DISTRIBUTION AND STATE OF ENZYMES IN CELLS 433 



Distribution of Enzymes in Cell Fractions 



Enzyme analyses of cell components separated from homogenates by 

 centrifugation have invariably shown differential distribution. It has been 

 known for some time that nuclei contain very little or no oxidizing en- 

 zymes, but do have enzymes involved in protein and nucleic acid metabo- 

 lism, such as proteinases, cathepsin, dipeptidases, and ribonuclease (Siebert 

 and Smellie, 1957). The enzymes for the tricarboxylic acid cycle are mainly 

 or entirely located in the mitochondria or related structures, although 

 recently the problem of different types of mitochondria, with varying 

 composition, has arisen (Novikoff, 1957). The different concentrations of 

 enzymes (succinic dehydrogenase, uricase, DPN-cytochrome c reductase, 

 and acid phosphatase) in differently sedimentable fractions from liver 

 indicate a degree of complexity previously unappreciated (Kuff and Ho- 

 geboom, 1956). The usual relegation of the glycolytic enzymes to the su- 

 pernate fraction has been questioned in brain where these enzymes have 

 been found associated in part with the mitochondria (Abood et al., 1959). 

 Although localization of enzymes in the cell undoubtedly occurs, care must 

 be taken not to assume the segregation between cell fractions to be complete. 

 Although much progress has been made in this field, it is evident that the 

 fractionations are still rather crude and errors due to redistribution of en- 

 zymes during the preparation are inevitable. Histochemical analyses have 

 also shown characteristic distributions of enzymes in cells but at present 

 the techniques have not advanced far enough to allow detailed localizations, 

 although these methods may eventually provide the desired answers. Re- 

 sults on the distribution of inhibitors within the cell are not of much value 

 unless we also know the enzyme pattern. 



Localization of Enzymes Relative to the Plasma Membrane 



Since the plasma membrane is the major barrier to the entrance of in- 

 hibitors into the cell, it is of primary importance to determine if the involved 

 enzymes are in the cell, within the membrane, or outside. A great deal of 

 indirect evidence has been presented that some enzymes are at or near the 

 cell surface and are susceptible to influences that do not extend within 

 the cell. If an enzyme is both at the membrane and inside the cell, a specific 

 inhibition of the membrane enzyme may be achieved when the inhibitor 

 does not penetrate into the cell, and a specific functional change in the 

 membrane may be produced. 



Yeast /5-fructofuranosidase (invertase) in extracts and suspensions of 

 intact cells is affected equally by changes in pH; it was therefore suggested 

 nearly 30 years ago that this enzyme must be in the outer region of the 

 cell where the pH follows the external pH (Wilkes and Palmer, 1932). 

 Comparison of pH effects on the hydrolysis and fermentation of sucrose 



