1. DNA prepared from non-gelable rat liver nuclei isolated in the presence of disrupted mitochondria at pH 6.0, has been compared with DNA obtained from gelable nuclei isolated at pH 4.0. The DNA of the non-gelable nuclei is partially depolymerized relative to the DNA of the gelable nuclei. 2. It has been found that sufficiently small quantities of crystallized DNAase I can cleave a very large part of the DNA of gelable nuclei isolated at pH 4 from the residual protein of these nuclei without causing extensive depolymerization of the DNA. At the same time the gelable nuclei are rendered non-gelable. 3. Partially purified DNAase II can also render gelable nuclei isolated at pH 4 non-gelable, and in so doing presumably also cleaves the DNA from the residual protein of the nuclei. 4. Mitochondrial DNAase I appears to be the enzyme responsible to a large extent for the cleavage of DNA from the residual protein of gelable rat liver cell nuclei with concomitant destruction of the gel-forming capability of these nuclei, when the nuclei are subjected to the action of disrupted mitochondria at pH 6.0 during the isolation procedure. 5. Mitochondrial DNAase II does not appear to exert appreciable action on nuclei during the course of isolation of the nuclei at pH 6.0 in the presence of disrupted mitochondria. 6. It is probable that DNAase I is not the sole enzyme responsible for destroying the gelability of nuclei isolated at pH 6.0 in the presence of disrupted mitochondria. Protease may be involved. 7. Sodium dodecyl sulfate at pH 6.0–6.3 cleaves the DNA of isolated gelable nuclei from the residual protein of these nuclei over a period of 2 to 3 hours. At pH 7.0–7.5, however, there is negligible cleavage over a period of 96 hours. 8. If non-gelable nuclei are isolated at pH 6.0 in the presence of disrupted mitochondria, DNA subsequently can be removed from them by the use of detergent at pH values ranging from 6.0–7.5 without the necessity of incubation in the detergent solution, since the DNA had already been detached from the residual protein by the action of the mitochondrial enzyme system during isolation of the nuclei.

1. Cytochemical studies of the intracellular distribution of alkaline phosphatase in rat liver have been made, using a fractionation procedure recently developed in this laboratory (8) and a similar but modified method not described previously. Aqueous media were used in both cases. 2. The alkaline phosphatase was found to consist of two forms, one of which is strongly activated by magnesium and one of which is not sensitive to this metal. 3. The form of the enzyme that is not activated by magnesium occurs mainly in the nuclear fraction, where it seems to be rather firmly bound. Some of this form of the enzyme is also found in the microsomes, but very little if any occurs in the soluble supernatant fraction. 4. The form of alkaline phosphatase which is activated by magnesium occurs mainly in the soluble supernatant fraction, but what is believed are significant amounts also occur in nuclei. A significant portion of this form of the enzyme can be extracted from the isolated nuclei with cold, isotonic saline solution. Some activity of this form of the enzyme is also found in the microsomal fraction. 5. Mitochondria appear to contain relatively little alkaline phosphatase of either kind. 6. The concept of a porous nuclear membrane has been invoked to explain some of the results obtained in this work. It is postulated that part at least of the form of the enzyme that is activated by magnesium is free to diffuse back and forth through pores in the nuclear membrane, whereas this is considered not to be possible for the form of the enzyme that is insensitive to magnesium as a result of the firm binding of the latter to nuclear substance.

1. Rat liver nuclei were isolated from normal rats and rats fasted for 36 hours by a slight modification of the Behrens technique. 2. The nucleus of the rat liver cell contains two types of alkaline phosphatase. This confirms the previous findings on rat liver nuclei isolated in aqueous media. 3. The one type of alkaline phosphatase is not activated by magnesium ions, and this enzyme is very strongly bound to structural material of the nucleus. The other type of alkaline phosphatase is activated by magnesium ions, and this enzyme is probably free to diffuse from cytoplasm to nucleus and vice versa through the nuclear membrane. 4. Fasting caused a pronounced decrease of protein in general and of the alkaline phosphatase which is activated by magnesium ions from the nucleus of the rat liver cell, while the alkaline phosphatase that is not activated by magnesium was less affected.

1. An improved type of ground glass homogenizer for soft tissues has been described which brings about a high degree of cell disruption and liberation of nuclei without causing appreciable damage to mitochondria. The gentleness and effectiveness of the new homogenizer in respect to isolation of mitochondria have been ascertained by comparing the ATP-ase activities of mitochondria isolated in 0.25 M sucrose solution without pH adjustment using a previous type of homogenizer with those of mitochondria isolated under the same conditions with the aid of the new homogenizer. In these experiments sucrose of 0.25 molarity without pH adjustment has been used in order to maintain the mitochondria in a rather sensitive state so as to make slightly deleterious effects of homogenization readily apparent. 2. A new method is described for the isolation of morphologically intact mitochondria and cell nuclei from the same homogenate. In this procedure the pH of the homogenate in 0.44 M sucrose is maintained at 6.0–6.2 with citric acid during the homogenization. An alternative method employing 0.44 M sucrose plus 0.005 M CaCl 2 is given for the isolation of nuclei from tumor cells. However, the latter method does not produce unaltered mitochondria. 3. The α-ketoglutarate, malate, succinate, and hexanoate oxidases of the "intact" mitochondria isolated in 0.44 M sucrose adjusted to pH 6.0–6.2 with very dilute citric acid as described in this paper have been investigated, and it has been shown that the mitochondria compare favorably to those isolated in 0.25 M sucrose by a previously described method. 4. Mitochondria have been found to contain an enzyme which causes nuclei to lose their ability to form gels in dilute alkali. This enzyme is released from the mitochondria when the latter are disrupted. 5. Some properties of nuclei isolated by the new method have been briefly discussed.

1. Known methods for isolating cell nuclei are divided into two classes, depending on whether or not the nuclei are capable of forming gels in dilute alkali or strong saline solutions. Methods which produce nuclei that can form gels apparently prevent the action of an intramitochondrial enzyme capable of destroying the gel-forming capacity of the nuclei. Methods in the other class are believed to permit this enzyme to act on the nuclei during the isolation procedure, causing detachment of DNA from some nuclear constituent (probably protein). 2. It is shown that heating in alkaline solution and x-irradiation can destroy nuclear gels. Heating in acid or neutral solutions can destroy the capacity of isolated nuclei to form gels. 3. Chemical and biological evidence is summarized in favor of the hypothesis that DNA is normally bound firmly to some nuclear component by non-ionic linkages.