582 12. RATES OF INHIBITION 



ably missed by determining the inhibition over a single arbitrary interval; 

 if here the inhibition had been observed only at 30 min, the results would 

 have been unimportant and indicative of nothing fundamental. Secondly, 

 one is struck by the different types of curves obtained for the three heavy 

 metals — the immediate action of HgCla, the definite latent period of Cu"*""*", 

 and the initial stimulation produced by Ag"^ — and these differences alone 

 would lead us to expect the kinetics to be complex. The biphasic action of 

 Ag"*" could be explained on the basis of two kinds of effects on the respi- 

 ration occurring at different rates, but the mechanism by which the stim- 

 ulation is produced is unknown. The immediate action of HgClg is not 

 unreasonable since a large fraction of the HgClg would be un-ionized and 

 would penetrate readily and then the latent period observed with Cu"*""*" 

 might be inferred as due to a delayed penetration into the cells. However, 

 Cook (1926 b) subsequently found that the explanation of the latent period 

 is not quite as simple as this. In the first place, Nitella and Valonia also 

 demonstrated a definite latent period and here the penetration of Cu"'""'" 

 could be measured; it was found that Cu"*"^ enters the cells quite rapidly 

 and this by itself could not be responsible for the lag in inhibition. In the 

 second place, experiments with Aspergillus, wherein the Cu"^+ was washed 

 out at varying times after exposure, indicated that some initial reversible 

 reaction occurred preliminary to the respiratory depression. Cook assumed 

 that there are two consecutive reactions, the first "activating" the Cu"*""^ 

 so that it was then able to block respiration. It was furthermore found that 

 the latent period (LP) was not constant but varied inversely with the con- 

 centration of Cu"*""*" according to an equation of the type LP = a(Cu"'""'")~^ 

 where a and b are constants. Thus the latent period is not merely a "bio- 

 logical lag" requiring certain events to occur before respiration is inhi- 

 bited. This problem cannot be solved today because the basic mechanism 

 by which Cu"*""^ inhibits respiration is unknown. 



(C) Effects of iodoacetate on respiration and glycolysis. The inhibition of 

 enzymes by iodoacetate has been known to be a relatively slow process 

 since the earliest work on this inhibitor (Lohmann, 1931). Thus at the 

 commonly used low concentration of iodoacetate (0.1 mM), depression of 

 lactate formation is muscle extracts progressively increased over 1 to 2 

 hr, although high concentrations (above 1 mM) inhibited fairly rapidly. 

 It is now known that the principal reaction of iodoacetate with these en- 

 zymes is an alkylation of sulfhydryl groups and that certain of these sulf- 

 hydryl groups react more rapidly than others. Inhibition of glycolysis in 

 intact muscle is also slow and the question arose whether this delay was 

 due in part to the penetration of iodoacetate into the tissue. Meyerhof and 

 Boyland (1931) determined the uptake of iodoacetate in frog sartorius 

 muscle simultaneously with measurements of glycolytic inhibition (Fig. 12- 

 26). Uptake was almost complete between 20 and 30 min but glycolysis 



