532 5. QUINONES 



role of the exposure duration. With any concentration used, 50-min ex- 

 posure is required if 100% mitotic inhibition is to be achieved. At a con- 

 centration of 0.00033 mM, the inhibition increases from 56% to 100% as 

 the exposure time is increased from 30 to 60 min. Even concentrations as 

 low as 0.0000033 mM definitely inhibit if allowed to act for 60 min. If the 

 cells are exposed to 0.00055 mM menadiol-diP for 30 min and then washed, 

 the antimitotic action is evident later, indicating that the inhibitor is either 

 bound tightly or produces some irreversible modification. 



It may also be noted that Meier and Allgower (1945) claimed that p-ben- 

 zoquinone is a typical premitotic inhibitor, in contrast to colchicine which 

 interferes with cleavage, and that at low concentrations near 0.000009 mM 

 the antimitotic action is selective, no toxic effects on fibroblasts being 

 observed (Meier and Schar, 1947). Although fibroblasts are very sensitive 

 to 2?-benzoquinone, lymphocyte cultures from rat lymph nodes are not, 

 50% inhibition being seen at 0.19 mM (Trowell, 1960). A general review 

 of the effects of quinones on mitosis has been given by Biesele (1958). 



We shall now turn our attention to the suggested mechanisms by which 

 the quinones inhibit mitosis, and especially to the evidence for and against 

 the participation of SH groups in the critical reaction, a theory which domi- 

 nated the early work of the Cambridge group. The major difficulty in accep- 

 ting this is that the antimitotic activities do not parallel the reactivities 

 with SH groups. One may argue that penetration or other factors are in- 

 volved, but the fair degree of activity of the 2,3-disubstituted 1,4-naphtho- 

 quinones (such as S-methyl-MDHg-diP and phthiocol), which do not react 

 with SH groups, cannot be explained on this basis. Also, if the phosphates 

 are directly active, and we have seen there is some evidence for this, the 

 SH reaction theory would not be applicable. The greater potencies of the 

 unsubstituted benzo- and naphthohydroquinones might be attributed to 

 a reaction with SH groups in addition to whatever mechanisms are involved 

 in the actions of the substituted compounds. On the other hand, some sub- 

 stances which react rather readily with thiols, such as 2-bromo-l,4-naphtho- 

 quinone or menadione, are not particularly potent antimitotic agents. 



Another approach to determining the role of the thiols in the actions 

 of the quinones is the examination of various conjugates of the quinones 

 with the thiols. Some of these conjugates have been shown to be quite 

 potently antimitotic and the results are very puzzling. The compound 

 formed by the addition of glutathione to menadione, for example, is more 

 active than menadione (Mitchell and Simon-Reuss, 1952 b). On the other 

 hand, the addition of mercaptoacetate to menadione yields an inactive 

 product, and the addition of both glutathione and mercaptoacetate to 

 1,4-naphthoquinone also gives inactive compounds (i. e., no mitotic in- 

 hibition or phase shifts at concentrations up to 0.006 mM). This work was 

 extended by Friedmann and Simon-Reuss (1956 a) and the results are 



