STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 71 



was present in the mitochondria (Williams, 1952). However, in this instance 

 the supernatant fluid was reported to contain a heat-labile inhibitor which 

 severely depressed this activity. 



Finally, we must note that any study attempting to determine the quan- 

 titative and qualitative metabolic relations of structural units must employ 

 methods in which damage to the components does not occur or is minimized. 

 Recent fractionation methods stress the controlling use of microscopic 

 examination, paying particular attention to the size, shape, internal structure, 

 and stainability of the organelle. It is no exaggeration to say that many 

 laboratories have wasted years of effort on materials subsequently shown to 

 be unsatisfactory from the point of view of homogeneity and structural 

 resemblance to the native state. 



Wlien due attention is paid to these elementary orienting considerations, 

 it is found that there are numerous enzymatic activities in homogenates that 

 are relatively unaffected by careful fractionation. Thus, the cytochrome 

 oxidase and succinoxidase of a homogenate are entirely limited to cytoplasm 

 and may be quantitatively recovered in mitochondria. These structures may 

 then be disrupted for purposes of further purification of the component 

 enzymes. 



The Mg++-activated ability to dephosphorylate ATP is also concentrated 

 in the mitochondria, but these bodies only contain about 65 % of the total 

 activity, the remainder being distributed among all other fractions. Systems 

 such as these, in which the total activity of a homogenate can be accounted 

 for, are useful standards in control of the fractionation procedure. However, 

 systems in which activators and inhibitors have been detected and in which 

 interactions of the components of the homogenate occur, can be expected to 

 tell us more about problems of cell regulation. 



B. Tables of Enzyme Distribution 



A number of important reviews have appeared on the biochemical pro- 

 perties of the cellular components. These include general discussions (Brad- 

 field, 1950), surveys of plant material (Van Fleet, 1952; Goddard and 

 Stafford, 1954), of bacteria (Alexander, 1956), and of animal tissues (Hoge- 

 hoom.etal., 1953; Dounce, 1955; Allfrey eiaL, 1955b; Hogeboom and Schneider, 

 1955). It is not the purpose of this discussion to summarize these contribu- 

 tions in detail, but rather to extract data which can reveal the patterns 

 directly relevant to the problems of polymer synthesis. The following tables 

 (Tables VIII, IX, and X) of distribution of enzymes in rat fiver components, 

 taken from Hogeboom et al. (1953) are useful examples for these purposes. 



A glance at these tables reveals that of the known enzymes assayed in rat 

 liver only a few are concentrated in the nucleus and microsomal fractions. 



