108 1. lODOACETATE AND lODOACETAMIDE 



Actually it is more accurate to think of lactate simply in equilibrium with 

 one intermediate in the aerobic oxidation of glucose, namely, pyruvate, just 

 as dihydroxyacetone-P is in equilibrium with 3-phosphoglyceraldehyde. The 

 concentration of lactate in a tissue will fluctuate with the pyruvate concen- 

 tration and an inhibitor affecting the formation or disposal of pyruvate will 

 modify its level. In addition, the NADH/NAD ratio is important in this 

 equilibrium. One would expect that the steady-state level of lactate would 

 be reached quite rapidly in a tissue and remain there as long as the rate of 

 glucose oxidation is constant. Measurements of aerobic glycolysis really in- 

 volve determinations of steady-state lactate concentrations in most cases. 

 An increase over a period in the lactate level is often interpreted as aerobic 

 glycolysis, whereas such increase is probably due to changes in the tissue 

 leading to impaired cycle operation. Looked at in this way, the effects of 

 iodoacetate fall into the category of pseudosteady-state modifications in a 

 multienzyme system. 



The effect of iodoacetate on tissue lactate in vivo, such as Lundsgaard's 

 early observations with muscle, is basically a matter of aerobic glycolysis. 

 No study has been made of the comparative effects of iodoacetate on dif- 

 ferent tissues after injection into animals, but reduction of lactate occurs in 

 the brain and kidney of dogs given iodoacetate, which is allowed to incubate 

 for 10 min (Haldi, 1932). In kidney, lactate formation is essentially abol- 

 ished; in the brain, the results are erratic but somewhat over 50% depression 

 seems to be the rule. There is thus evidence that iodoacetate can produce 

 changes in intact tissue metabolism similar to those seen in slices or extracts. 



The effects of iodoacetate on lactate formation and respiration of brain 

 slices are very interesting and present some unexplainable phenomena 

 (Heald, 1953). The results are summarized in Fig. 1-10. Three things are 

 immediately evident: (1) aerobic glycolysis is inhibited more than respira- 

 tion, (2) electrically stimulated slices are inhibited more than resting slices, 

 and (3) the increase in inhibition with iodoacetate concentration is complex. 

 The first point will be discussed later (page 122). Stimulation increases 

 aerobic glycolysis some 50% and iodoacetate reduces this extra lactate for- 

 mation. Stimulation apparently increases the contribution of glucose to O2 

 uptake, so the greater inhibition of stimulated slices is not surprising. The 

 shapes of the curves are not so easy to explain. It would appear offhand 

 that more than one reaction is inhibited, the first being completely inhibited 

 by 0.01 mM iodoacetate and the second being inhibited only above 0.04 

 mM. These might be 3-PGDH and the hexokinase reaction. However, in 

 such a complex multienzyme system one must beware of facile interpreta- 

 tions. Another explanation might be the following. At the high glycolytic 

 rate of stimulated tissue the oxidation of pyruvate is saturated, i.e., the 

 pyruvate concentration is high enough so that its reduction by partial inhi- 

 bition of the EM pathway does not reduce its oxidation, which is substan- 



