maximal P/O ratios (1,(12)(13)(14). Although preliminary experiments indicate that the ATP-ase activity of mitochondria in 0.88 M sucrose may sometimes be increased by freezing and thawing (12) there are no reports of detailed experiments showing that this enzyme is latent in this preparation. Finally, the properties of the A'l~P-ase of mitochondria isolated in solutions of sucrose concentration other than 0:88 ~ or 0.25 x~ have not been investigated.An attempt has been made, therefore, to correlate changes in morphology of mitochondrla brought about by different methods of isolation with character-

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