78 NITROGEN METABOLISM 



that the C in position 2 of the imidazole ring of histidine 

 is derived exclusively from formate [33, 44]. 



Isotopically labelled compounds such as amino-acids can 

 be readily prepared by isolating them from organisms which 

 have been grown in media containing substances enriched 

 with the appropriate isotope [2]. Non-exacting organisms 

 are especially useful since they utilize simple substances 

 (HC07,NHt, SOI") as sources of C, Nand S and such 

 substances enriched with the appropriate isotopes are 

 readily available. 



REFERENCES 



la. Abelson, P. H. (1954), jf. hiol. Chem., 206, 235 

 \h. Aldelberg, E. A. (1951), J. Bact., 61, 365 

 \c. Altenbern, R. A. and Housewright, R. D. (1953), J. hiol. 

 Chem., 204, 159 



2. Aquist, S. E. (195 1), Acta chem. Scand., 5, 103 1 



3. Bonner, D. (1946), Cold Spr. Harb. Symp., 11, 14 



4. Braunstein, A. E. and Kritzmann, M. G. (1937), Enzymologia, 



2, 129 



5. Catcheside, D. G. (195 1), The Genetics of Micro-organisms 



(Pitman, London) 



6. Cutinelli, C, Ehrensvard, G., Reio, L., Saluste, E. and 



Stjernholm, R. (195 1), Acta chem. Scand., 5, 353 



7. Davis, B. D. (1950), Experientia, 6, 41 



8. (1952), Intermediates in the Biosynthesis of Proline, etc., 



in Biological Systems, 2nd Internat. Congress of Biocheni. 



9. (1952), Nature, 169, 534 



10. (1952), J. Bact., 64, 729, 749 



11. Delbruck, M. (1946), Cold Spr. Harb. Symp., 11, 22 



12. Ehrensvard, G. (1952), On the Origin of Aromatic Structures 



in Biological Systems, 2nd Internat. Congress of Biochem. 



13. Reio, L., Saluste, E. and Stjernholm, R. (1951), J. hiol. 



Chem., 189, 93 



14. Feldman, L. I. and Gunsalus, I. C. (1950),^. biol. Chem., 187, 



821 



15. Fildes, P. (1945), Brit. J. exp. Path., 26, 416 



16. and Rydon, H. N. (1947), Brit. J. exp. Path., 28, 211 



i7<3. Fincham, J. R. S. (1950), J. biol. Chem., 182, 61 



176. (1953), Biochem. J., 53, 313 



18. Fries, N. (1947), Nature, 159, 199 



19. Gladstone, G. P. (1939), Brit. J. exp. Path., 20, 189 



20. Good, N., Heilbronner, R. and Mitchell, H. K. (1950), Arch. 



Biochem., 28, 464 



21. Gordon, M., Haskins, F. A. and Mitchell, H. K. (1950), Proc. 



Nat. Acad. Set., Wash., 36, 427 



