242 GEORGE H. HOGEBOOM AND WALTER C. SCHNEIDER 



Chantrenne,-"" who prepared cytoplasmic extracts of mouse liver and 

 centrifuged them at speeds arbitrarily selected to yield 5 fractions of par- 

 ticles with different sedimentation rates. Analysis of the fractions revealed 

 that their content of PNA, phospholipid, and phosphatases was different 

 and that none of the properties studied was a sole constituent of a single 

 fraction. The results suggested, in fact, that the cytoplasm consists of a 

 continuous spectrum of particles, heterogeneous with respect to size, chemi- 

 cal composition, and enzymic pattern. One objection to Chantrenne's 

 experiments is that a salt solution was used in the preparation of the ex- 

 tracts, and that most of the mitochondria were therefore probably lost as a 

 result of aggregation and sedimentation on removal of nuclei and intact 

 cells. Another shortcoming of the experiments, which was recognized by 

 Chantrenne, lay in the fact that the total quantity of enzyme or chemical 

 compound in each fraction was not determined. Furthermore, subsequent 

 work has shown that none of the properties studied is localized exclusively 

 in a single cell structure. These considerations, together with- the probability 

 that several of the fractions were mixtures of mitochondria and microsomes, 

 make it evident that the experiments could indicate a greater degree of 

 heterogeneity than actually existed. The objections to Chantrenne's investi- 

 gation were avoided by Novikoff et al.,^^ who fractionated liver homogenates 

 prepared in 0.25 M sucrose. The latter workers isolated 8 cytoplasmic frac- 

 tions and found that those composed mainly of mitochondria exhibited 

 minor degrees of heterogeneity while the microsomal fractions were mark- 

 edly heterogeneous. 



That submicroscopic particles are heterogeneous had already been indi- 

 cated by the work of Barnum and Huseby,^*'' who studied the chemical 

 composition of two sedimentable fractions obtained from liver extracts 

 after removal of mitochondria and larger elements. In this respect, it has 

 been observed in the writers' laboratory that similar cytoplasmic extracts 

 of liver, when examined in the analytical ultracentrifuge or the electrophore- 

 sis apparatus, yield refractive index patterns indicating the presence of 

 several classes of particles, differing both with respect to sedimentation 

 rate and electrical charge (unpublished experiments). Petermann and 

 Hamilton-^i and Petermann et al.,^''- in a similar study of the sedimentation 

 pattern of submicroscopic particles obtained from liver and from normal and 

 leukemic spleen, report the presence of 5 to 7 groups of particles with 

 characteristic sedimentation rates. 



A recent investigation by Kuff and Schneider^ '^^^ suggests that the 



ao" H. Chantrenne, Biochem. et Biophys. Acta 1, 437 (1947). 



201 M. L. Petermann and M. G. Hamilton, Cancer Research 12, 373 (1952). 



202 M. L. Petermann, N. A. Mizen, and M. G. Hamilton, Cancer Research 13, 372 

 (1953). 



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