496 J. BRACKET 



different in their PNA content and enzymic constitution (Chantrenne,"^ 

 Barnum and Huseby,"^ Huseby and Barnum"^). According to Chan- 

 trenne,"^ all intermediate stages can be found between very small micro- 

 somes and large mitochondria, and it would appear that the latter grow 

 out of microsomes by a process of progressive complication. 



More recently, however, Novikoff et a/.^^" have separated 8 different 

 fractions from liver homogenates and concluded that 3 of them were purely 

 mitochondrial and 2 entirely microsomal ; the remaining 3 were mixtures of 

 mitochondria and microsomes. They conclude that the cell particles isolated 

 by differential centrifugation are mitochondria, and large and small micro- 

 somes. A somewhat similar opinion is expressed by Smellie et al.,^^^ who 

 find by electron microscopy of tissue extracts only mitochondria and 

 microsomes, the latter being less homogeneous in appearance than the 

 former. 



One recent finding which is in agreement with Chantrenne's"^ suggestion 

 that microsomes might represent early stages in the development of mito- 

 chondria is the fact that, in rat liver at least, the PNA's in both types of 

 particle and in the nonsedimentable fraction (cell sap) have the same 

 composition in terms of molar proportions of bases (Elson and Chargaff,'^- 

 Crosbie et al.^-^). In this respect, they differ markedly from nuclear PNA, 

 a fact which will be discussed in the next section. 



However, results on the incorporation of various labeled precursors in 

 the PNA of the different cellular fractions do not, on the whole, agree with 

 Chantrenne's"^ hypothesis. According to this hypothesis, microsomes 

 should show a higher specific activity than mitochondria, in short-duration 

 experiments at any rate. Such results have been reported by Jeener and 

 Szafarz'2* in rat liver with P^'-; within the cytoplasm, the specific radio- 

 activity decreases progressively from the supernatant to the mitochondria, 

 the microsomes giving intermediate values. In growing tissues (mouse 

 embryos), however, the mitochondria have an activity almost as high as 

 that of the supernatant. 



Jeener and Szafarz's^^* results with liver are similar to those reported a 

 little earlier by Marshak and Calvet,'-^ who also found a higher specific 



'^' H. Chantrenne, Biochim. et Biophys. Acta 1, 437 (1947). 



i'8 C. P. Barnum and R. A. Huseby, Arch. Biochem. 19, 17 (1948). 



"« R. A. Huseby and C. P. Barnum, Arch. Biochem. 26, 187 (1950). 



'20 A. B. Novikoff, E. Podber, J. Ryan, and E. Noe, Federation Proc. 11, 265 (1952). 



'=" R. M. S. Smellie, W. M. Mclndoe, R. Logan, and J. N. Davidson, Biochem. J. 64, 



280 (1953). 

 '" D. Elson and E. Chargaff, in "Phosphorus Metabolism" (McElroj^ and Glass, 



eds.). Vol. 2, p. 329. Johns Hopkins Press, Baltimore, 1952. 

 '" G. W. Crosbie, R. M. S. Smellie, and J. N. Davidson, Biochem. J. 54, 287 (1953). 

 '24 R. Jeener and D. Szafarz, Arch. Biochem. 26, 54 (1950). 

 '" A. Marshak and F. Calvet, J. Cellular Comp. Physiol. 34, 451 (1949). 



