490 DANIEL MAZIA 



does exist but it is not a fluctuation of glutathione. Rather, it involves a 

 protein or polypeptide that is soluble in TCA but is precipitable by other 

 protein precipitants (Fig. 3). 



Thus it can be confirmed that there is a major fluctuation, during 

 division, of an SH-carrying molecule which may very well be viewed as 

 "environmental" and not part of the structure of the mitotic apparatus. 

 Other and equally interesting fluctuations of the intracellular medium may 

 well be found, but this is the one we now know and can speculate about. 

 One obvious speculation is that the fluctuation is a controlling factor in the 

 mechanochemical operations of the mitotic apparatus, in the same sense 

 that other mechanochemical systems, non-biological systems or "models" 

 of biological derivation, can be driven by appropriate changes in their 

 surroundings. 



A second speculation, and one more relevant to the problems we have 

 been discussing, is that the Sakai-Dan cycle, the successor to the gluta- 

 thione cycle, may account for the assembly and stability of the mitotic 

 apparatus, during the division period, and its instability at the end of the 

 division period. Some years ago I proposed a mechanism of how this could 

 take place by the reduction of intramolecular disulphide, followed by 

 reoxidation to form intermolecular disulphide links [30, 30a]. For reasons 

 given above, I would no longer stress the importance of conventional S — S 

 bonds as such, but the principle may yet hold up in a more refined 

 version involving other intermolecular associations through thiol groups. 

 I will return to this point below. 



9. The mitotic apparatus and ATP : the 

 energetics of cell division 



Since cell division involves the movement of the chromosomes as well 

 as the formation of a rather elaborate structure, we can certainly assume 

 that it has its price in energy. Attempts to assess this price as an excess 

 oxygen consumption have led to the conclusion that it is probably not very 

 great, but in any case the payment does not seem to be made during the 

 visible phases of division but beforehand. An increased oxygen consump- 

 tion during division itself is not observed; indeed, Zeuthens' extensive 

 experiments (summarized by Zeuthen [44]) show a slight decline in 

 respiration during the division period. Similarly, inhibitors of respiration, 

 glycoloysis, or oxidative phosphorylation do not block division once 

 mitosis has begun, but can prevent it if imposed before a "point of no 

 return" just before the active phases of division. These findings have led 

 to the valuable hypothesis of an "energy reservoir" (Swann [31] and 

 earlier). In some kinds of cells, such an energy reservoir has not yet been 

 detected as a pool of a known high-energy compound. In one case, the 



