DISTRIBUTION AND STATE OF ENZYMES IN CELLS 435 



two membranes, each about 100 A thick and separated by a space 

 that varies in thickness from 100 to 500 A. In addition, certain cells 

 may possess dense regions a few hundred angstroms thick on the inner or 

 outer sides of these membranes. Indirect evidence has been adduced for 

 two membranes of different conductivities at the nerve surface, so that an 

 ion, such as K^, leaving the cell may be temporarily retained between the 

 tw^o membranes (Frankenhaeuser and Hodgkin. 1955). The implications 

 of such double membranes for enzyme inhibition in the cell have never 

 been discussed but it is clear that the rates of penetration into the cell, 

 the localization of the inhibitor, and the specificity of effect upon the cell 

 may all be dependent on these structures. One must now abandon the sim- 

 ple concept of '"outside" or "inside" enzymes, since the enzyme may be 

 situated in a variety of regions all near the cell surface. Actually, nine 

 different zones may be imagined in the general case, as illustrated in Fig. 9-1. 



Fig. 9-1. Possible locations of an enzyme near the surface of a cell. B and D rep- 

 resent the double membrane with a space, C, between, while A and E are the special 

 regions adjacent to these membranes and are not necessarily present in every cell. 

 The enzyme may be at: 1, the extreme outer surface; 2, in the outer differentiated 

 layer; 3, at the outer surface of the outer membrane; 4, at the inner surface of the 

 outer membrane; 5, in the space between the membranes; 6, at the outer surface of 

 the inner membrane; 7. at the inner surface of the inner membrane; 8, in the inner 

 differentiated layer; or 9, in the interior of the cell. 



State of Intracellular Enzymes 



Some instances have been noted where enzymes within the living cells 

 seem to have different properties from the isolated enzymes. Thus acon- 

 itase, with respect to its inhibition by fluorocitrate, behaves differently 

 when isolated than in intact mitochondria (Peters, 1955), and trypto- 

 phanase isolated from E. coli has an apparent Michaelis constant some 

 twenty times greater than the enzyme in the cells (Rydon, 1955). The latter 

 observation would indicate a much greater affinity of the enzyme for its 

 substrate within the cell and if this is a common phenomenon it would 



