THE CYTOPLASM 241 



formation by the particulate fractions is not readily explained but probably 

 comprises another example of the integrated activity of several cell struc- 

 tures in carrying out a complicated metabolic process. A complete picture 

 of the intracellular locale of glycolytic enzymes will be available, however, 

 only when studies are made of the distribution among cell fractions of each 

 individual enzyme. Some recent steps in this direction all lend support to 

 the view that glycolysis is largely a function of the soluble fraction of the 

 cell. Thus Hers, de Duve, and their collaborators^^^-^^* have shown that 

 hexokinase, phosphorylase, and phosphoglucomutase, and Kennedy and 

 Lehninger^"' that aldolase, are mainly recovered in this fraction. Evidence 

 that lactic dehydrogenase can also be included in the group of nonparticulate 

 enzymes has been obtained in unpublished experiments in the wTiters' 

 laboratory. It may be mentioned, however, that Crane and Sols^'® have 

 found that a significant proportion of the hexokinase of homogenates of 

 various tissues is sedimented at 18000 g. The recovery of hexokinase in this 

 mixed particulate fraction ranged from 35 % in the case of liver to over 90 % 

 in the case of brain. A somewhat similar situation apparently exists in cer- 

 tain plants. ^^* 



The soluble fraction of the liver cell, through its content of isocitric de- 

 hydrogenase** (Table I) and aconitase,'^* apparently takes part in the reac- 

 tions of the Krebs cycle. The presence in the fraction of 90% or more of 

 the adenosine deaminase and nucleoside phosphorylase activity of liver 

 homogenates"^ also demonstrates a probable role in the metabolism of 

 purines. The occurrence of certain other enzymes and compounds, including 

 acetylase,^" glyoxalase,^^* xanthine oxidase, ^^' hexose diphosphatase,^^^ 

 alkaline phosphatase,*" and cytochrome c,^"* as well as the numerous in- 

 stances in which the supernatant has been used as the starting material in 

 the purification of enzymes, attest further to the biochemical complexity 

 of the fraction. 



5. On the Biochemical Homogeneity of Isolated Cytoplasmic 



Structures 



It is evident from the observations described above that conclusions as 

 to the localization of biochemical properties in specific cell structures are 

 in most instances based on cytological determinations of the homogeneity 

 of isolated fractions. That enzymes or other compounds could be confined 

 to a single particulate component was challenged by the experiments of 



1" C. de Duve, H. G. Hers, and J. Berthet, Ind. chim. beige 17, 143 (1952). 

 1S6R. K. Crane and A. Sols, J. Biol. Chem. 203, 273 (1953). 

 '" J. Chauveau and L. V. Hung, Compt. rend. 235, 1248 (1952). 

 1S8 E. Kun, Euclides 10, 251 (1950). 



!«' V. Meikleham, I. C. Wells, D. A. Richert, and W. W. Westerfeld, J. Biol. Chem. 

 192, 651 (1951). 



