l8 SUBCELLULAR PARTICLES 



POTENTIALITY VS. ACTIVITY 



One may raise the question whether tetrazolium salts, added in this artificial 

 manner, truly indicate the intracellular sites of DPNH reduction in vivo. This 

 is essentially the question, raised earlier, of potentiality as opposed to actuality. 

 We may ask the same about the apparent ATPase activity shown in figures 10-13. 

 Does the enzyme function, in vivo, as a phosphatase; or does it, as the evidence 

 suggests for isolated mitochondria, catalyze a reaction linking oxidation and 

 phosphorylation ? 



Similar questions may, and should, be raised regarding enzyme activities as 

 measured in isolated subcellular fractions, in submitochondrial fragments, or in 

 purified enzymes. Whether an added dye will accept electrons may, for example, 

 depend upon the degree of coupling of the electron transport chain. In a tightly 

 coupled system, the dye may be unable to reveal the DPNH-cytochrome c re- 

 ductase. Perhaps only if the system is loosened, to begin with, by treatment either 

 with the dye itself or by some other means, will the dye gain access to electron- 

 transmitting enzymes. It would be ironic if only that dye is capable of showing 

 enzyme activity in an organelle which uncouples the organization of that 

 organelle and thereby alters its structure. 



We arc here in an area where we can do little more than ask questions. Our 

 information is still too meagre with respect to control mechanisms within the 

 cell, or influences of substrate and coenzyme concentrations on rate-limiting 

 reactions and enzyme-forming systems, or effects upon enzyme activity of its 

 integration into a film or membrane within the cell (as Dr. McLaren emphasizes 

 in his chapter (23)) or of the milieu in which it acts (as Dr. Siegel suggests in 

 his chapter (47)). Limitations of thermal collisions of cytochromes by structural 

 factors in the mitochondria are stressed by Green in this volume (17), as is the 

 vital role of lipoprotein in electron transport. These are some of the most fun- 

 damental issues in relation to the in vivo function of cell particles. In this 

 connection, an article by Korr, "Oxidation-Reductions in Heterogeneous Sys- 

 tems," written some 20 years ago for a Cold Spring Harbor Symposium (20), 

 makes rewarding reading. Consideration of his generalizations is particularly 

 interesting when viewed in current knowledge of multi-enzyme systems of 

 mitochondria, intracellular distributions of enzymes, the fine structure of cells, 

 the capacity of mitochondrial fragments to carry out oxidative phosphorylation, 

 and the 'reconstruction' of the glycolytic sequence with soluble enzymes. It seems 

 to this reader that relatively litde advance has been made, in these two decades, 

 toward the 'formulation of the rules,' as Korr (20) puts it, which govern the 

 properties of enzymes and other cell constituents when incorporated in an 

 interface or film. 



Another general point may be made in relation to evaluating biochemical 

 data in terms of /;; vivo tissue. I believe we are all aware that 'the liver cell' is 



