IV BIOSYNTHESIS OF AMINO ACIDS 57 



isolated from the culture medium of E. coli and Neurospora mutants which require histidine. 

 The histidinol may be utilized for growth by a second histidineless E. coli mutant (Davis, 



1955a). 



2) An enzyme exists in cell free preparations of Neurospora which conv'erts imidazole- 

 glycerol phosphate to imidazoleacetol phosphate and a second enzyme which catalyzes 

 the transamination of the latter to histidinal phosphate (Ames and Mitchell, 1955). 

 Glutamic acid and pyridoxal phosphate are required for the transamination reaction 

 (Ames and Horecker, 1956). The phosphate esters are hydrolyzed by crude potato phos- 

 phatase. Since free imidazole compounds have been found in the mutant media, it would 

 seem that the hydrolysis of histidinal phosphate to histidinol is not improbable. It is to be 

 noted that the configuration of the terminal carbon atoms of D-erythroimidazoleglycerol 

 phosphate is similar to that of D-ribose phosphate. This suggests that ribose phosphate 

 might be a precursor of the former compound. 



3) Soluble enzymes obtained from E. coli, yeast or Arthobacter, catalyze the conversion 

 of histidinol to histidine. Two moles of DPN* are reduced per mole of histidinol oxidized 

 (Adams, 1955a, b). L-histidinal is active in the enzyme system which catalyzes the above 

 conversion. Histidinal is reduced to histidinol by DPNH. In those E. coli mutants which 

 are incapable of forming histidine from histidinol. histidinal is not active as a histidine 

 precursor. 



In an effort to throw light on the precursors of imidazoleglycerol phosphate, E. coli 

 mutants which accumulated histidinol were incubated with glucose- i-'-'C or glucose-G-'-iC. 

 The histidinol was isolated and degraded (Westley and Ceithaml, 1956). In each case, 

 the radioactivity was almost entirely confined to carbons 2, 6, and 8 of the histidinol 

 molecule. This pattern of labelling is consistent with a mechanism involving a pentose 

 compound formed by a condensation of glycolytic intermediates but not on the basis of a 

 pentose compound derived by way of the hexosemonophosphate shunt. The results obtained 

 in experiments with E. coli cells are however at variance with these found in other experi- 

 ments with yeast cells, in which the histidine of the protein was isolated and degraded. 

 In this case, approximately half of the label from glucose- i-'-'C was attributable to carbon 

 four of the histidine molecule while the remainder of the radioactivity was distributed 

 between carbons 5-8 and carbon two of the imidazole ring (Levy and Coon, 1954). 

 Non-labelled glutamic acid depressed the incorporation of labelled acetate but not labelled 

 glucose into histidine which constitutes further evidence that the intermediates in histidine 

 synthesis are not derived directly from tricarboxylic acid cycle metabolites. 



In lactic acid bacteria as in yeast, formate is a precursor of the C2 of the imidazole ring 

 of histidine (Mitoma and Snell, 1955). It is probable that glucose- i-i^C and glucose-G-'-iC 

 are precursors of C2 by virtue of the fact that they are potentially convertible to "active 

 formate" compounds. It is of interest that in certain bacteria, carbon two of guanine and 

 the amino group attached to carbon two are precursors, respectively, of carbon two and 

 nitrogen one of histidine. Aminoimidazolecarboxamide ribotide is a product of this reaction 

 (Mitoma and Snell, 1955; Magasanik, 1956: Magasanik et al., 1956). 



The transfer of the second carbon and the amino group of guanine may represent 

 an example of a formimino group ( — CH = NH) transfer reaction. FoHc acid deriva- 

 tives are apparently coenzymes in formimino group transfers, two examples of 

 which have been studied. Formimino glutamic acid (FGA), an intermediate in 



i) FGA + THFA - — . N'«-(formimino)-THFA + glutamic 



H2O 



N'O-formyl-THFA + NH3 



histidine catabolism, accumulates in the livers of folic acid deficient rats. Liver 

 enzymes from normal rats catalyze the transfer of the formimino group from 

 FGA to tetrahydrofolic acid. The same enzymes in the presence of THFA catalyze 



Literature p. 124 



