Phenylalanine and Tyrosine Synthesis 225 



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carbon atom 1 arises by the C5-C1 mechanism that Dr. Bloch has been 

 discussing; we have httle information about that at the moment, except 

 that carbon dioxide and formic acid are not incorporated into the kojic 

 acid to any extent. 



Arnstein: I would hke to add one experiment which Dr. Bentley 

 did not mention, and that is that in these fermentation reactions we 

 have collected the CO 2; the specific radioactivity of the CO 2 during the 

 early part of the fermentation is considerably higher than that from 

 the later periods. That would indicate a preferential oxidation of the 

 1 -carbon of glucose and not an equal oxidation of all carbon atoms. 



Pochin: As most of the glucose in themedium is unlabelled, Dr. Block, 

 you presumably would get only a very small proportion of your recombined 

 trioses in the phenyl ring with two labellings, and does not your argu- 

 ment depend upon such double labelling? 



Rittenberg: I think Dr. Rloch's line of reasoning is perfectly correct, 

 because he does not insist that in his single phenylalanine two positions 

 he labelled, but merely that in the mixture he finds both positions 

 labelled. 



Dr. Bloch, as I remember your tables, the j8-position is much more 

 active than the 2,6-position, and offhand I should think they should 

 be the same. 



Bloch: This is correct, The ^-position has a specific activity very 

 close to the value which one would expect from a 3>-carbon unit formed 

 in glycolysis (1900 as compared to a calculated value of 2400). The 

 activity in the ring, when recalculated for undiluted material, is very 

 similar to this value, although it should be twice as high if the benzene 

 ring were formed directly from glucose. It should have one labelled 

 carbon atom with the same specific activity as that of glucose. The 

 count in the j3 carbon of the side chain indicated that the ^ carbon is 

 derived from a 3-carbon unit, which has half the isotope concentration 

 of Ci in glucose. This is in agreement with the glycolytic scheme. On 

 the other hand we cannot explain the finding that the activity of the 

 benzene ring is only 50-60 per cent of what it should be on the basis 

 of the cyclization scheme. 



Rittenberg: I would like to ask Dr. Krebs what his reaction is to 

 the manner in which his citric acid cycle is invoked to explain much of 

 current biochemical reactions. 



Krebs: The reactions which make up the tricarboxylic acid cycle 

 can certainly occur in yeast cells and in many types of biological 

 material, but I doubt whether in yeast cells they are the main mechanism 

 of the oxidation of acetate or carbohydrate. In cells like yeast which 

 grow rapidly there are two types of major reactions going on, those 

 which build up the cell material and those which supply energy. If we 

 find that reactions take place at a fairly fast rate it remains to be 

 investigated whether they are required for one or the other type of 

 reaction. Many facts are in favour of the assumption that the synthesis 

 of citric acid in yeast cells is a link in synthetic processes, being con- 

 cerned, for example, with the supply of a-ketoglutaric acid for the 

 synthesis of glutamic acid. One reason in favour of this view is the 



ISOTOPES 16 



