66 David Shemin and Jonathan Wittenberg 



glycine was included in these experiments to check this point. The 

 hsemin samples isolated in these experiments had the same ^^N con- 

 centrations, demonstrating the same rate of synthesis. The members 

 of the citric acid cycle merely dilute the acetate utilization. 



Bloch: If I understand you correctly, you assume a minimum of three 

 revolutions of the cycle in order to account for the data and for the 

 similarity of the methyl and the carbonyl atoms of pyruvate? 



Shemin: I am only assuming that it is a finite number. I wouldn't 

 hazard a guess how many cycles it went around. 



Block: On the other hand, if there is more than one revolution, 

 it would appear that the two carboxyl groups of succinate should become 

 equilibrated, whether you have an unsymmetrical intermediate or not. 

 Would you not then expect a larger incorporation of the carboxyl 

 carbon of acetate into one of the pyrrole rings than you actually find? 



Shemin: No, because the labelled carboxyl group of succinate would 

 never get into the porphyrin with forward revolutions of the cycle. 

 It would have to go backward for it to get in. I hope we shall be able 

 to demonstrate this by the time I get back. We shall have carboxyl 

 labelled succinate. Theoretically, if we find any labelling in the hsemin 

 it would show the reversibility of this step of the cycle. 



Neuberger: Mightn't you get a reduction of succinate to your 

 intermediate? 



Shemin: Yes. That implies a backward step. 



Neuberger: That would get it into the ring too. But it wouldn't 

 have to go back to ketoglutarate. 



Shemin: No. To the 4 carbon compound. 



Rimington: The position of uroporphyrin III is a very important 

 one in the whole scheme of biosynthesis, and it is worth while reporting 

 some information which will help to clear up some of the difficulties 

 which have surrounded this substance in the past. Authentic uropor- 

 phyrin III has not up to the present been isolated from natural sources. 

 The uroporphyrin present in congenital porphyria urine is the series I 

 type. In acute porphyria Waldenstrom and Mertens reported that they 

 had isolated uroporphyrin III, because on decarboxylation they got 

 coproporphyrin III; Watson and his co-workers were able to show, 

 however, that in samples they studied, decarboxylation gave rise to a 

 mixture of isomers. We have approached this problem from two 

 different lines. We have isolated quite unequivocal uroporphyrin III, 

 which we have been able to characterize, from turacin. I showed some 

 years ago that turacin on decarboxylation gave only pure coproporphyrin 

 III. We have gone into the question of the existence of uroporphyrin 

 III in pathological urines and I think the position may be summarized 

 fairly by saying that in some cases the excretion is almost entirely 

 uroporphyrin III, agreeing ^ith Waldenstrom and Mertens, but in 

 other cases there is undoubtedly a mixture of the two isomers. Finally, 

 we have been able to detect uroporphyrin in normal urine, working up 

 large quantities and examining it by paper chromatography. We are 

 now working on larger amounts in order to try and identify the isomeric 

 series. 



