672 



Martin Gibbs 



inhibit muscle and yeast glyceraldehyde-3-P dehydrogenase. Park 

 et al. ^ ^ observed that threose-2,U-diP inhibited COp fixation 

 by sonically ruptured spinach chloroplasts and assigned the site 

 of inhibition/agxthe TPN-linked triose-P dehydrogenase. Gibbs 

 and Bamberger^ ' have repeated these experiments using the 

 intact spinach chloroplasts. Threose-2,4-diP (10"^ M) did not 

 affect C0_ assimilation. The diphosphate may not penetrate the 

 chloroplast but evidence obtained in this laboratory shows that 

 other diphosphates, sedoheptulose and fructose, can affect COp 

 fixation. Indeed, when threose-2,4-diP (l.l x 10' M) was in- 

 cubated with the rabbit muscle, DPN- spinach or TPN- spinach 

 triose-P dehydrogenase, the mammalian and the DPN- linked plant 

 enzymes were inhibited completely while the TPN-enzyme was 

 unaffected. These data indicate that the DPN-dehydrogenase is 

 not a rate-limiting step in the chloroplast carbon cycle. 

 Threose-2,4-diP would appear to be of little value in elucidating 

 the enzymes involved in the carbon reduction cycle in the intact 

 chloroplast . 



The inhibition of the enzyme is apparently due to the simi- 

 larity between threose-2,4-diP and 1,3-diPGA molecules. The 

 data obtained here suggest that the mechanism of the TPN-enzyme 

 may differ from that of its nucleotide counterpart. Perhaps, 

 1,3-diPGA is not the real substrate for this enzyme and there- 

 fore, inhibition does not occur. 



An unexpected finding was the inhibition by sedcheptulose-T-P 

 of COp fixation by the intact chloroplast. In contrast to 

 threose-2,i+-diP, sedoheptulose-T-P inhibited the rabbit muscle 

 enzyme as well as both triose-P dehydrogenases from spinach 

 chloroplasts (Table 6) . 



Table 6. EFFECT OF D- SEDOHEPTULOSE-T-P ON GLyCERALDEHYDE-3-P 

 DEHYDROGENASE FROM RABBIT IWSCLE AND SPINACH CHLOROPLAST 



Enzyme source 



muscle- DPN 

 muscle- DPN 

 muscle- DPN 

 chloroplast- DPN 

 chloroplast- DPN 

 chloroplast-TPN 

 chloroplast-TPN 



S-7-P 



-i+ 

 5x10 ^M 



IxlO'^M 



2xl0"^M 



5xlO" M 



IxlO'j^M 



5xlO"^M 



lxlO"^M 



Back reaction Forward reaction 

 ^ inhibition 



14 

 37 

 53 

 33 

 66 

 10 

 50 



28 



33 

 40 



