the early stages of mitosis. At the time of active spindle formation 

 (prometaphase, metaphase) the concentration of soluble — SH then in- 

 creased rapidly until it reached its initial level at about the time of 

 cytokinesis. These fluctuations in concentration of soluble — SH were 

 interpreted by Rapkine as due to changes in the levels of the oxidized and 

 reduced form of glutathione, and were thought to be accompanied by 

 corresponding changes in the amounts of fixed — SH brought about by a 

 reversible denaturation of proteins during spindle formation. More re- 

 cently, Nasatir and Stern (1959) have described variations in amounts of 

 soluble — SH during the mitotic cycle of lily microspores similar to those 

 observed by Rapkine. On the basis of Rapkine's view, the decrease in 



r^ 



(I) 



(11) 



+ 4GSH 



rVSH HS 



+ 

 L>SH HS\J 



+ 2GSSG 



SH HS-T^ 



\^^ 



+ 2GSSG 



r>S SY^ 



+ 4GSH 





Figure 6-5. Schematic Representation of One Possible Mechanism of 

 Polymerization of Protein of the Mitotic Apparatus Through Oxidation- 

 Reduction Cycles of Sulfhydryl Protein and Glutathione. Essentially, intra- 

 molecular — SS — is converted to intermolecular — S — . According to this 

 mechanism, oxidized glutathione should be found during the first part of the 

 cycle. (From Mazia, D., 1955. "The Organization of the Mitotic Apparatus," 

 Syinp. Soc. Exptl. Biol.. 9, Fig. 7, p. 349.) 



soluble — SH during the initial stages of mitosis should be accompanied 

 by a corresponding increase in the amount of fixed — SH of the spindle. 

 Likewise, the rise in soluble — SH occurring at metaphase should in- 

 volve a concomitant decrease in the amount of fixed — SH, presum- 

 ably as the result of spindle polymerization taking place at this stage. 

 According to Mazia (1955), the variations in protein-bound — SH 

 and soluble — SH actually do follow a course during cell division 

 similar to that proposed by Rapkine; that is, when the protein-bound 

 — SH is high, the soluble — SH is low, and vice versa. This finding has 

 led Mazia (1955) to propose that spindle formation involves an oxida- 

 tion-reduction cycle between glutathione and protein-bound — SH groups 

 of the spindle protein, and that formation of the gelated spindle is 

 essentially the result of conversion of intramolecular to intermolecular 

 — SS — linkages. In this scheme (Figure 6-5), a decrease in concentra- 



MECHANICS AND PHYSIOLOGY OF CELL DIVISION / 147 



