208 1. lODOACETATE AND lODOACETAMIDE 



farlane and Meares, 1958; Miischoll, 1959). It is doubtful if any of these 

 changes relates to rigor, but they may well play a role in the modifications 

 of twitch pattern. These effects on membrane electrical behavior are of 

 course due to alterations in ion transport and flux rates (see page 174), 

 but the only ion which may be directly involved in rigor is Ca++. Kutscha 

 (1961) has pointed out that rigor in frog muscle is dependent on the exter- 

 nal Ca++ concentration, whatever the agent used to induce the rigor. lodo- 

 acetate at 0.2 mM causes contracture when the Ca++ is 15 mM but does 

 not when Ca++ is absent. Furthermore, the contracture is relaxed somewhat 

 if Ca++ is removed from the medium. Since Ca++ is necessary for normal 

 muscle contraction, the relationship here may not be one particularly perti- 

 nent to rigor, but it is very interesting and should be pursued, especially 

 since a dependence on Ca+^" might provide evidence that rigor is related 

 to contraction. Incubation of frog muscle with 1 mM iodoacetamide for 1 hr 

 has no direct effect on Ca^^ influx or efflux, but the efflux suddenly increases 

 when rigor begins to develop (Bianchi, 1963). These results are contrary to 

 what one might expect from other work, particularly on the heart. 



(I) Direct effects of iodoacetate on contractile systems. Inasmuch as iodo- 

 acetate reacts with SH groups, and since actomyosin contains SH groups 

 believed to be important in contraction, it is necessary to inquire whether 

 rigor may to any degree be explained by such a nonglycolytic site of action. 

 Bailey and Perry (1947) have emphasized that both the interaction of 

 myosin with actin and the ATPase activity are dependent on SH groups; 

 they also point out that iodoacetamide reacts very slowly with such SH 

 groups. The sluggish reaction of myosin SH groups with both iodoacetate 

 and iodoacetamide has been noted by Needham (1942), Engelhardt (1946). 

 Polls and Meyerhof (1947), and Barany and Barany (1959 a). A decrease 

 in the extractable or soluble fraction of muscle proteins brought about by 

 injections of bromoacetate was observed by Embden and Metz (1930), but 

 this need not result from a direct reaction of bromoacetate with the pro- 

 teins, since it could also presumably derive from a depletion of ATP and 

 the formation of protein complexes. Jacob (1947) found changes in the 

 myosin fractions of rabbit muscle during rigor, whereas other soluble muscle 

 proteins are unaffected, but again the mechanism is unclear. The viscosity 

 and Ca^^ binding of G-actin and F-actin are not significantly changed by 

 iodoacetamide at 2 mM (Barany et al., 1962). Results with glycerol-extrac- 

 ted muscle fibers are variable. Hasselbach (1953) claimed there to be no 

 inhibition of the contraction upon adding ATP, whereas relaxation is in- 

 hibited; Watanabe and Sleator (1958) found no effect on relaxation but an 

 acceleration of the contraction; and Procita (1960) observed some inhibition 

 of the contraction. In any event, fibers contracted with iodoacetate are not 

 relaxable with the usual agents. It is very difficult to decide whether all of 

 these data taken together imply some direct action on the contractile ele- 



