70 S. S. COHEN 



the cell fragments, nucleated and enucleate, into which a cell is cut. Or the 

 synthetic capacities of a cell may be studied under the influence of metaboUc 

 blocks, imposed genetically, nutritionally, or by means of more or less 

 specific inhibitors. However, in the immediate sections to follow we may 

 consider the result of apparently more direct techniques, namely, the histo- 

 chemical method and the method of isolation via differential centrifugation 

 of cell components from homogenates. 



Little will be said concerning the first of these; at the present time it 

 appears strongly desirable to check results obtained by this approach by 

 centrifugal fractionation or by other techniques. For example, it has been 

 claimed on histochemical gxounds that alkalme phosphatase is concentrated 

 in the nucleus. Data on isolated fractions have failed to support this conten- 

 tion (Novikoff, 1951). A similar discrepancy has been observed in the data on 

 the localization of acid phosphatase (Palade, 1951). At first, then, we shall 

 concentrate on our knowledge of the distribution of enzymes obtained with 

 isolated organelles. 



An investigation into mtracellular enzyme content and distribution 

 should attempt to correlate actual cellular activity and potential cellular 

 activity. The observed activity of a cell is not merely equal to the activity 

 of the component parts. For example, particular activities of hver may 

 either be lower or greater than the activities detectable in a homogenate. In 

 addition, the activities of intact cells must be understood m terms of the 

 external natural miheu of the cell, which would include hormones, substrates, 

 etc., as weU as in the interactions of the internal enzymatic equipment which 

 functions in many different states of organization and complexity. 



The enzyme activity of a homogenate is not always the sum of its com- 

 ponent parts. The mere preparation of a homogenate, assuming enzymes are 

 not being damaged in the process, may dilute or affect previously optimal 

 concentrations of coenzymes and various ions. Furthermore, the individual 

 activities of the components will not necessarily add up to the activity of the 

 homogenates. Thus, the interactions of the separated cell components may be 

 essential to the demonstration of maximal reactivities. For example, the rate 

 of oxidation of various substrates by mitochondria can be increased several 

 fold by addmg the glucose-hexokinase system to the medium in order to trap 

 liigh energy phosphate being generated at the organized enzyme sites (Lardy 

 and Welhnan, 1952). The rate-limiting factors affecting oxidation by mito- 

 chondria include not only the enzymes of this organelle but also its supply of 

 phosphate acceptor. Hexokinase is normally in the fluid surrounding the 

 mitochondria. 



There are many systems in which fractionation reveals the dependence of 

 maximal metabolic activity upon a considerable order of complexity. In a 

 study of the choline oxidase of rat liver homogenate, most of the activity 



