58 David Shemin and Jonathan Wittenberg 



at first suggest that the tricarboxyhc acid cycle is not func- 

 tioning as postulated theoretically above. It was found, 

 however, that carbon atoms numbered 5 and 3 are also in 

 part derived from the carboxyl group of acetate (Fig. 4). 

 Correcting for this dilution, since in the methyl labelled ace- 

 tate experiment the carboxyl group of the acetate contains 

 no ^*C, the average activities of these two pairs of adjacent 

 carbon atoms are equal. The contribution of the carboxyl 

 groups of acetate to positions 5 and 3 is 100 counts on the 

 average. The addition of these 100 counts to these carbon 

 atoms having an average activity of 788 cpm gives a total 

 of 888 cpm, a figure close to the 877 cpm found for the average 

 activity of carbon atoms 4 and 8 in the product from methyl 

 labelled acetate. 



It would appear therefore that acetate is utilized through 

 the tricarboxvlic acid cvcle, and the relative activities found 

 fit those theoretically predicted on the basis of the distribution 

 of activities in a-ketoglutarate. This also supports the 

 previous conclusion that the same carbon compound is utilized 

 for the methyl side of the pyrrole as for the vinyl and propionic 

 acid sides. On the propionic acid side a compound containing 

 not less than four carbon atoms must have been utilized, since 

 carbon atoms C2 and D2 arise from the a-carbon atom of 

 glycine. If originally a three carbon compound (e.g. pyruvic 

 acid), rather than a four carbon compound were utilized for 

 the methyl side or vinyl side, the methyl group numbered 

 6 and carbon atom 4 of the pyrrole to which it is attached 

 would have had equal activities, whereas carbon atom 5 of 

 the pyrrole w^ould have been much less active than carbon 

 atom 4. The same relationship would also exist for carbon 

 atoms 3, 8 and 9. The distribution of ^^C in pyruvic acid 

 from methyl labelled acetate can be gathered from Fig. 5 and 

 Table V and has been demonstrated by Wood (1948). The 

 non-utilization of pyruvic acid can also be concluded from 

 the data of the experiment with carboxyl labelled acetate. 

 If pyruvic acid were utilized, carbon atoms 5 and 3 would 

 have been similar in activity to carbon atoms numbered 10. 



