1764 



Bassham, Benson, Kay, Harris, Wilson and Calvin 



Vol. 76 



the easily purified guloheptulose was used for subsequent 

 degradations witli cerate ion, despite its much poorer yield. 



Oxidation of Sedoheptulosan. — The radioactive sarnple 

 and carrier were treated with sodium periodate as described 

 by Pratt, Richtmyer and Hudson" and allowed to stand at 

 room temperature for 3-4 days to give time for most of the 

 formate to be released from the intermediate ester. Then 

 the mixture was acidified with iodic acid and the formic 

 acid was distilled in vacuo. This was then counted as 

 barium formate. 



Results 



In Fig. 2, the radiocarbon fixed in a "steady 

 state" photosynthesis with Scenedesmus is shown as 

 a function of time of exposure of the plant to C'^Oj. 



glucose monophosphate and fructose monophos- 

 phate ciu-ves although individual points are more 

 erratic, probably due to the relative instability of 

 the ribulose diphosphate.' The appearance of 

 compounds other than PGA with a finite rate of 

 labeling at the shortest times is demonstrated in 

 Fig. 4 in which the percentage distributions of 

 PGA and of the total sugar phosphates are shown. 



TIME (SECONDS}. 



Scent 



e e 10 



TIME (SECONDS), 



Fig. 2. — Radioactivity incorporated in "steady state" photo- 

 synthesis with Scenedesmus. 



The rate of incorporation of C'*Os appears to be 

 reasonably constant over the period of the experi- 

 ment. The distribution of radioactivity among 

 various labeled compounds is shown in Fig. 3. The 



a 10 12 



TIME (SECONDS) , 



Fig. 3. — Distribution of radioactivity among compounds formed during "steady 

 state" photosynthesis with Scenedesmus. 



curve for the sugar diphosphates, principally ribu- 

 lose diphosphate, is not shown but lies between the 



(17) J W. Pratt, N. K. Richtmyer aod C. S. Hudson, Tais Joumnal, 

 T4, 2200 (ieS2). 



Fig. 4. — Distribution of activity in "steady state" 

 desmus. 



The extrapolations of the PGA and sugar phos- 

 phates to zero time would give about 75 and 17%, 

 respectively. The remaining 8% not shown is dis- 

 tributed among malic acid (3%), free glyceric acid 

 (2%) and phosphoenolpyruvic acid (3%).' The 

 percentage distribution among the sugar phos- 

 phates is shown in Fig. 5 where it is seen that no 

 single labeled sugar phosphate predominates at 

 the shortest times. 



These data alone do not permit 

 assignment of an order of preced- 

 ence of the various labele(l com- 

 pounds in the path of carbon reduc- 

 tion. In order to make such an 

 assignment it would be necessary to 

 measure the relative rates of in- 

 crease in specific activity of the 

 various compounds. If the slopes 

 of the ciu^es shown in Fig. 3 are 

 measured between 2 and 10 sec, 

 rates of increase in total radioactiv- 

 ity are obtained. If these rates are 

 divided by the cellular concentra- 

 tion of the compounds involved, 

 rates of specific activity increase are 

 obtained. This has been done using 

 measurements of concentrations 

 made by two independent'" meth- 

 ods which agreed fairly well in rela- 

 tive Older {i.e., PGA concentration: 

 GMP concentration = 4:1). The 

 resulting values ranged from 0.3 for 

 GMP to 1.0 for PGA, with FMP, 

 DHAP, RDP and SMP falling be- 

 tween these values when the rates 

 for these compounds were divided by 2, 1, 2, 1, 1 

 and 3, respectively, to allow for the number of 

 carbon atoms which degradation data reported be- 



(18) A. A. BeiuoD. Z. BUUrochtm. U, 848 (19&2). 



96 



