244 



GEORGE H. HOGEBOOM AND WALTER C. SCHNEIDER 



in one liver cell differs from those in neighboring cells? The answers to 

 questions of this type obviously would depend upon methods more refined 

 than the present cell fractionation technic. If differences do exist between 

 the mitochondria in the same or adjacent cells, it would seem likely that 

 some differences in the microscopic appearance of these granules should 

 have been noted. Perhaps, with the biochemical results as a stimulus, it 

 may become possible to demonstrate cytologically that differences in mito- 

 chondria exist and to establish where the variations occur. The biochemical 

 studies may also provide a basis for the study of mitochondrial duplica- 

 tion, both with respect to the mechanism of reproduction of the granules 

 themselves and also with regard to the synthesis of the enzymes and other 

 compounds residing in the mitochondria. 



6. The Pentose Nucleic Acid of Isolated Cell Fractions 



Deoxypentose nucleic acid (DNA) does not occur normally in the cyto- 

 plasm. Although small amounts of DNA have occasionally been found in 

 isolated mitochondria and microsomes, its occurrence in these fractions 

 probably results either from the rupture of some nuclei during the prepara- 

 tion of homogenates^" or from inadequacies in the method of determining 

 DNA.«» 



The presence of PNA in mitochondria and microsomes was first suggested 

 by Claude's** determinations of the ultraviolet absorption spectra of ex- 

 tracts of the particles. His measurements did not, however, distinguish be- 

 tween PNA and other compounds with similar absorption characteristics. 

 Quantitative studies of the distribution of PNA in liver fractions showed 

 that only small amounts were present in the isolated nuclear and mito- 

 chondrial fraction and that the major proportion was associated with the 

 unfractionated supernatant remaining after removal of these two particu- 



TABLE VIII 

 Pentose Nucleic Acid Content of Mouse Liver Fractions** 



PNA phosphorus 



Total," Recovery, 



Preparation fig. per cent 



Homogenate 92.9 100 



Nuclear fraction 10.2 11.0 



Mitochondria 15.7 16.8 



Microsomes 48.7 52.4 



Soluble fraction 15.3 16.5 



" Per 100 mg. of whole tissue or an equivalent amount of each fraction. 



Concentration, 

 ixg. per mg. 

 of nitrogen 



28.1 

 19.2 

 18.4 

 64.2 

 12.0 



