EFFECTS ON MITOSIS, GROWTH, DIFFERENTIATION 259 



3-PGDH of marine eggs is much less sensitive than the mammalian enzyme 

 to iodoacetate. Concentrations above 1 mM, and often 10 mM or above, 

 are required for marked inhibition of respiration and glycolysis. 



It is quite clear that fertilization of marine invertebrate eggs is not af- 

 fected by iodoacetate, even at concentrations of 20-30 mM or when the 

 pH of the sea water is reduced to 6, as was shown for Urechis caupo (Tyler 

 and Schultz, 1932), Arbacia pimctulata (Runnstrom, 1935), and Echinus 

 esculentus (Kriszat and Runnstrom, 1952); it was concluded that glycolysis 

 is not necessary for fertilization. Indeed, iodoacetate and iodoacetamide at 

 2 mM promote elevation of the fertilization membrane, due, it was thought, 

 to reaction with SH groups on the plasma membrane with disruption of 

 hydrogen bonds (Isaka and Aikawa, 1963). Early cleavage is also relatively 

 resistant to iodoacetate. When Urechis eggs are fertilized and then placed 

 in iodoacetate-containing sea water, there is no effect on the cleavage to 

 the 8-cell stage (Ellis, 1933). However, if the pH of the sea water is lowered 

 to 6, some inhibition is observed: cleavage is depressed 50% and 90% by 

 1.7 mM and 3.4 mM iodoacetate, respectively, in Arbacia eggs (Clowes 

 and Krahl, 1940) and 30 mM causes separation of the cells, due to some 

 action on their surface membranes (Runnstrom, 1935). We see again a 

 stimulation of division at low concentrations (0.2-0.4 mM) of iodoacetate 

 (Krahl and Clowes, 1940). The egg of the marine snail Ilyanassa obsoleta 

 does not cleave readily in 5.4 mM iodoacetate at pH 6.4, the protoplasmic 

 viscosity is doubled, and the egg surface loses its smoothness and becomes 

 pointed and wrinkled (Butros, 1956). Iodoacetamide may penetrate much 

 better than iodoacetate, since 0.1 mM inhibits cleavage of Echinvs eggs 

 and causes structural changes in the cortex (Kriszat and Runnstrom, 1952). 

 There is also a decreased protoplasmic viscosity, as determined by cen- 

 trifugal displacement of mitochondria, and a shortening of the spindle. The 

 most sensitive period is immediately after fertilization, and this was said 

 to correspond to a peak in the glutathione level. Ellis (1933), however, 

 found that whereas iodoacetate at 10 mM reduces the glutathione content 

 of Urechis eggs 60%, there is no significant effect on early cleavage, so that 

 he doubted the SH theories of cleavage. Of course glutathione levels are 

 not very reliable indicators of other SH-containing cell components. Bra- 

 chet (1938) reported that 17-33 niM iodoacetate does not alter early cleav- 

 age or development, but arrests frog development at the gastrula stage; 

 we have observed similar behavior in the development of Strongylocentrotus 

 purpuratus. 



Later developmental abnormalities induced by iodoacetate have not been 

 extensively studied. Runnstrom (1935) exposed Arbacia eggs to 30 mM 

 iodoacetate for 2-6 hr, and they were then fertilized and returned to normal 

 sea water. These eggs developed into larvae with morphological defects: 

 poor development of stomodeum and oral arms is the commonest abnor- 



