434 9. INHIBITION IN CELLS AND TISSUES 



also pointed to /?-fructofuranosidase being external, while the glycolytic 

 enzymes are within the cell (Myrback and Vasseur, 1943). These conclu- 

 sions have been supported by kinetic studies on /9-fructofuranosidase and 

 hexokinase in intact yeast (Best, 1955 c). The results indicated that /5-fructo- 

 furanosidase was external to the major diffusion barrier and perhaps as- 

 sociated with the polysaccharide of the capsule in the outer space of Con- 

 way and Downey (1950). However, some barrier to diffusion seems to exist 

 outside the /?-fructofuranosidase region. Finally, the differential inacti- 

 vation of /5-fructofuranosidase by bombardment of yeast cells with low 

 voltage electrons of differing penetration powers localized the enzyme to 

 a zone lying between depths of 500 and 1000 A from the outer surface 

 (Preiss, 1958). Yeast phosphatase also seems to be at the cell surface be- 

 cause, although antibodies inhibitory to hexokinase and pyruvic decar- 

 boxylase showed no effect in the intact cell, antiphosphatase was able to 

 inhibit in yeast suspensions (Derrick et al., 1953). Mention may also be made 

 of the evidence for trans])ort enzymes at the yeast cell membrane from 

 studies on uranium inhibition (Rothstein, 1954). The loostulated membrane 

 transport enzymes, permease (Monod, 1956; Gale, 1957) and translocase 

 (Mitchell, 1957), must be situated somewhere within the membrane in 

 bacteria but exact localization and susceptibility to inhibition have not 

 been determined. In micrococci the following enzymes appear to be lo- 

 calized in the membrane: succinic dehydrogenase, lactic dehydrogenase, 

 malic dehydrogenase, formic dehydrogenase, acid phosphatase, and the 

 cytochrome system (Mitchell and Moyle, 1956). Enzyme activity of the 

 erythrocyte stroma has been established, especially for the synthesis of 

 ATP (Prankerd, 1956) but most of the evidence for membrane enzymes 

 in mammalian cells is very indirect. It would be reasonable to expect the 

 enzymes involved in active transport and ionic pumps to be situated in 

 or near the membrane. Inhibitors exert actions on cardiac cell membrane 

 potentials most readily-interpreted by assuming that the enzymes at- 

 tacked are in the membrane (Webb and Hollander, 1959). The effects of 

 Ca++ on brain glycolysis, as related to changes in pH, have led to the sug- 

 gestion that the neuronal surface is an important site o j glucose meta- 

 bolism (Adams and Quastel, 1956). Finally, it would appear that there are 

 two glycolytic systems capable of forming lactate in diaphragm muscle, 

 one intracellular and able to form glycogen and the other at the membrane 

 and unable to form glycogen (Shaw and Stadie, 1959). Only the former 

 pathway is dependent on insulin, inasmuch as insulin is involved in the 

 inward transport of glucose and the membrane system requires no such 

 transport. 



The problems of enzyme localization have been made more complex 

 by the demonstrations of double membranes throughout the cell (Robert- 

 son, 1957). The classic plasma membrane seems to consist generally of 



