100 B. L. HORECKER, P. Z. SMYRNIOTIS VOL 12 (1953) 



6-Phosphogluconate + TPN -> ribulose-5-phosphate + CO2 + TPNH + H+ 

 6-Phosphoglucono-S-lactone + TPNH + H+ -^ gliicose-6-phosphate + TPN 

 Sum: 6-phosphogluconate +6-phosphoglucono-S-lactone^ribulose-5-phosphate + 

 CO 2 + glucose-6-phosphate 

 The formation of glucose-6-phosphate in this system is shown in Table II. No reduc- 

 tion to glucose-6-phosphate could be detected in the absence of the lactone or if the lactone 

 formed by heating in acid were added. Only catalytic amounts of TPN were required; 

 2 fjiM of glucose-6-phosphate were formed in the presence of 0.14 fiM of TPN. 



Similar results were obtained with isocitrate and isocitric dehydrogenase as the 

 TPN reducing system (Table III). In a control experiment with no ^socitrate added no 

 glucose-6-phosphate was formed; the small decrease in glucose-6-phosphate can be 

 attributed to oxidation by the TPN added. 



TABLE II 



THE REDUCTION OF 6-PHOSPHOGLUCONOLACTONE COUPLED WITH THE OXIDATION 



OF 6-PHOSPHOGLUCONATE 



In Experiment i, the reaction mixture contained 17.7 fiM of 6-phosphogluconate, 14.8 /lAI of 



which was present initially as the ^-lactone, 0.14 /nM of TPN, about 0.25 mg oi Zwischenferment and 



3.5 mg of phosphogluconic dehydrogenase in 0.07 M glycylglycine buffer, pH 7.4, in a total volume of 



I.I ml. Experiment 2 was similar except that 0.28 jliM of TPN and 14.5 fiM of 6-phosphoglucono-(5- 



lactone were present. Samples for glucose-6-phosphate assay were diluted with 4 volumes of water 



heated for 2 minutes at 100°, and centrifuged. In Experiment 3, the reaction mixture was similar 



except that it contained 11.7 /nM of 6-phosphogluconate, 3.8 /nM of which was present initially as the 



y-lactone. 



p. . . Time Glucose-6-phosphate 



" minutes micromoles 



o 3-51 



(i) With ^-lactone 10 5.54 



A +2.03 



o 2.07'' 



(2) With (5-lactone 10 4.35 



o 0.39 



(3) With y-lactone 10 0.41 



A -1-2. 2^ 



o 0.3c 



10 0.41 



A +0.02 



* Present initially in the phosphogluconolactone preparation. 



TABLE III 



THE REDUCTION OF 6-PHOSPHOGLUCONOLACTONE COUPLED WITH THE OXIDATION 



OF ^-ZSOCITRATE 



The reaction mixture contained 17.6 /<A/ of 6-phosphogluconate, 12.2 fiM of which was present 

 initially as the (^-lactone, 0.14 /iM of TPN, mg oi Zwischenfei'ment, 4 mg of Z50citric dehydrogenase, 

 5.0 [A,M of d-isocitra,t& and 2 ^M of MnClg in 0.03 M glycylglycine buffer, pH 7.4. The total volume 

 was 2.5 ml and the final pH was 6.9. 



Experiment 



Time Glucose-6-p)iosphate 



{min.) fiM 



Complete system 



/socitrate omitted 



References p. 102. 



