THE CYTOPLASM 217 



chondria, are sedimented by low-speed centrifugation, leaving a super- 

 natant containing the cytoplasmic elements of the cell. The sediment has 

 been termed the "nuclear fraction," and because of its heterogeneity is 

 not suitable for detailed studies of the biochemistry of the cell nucleus. 

 Although the latter subject has been discussed at length in Chapter 18, 

 it may be mentioned that analyses of the nuclear fraction have, despite 

 its inhomogeneity, shed considerable light on the localization of enzymes 

 within the nucleus. In the case of practically every enzyme system studied, 

 some activity was recovered in the nuclear fraction. With a few notable 

 exceptions, however, most if not all of the activity could be accounted for 

 by the cytoplasmic elements present. The enzymes that were present in a 

 sufficiently high concentration to indicate actual localization within the 

 cell nucleus were arginase,- -^^ a calcium-activated apyrase**-*® adenosine-5- 

 phosphatase," and the enzyme catalyzing the synthesis of diphospho- 

 pyridine nucleotide (DPN) from ATP and nicotinamide mononucleo- 

 tide.^"-** The concentration of the latter enzyme in the nuclear fraction 

 (as well as in essentially homogeneous preparations of isolated nuclei) 

 was unique in that it approached the concentration of DNA itself, sug- 

 gesting that the synthesis of DPN, at least by this reaction, is an exclusive 

 property of the cell nucleus. 



2. The Mitochondrial Fraction 



a. Biochemical Data Relating to the Integrity of Isolated Mitochondria 



A number of observations have provided evidence that the technique 

 of cell fractionation is a sound tool for studies of the biochemical properties 

 of the rnitochondria of the liver cell. One of the most important questions 

 that had to be answered on an experimental basis related to the properties 

 of the mitochondrial membrane. Thus, although the early biochemical 

 investigations of the mitochondrial fraction revealed a clear-cut localiza- 

 tion of certain enzyme systems in this cytoplasmic structure, it had to be 

 recognized that these enzymes were "insoluble," i.e, easily sedimented from 

 extracts of practically any tissue, not obtainable in a monodisperse state 

 in true solution, and thus refractive to purification. Notable examples were 

 cytochrome oxidase, succinic dehydrogenase, adenosinetriphosphatase 

 and DPN-cytochrome c reductase (Table II). It seemed possible, there- 

 fore, despite the cytological evidence for the integrity of isolated mito- 



8^ A. H. Schein and E. Young, Exptl. Cell Research 3, 383 (1952). 

 86 W. C. Schneider, G. H. Hogeboom, and H. E. Ross, J. Natl. Cancer Inst. 10, 977 

 (1950). 



86 A. B. Novikoff, L. Hecht, E. Podber, and J. Ryan, /. Biol. Chem. 194, 153 (1952). 



87 A. B. Novikoff, E. Podber, and J. Ryan, Federation Proc. 9, 210 (1950). 



88 G. H. Hogeboom and W. C. Schneider, Nature 170, 374 (1952). 



