492 DANIEL MAZIA 



splitting of ATP. On the other side, such a reaction can be only an exiguous 

 part of a complex biochemical picture, and it is not obvious where to 

 turn next. 



10. A speculation on the structure of the mitotic apparatus 

 and on cellular structure 



The instability of the mitotic apparatus is remarkable, the more so 

 when we consider that its job is to move massive chromosomes over long 

 distances. As further evidence of chemical instability, I may cite the 

 experience of those who have attempted to fix it for electron microscopy 

 with osmium tetroxide or other conventional fixatives. While beautiful 

 results have been obtained in some cases, there are others in which fixation 

 is capricious and still others where it seems to be impossible to preserve 

 fine structure. It is suggested that the mitotic apparatus, especially in 

 larger cells, will not always " stand still " long enough following damage to 

 the cell to be properly fixed before disintegrating. 



As we have seen, the idea that the mitotic apparatus was bonded 

 through protein-sulphur has had a certain predictive success, but the theory 

 that it was a simply vulcanized system, bonded through conventional and 

 stable S — S links, has not been substantiated. In the development of the 

 dithiodiglycol procedure to stabilize the apparatus for isolation, w^e turned 

 to a more dynamic conception of sulphur bonds. This was founded on the 

 growing body of evidence (reviewed by Jensen [41]) that (SH)-(S — S) 

 interchange existed and was perhaps a common phenomenon. The mitotic 

 apparatus was viewed as a massive aggregate in which there were numerous 

 pairs of S atoms located close to each other, and in which S-to-S linkages 

 were opening and closing all the time. As a statistical disulphide structure, 

 its stability would depend on the probability of the existence of a sufficient 

 number of S-to-S linkages at a given time, and it was imagined that this 

 probability could be "poised" at a given level by introducing, in the total 

 system, and S — S compound such as dithiodiglycol. This would influence 

 the level of S-to-S linkage in the protein, acting not quite as a conventional 

 stoicheiometric oxidant but as a kind of "buff^er" determining the trend 

 of electron flow to and from the protein-SH. Such a view may be out- 

 rageously naive, but it was in fact the basis for the isolation of the mitotic 

 apparatus with dithiodiglycol. 



This is not the place to review the body of evidence concerning 

 disulphide interchanges. They can take place in systems containing SH 

 and S- — ^S [46, 41] and can take place between two S — S compounds under 

 the action of ionizing radiation [47]. The point is that a structural system 

 based on S-to-S interactions between proteins can be viewed as a dynamic, 

 sensitive, and unstable one given the right conditions. One imagines that 



