206 



1. lODOACETATE AND lODOACETAMIDE 



more inhibited as the concentrations of its substrates fall, i.e., that in ad- 

 vanced rigor the enzyme could be inactivated. At the present time, how- 

 ever, there is no evidence that this enzyme is primarily involved in rigor. 



(H) Muscle membrane potentials and rigor. Depolarization of muscle cell 

 membranes can under certain circumstances produce a reversible contrac- 

 ture, so that it is necessary to inquire if iodoacetate rigor is in any way 

 dependent on depolarization. Iodoacetate unquestionably lowers the rest- 

 ing membrane potential, but unless this lowering is quite marked it could 

 not be construed as the primary causative factor in rigor. Jahn (1935) found 

 that frog muscles in iodoacetate rigor show some depolarization, and likened 

 this rigor to that produced by depolarizers such as acetylcholine, but it 

 is difficult to determine how much reduction in membrane potential occur- 

 red. Liu et al. (1948) treated half of a toad sartorius muscle with iodoacetate 



WASHED (K*-FREE MEDIUM) 



100 



Fig. 1-19. Effects of 0.27 mM iodoacetate on the toad sar- 

 torius muscle membrane potential (measured between nor- 

 mal and poisoned halves of the muscle). Potential changes 

 given in absolute figures; i.e., negative values indicate de- 

 polarization. (From Liu et al., 1948.) 



and determined the potential between this half and the normal half: sus- 

 pended in air, there was evidence of depolarization, but in Ringer medium 

 this did not occur, although rigor developed. In K+-free medium, iodoace- 

 tate produced rigor while the potential was actually somewhat greater than 

 normal (Fig. 1-19). Small to moderate degrees of depolarization have been 



