74 1. lODOACETATE AND lODOACETAMIDE 



of the triose-P's is given by (dihydroxyacetone-P)/(3-phosphoglyceralde- 

 hyde) = 22, so that most of the triose-P that did accumulate would be in 

 the form of dihydroxyacetone-P. Also the aldolase reaction is reversible 

 with the equilibrium favoring fructose-l,6-diP, according to (fructose- 1, 6- 

 diP)/(3-phosphoglyceraldehyde)2 = 700. Although the phosphofructokinase 

 reaction is not reversible, fructose-6-P will be reformed, if there is fructose- 

 1,6-diphosphatase activity, and hence glucose-6-P, since (glucose-6-P)/(fruc- 

 tose-6-P) = 2.3. One would predict a block of 3-PGDH to lead to the ac- 

 cumulation to varying degrees of several phosphorylated compounds, fruc- 

 tose-l,6-diP probably being dominant in most instances. 



Inhibition of 3-PGDH should bring about changes in the NAD : NADH 

 ratio. The addition of glucose-6-P or fructose-l,6-diP to extracts of Sac- 

 charomyces carlsbergensis causes a sinusoidal variation in NADH level for 

 several minutes until a steady state is reached (Chance et al., 1964). These 

 oscillations are prevented by 3 mM iodoacetate, and this is attributed to 

 an inhibition of 3-PGDH with consequent oxidation of NADH to NAD by 

 the accumulated acetaldehyde. This also provides evidence that the fluc- 

 tuations in the NAD : NADH ratio are mediated through 3-PGDH. 



Another factor to be considered in accumulation of hexose phosphates is 

 the supply of ATP and other high-cuergy phosphate. If 3-PGDH is com- 

 pletely inhibited, phosphorylation will proceed until ATP is depleted to a 

 low level, so that the total amount of hexose phosphates accumulated will 

 depend on the phosphorylation potential of the particular tissue or prepa- 

 ration. When glycogen can serve as a substrate, hexose monophosphates can 

 form readily if the supply of inorganic phosphate is adequate and phosphor- 

 ylase is active. Possibly the patterns of accumulation during iodoacetate 

 block will be somewhat different when glucose or glycogen is the initial 

 substrate for the EM pathway. 



In his initial work with iodoacetate, Lundsgaard (1930 a) observed the 

 increase in "lactacidogen," which he interpreted as hexose phosphates, in 

 iodoacetate-poisoned muscle, this increase corresponding to a decrease in 

 "phosphagen," which we now know to be ATP and creatine-P. Lohmann 

 (1931) soon showed that hexose mono- and diphosphates do indeed accumu- 

 late in the muscles of frogs given iodoacetate, and Meyerhof and Kiesling 

 (1933) succeeded in isolating a small amount of triose-P from muscle ex- 

 tracts incubated with iodoacetate. The situation in glycogenolysis is well 

 seen in the work of Ostern et al. (1939), who found glycogen to increase the 

 esterification of inorganic phosphate in the presence of iodoacetate. The 

 first quantitative analyses of accumulation were reported by Geiger (1940) 

 in rat brain extracts (see accompanying tabulation). Fructose-l,6-diP in- 

 creased almost 10-fold and 3-phosphoglyceraldehyde 4-fold during 30-min 

 incubation with 0.2 mill iodoacetate. The gain in inorganic phosphate indi- 

 cates the activity of phosphatases. It is evident that the disappearance of 



