520 



11. LOCALIZATION OF THE SITE OF INHIBITION 



thesis. Arnon (1952) believed that insufficient time had been allowed for 

 the iodoacetamide to act and that the original data pointed to a progres- 

 sive decline in sucrose formation with time; even if this were true, it is 

 evident that the inhibitor blocks more readily some other enzyme initially. 

 The situation is complicated still more by the determination of carbon-14 

 distribution in tobacco leaves, infiltrated with 0.15 mM iodoacetamide 

 and illuminated for periods of 4 and 30 sec, reported by Newburgh and 

 Burris (1954). The fixation of C^^O.^ was again depressed strongly and the 

 distribution of the label was markedly altered (Table 11-1). The reduction 



Table 11-1 



Effect of Iodoacetamide (iaam) on the Pattern of Carbon-14 Incorporation 



IN Tobacco Leaves Exposed to 01*0, " 



" The time intervals are the durations of exposure to light. Iodoacetamide was 

 0.15 mil/. The specific activity is given as counts/minute/m/<moles of compound. The 

 "phosphate esters" consisted of several compounds of about equal activity including 

 fructose phosphate and 6-phosphogluconate. The total incorporation of C^Oj was 

 inhibited about 75°o by this concentration of iodoacetamide. 



in the specific activity of phosphoglycerate indicates either an inhibition 

 of the carboxylation reaction or of the supply of ribulose diphosphate for 

 carboxylation. The increased formation of glucose phosphate and the high 

 specific activity at 30 sec point to the reversal of the pentose phosphate 

 shunt pathway; the great rise in "phosphate esters" (which include fruc- 

 tose phosphate and 6-i3hosphogluconate) may support this but more de- 

 tailed analysis of this fraction would be necessary. These experiments in 

 any event have demonstrated multiple sites for the inhibition and have 

 made it unlikely that the effect on phosphoglyceraldehyde dehydrogenase 

 is of major importance. 



