Carbohydrates in Aromatic Compound Biosynthesis 265 



A further consequence of these considerations is that attachment 

 between the two intermediates utilized in SA formation involves car- 

 bon 1 of the tetrose and carbon 3 of the triose. As will be pointed out 

 later this does not correspond to the known ways of forming heptoses. 



A complete series of experiments on the biosynthesis of tyrosine, 

 phenylalanine, the tryptophan from variously labeled glucose is un- 

 available for comparison with the present studies. However, several 

 relevant investigations are known. 



The incorporation of approximately 0.5 of an atom of G-l into 

 carbons 2, 6 of tyrosine and phenylalanine in yeast 5 agrees with the 

 presence of 0.65 G-l in S-2, 6. In agreement also are the incorpora- 

 tions, in E. coli, of 1.0 to 1.1 atoms of G-6 into carbons 2, 6 of both 

 tyrosine 13 and SA. The earlier results of Ehrensvard and collab- 

 orators 2 on tyrosine formation from acetate-1-C 14 (showing a high 

 incorporation of label into carbons 4 and 3/5 and essentially none 

 elsewhere in the ring) are in agreement too, if it is assumed that this 

 compound is incorporated via glucose-3,4-C 14 . 



In contrast, subsequent results of Ehrensvard et al. have indicated 

 that carbon 1 of acetate is extensively incorporated into at least three 

 atoms of the ring of tyrosine, 3,4 and into three consecutive atoms of 

 the benzene ring of tryptophan. 14 A similar incorporation of G-3, 4 

 into tryptophan has been observed by Rafelson. 15 These results appear 

 to be in conflict with the finding (Fig. 2) that G-3, 4 enters significantly 

 into only two atoms of the ring of SA. The tryptophan data have 

 led to the suggestion 15 - 16 that glucose-3,4-C 14 is converted to heptose- 

 3,4,5-C 14 , 8 ' 12 and that the intact carbon chain of the latter is cyclized 

 to SA-4,5,6-C 14 . However, utilization of the intact carbon chain of 

 such a heptose or of heptose formed by any known mechanism seems 

 excluded by the results presented above, as well as by enzymatic studies 

 to be discussed later. 



In order to test various intermediates of the glycolytic as well as 

 the pentose phosphate pathways as precursors of SA, a cell-free test 

 system was developed. In most cases the formation of 5-dehydro- 

 shikimate (DHS) , the precursor of SA, 17 rather than SA was studied, 



reactions) should yield the triose G-l, 2, 1 (or G-l > 6, 2 > 5, 1 > 6) and the 

 tetrose G-l, 3 = 4, 2 = 5, 1 = 6 (or G-l > 6, 3 = 4, 2 = 5, 1=6). From the 

 observed insignificant incorporation of these fragments into SA, it appears that 

 under these experimental conditions the contribution of reactions 3 to 5 to the 

 F-6-P pool is small. 



It has been assumed throughout that in triose phosphate G-6, 5, 4 = G-l, 2, 3. 

 This is a simplifying assumption, since S-2, 1, 7, which are derived from a three- 

 carbon intermediate of glycolysis, appear to show a small excess of G-6, 5, 4 

 over 1, 2, 3. 



