IV. BIOCHEMICAL SYSTEMS 21 



cystine was not an indispensable amino acid as had been believed since the 

 evidence of its supplementary potency presented by Osborne and Mendel 

 over twenty years earlier.^* The experiments in Rose's laboratory showed 

 the essential character of the methionine requirement" and that this amino 

 acid was a precursor of cystine in vivo.^^ It was evident also that cystine 

 could spare the methionine requirement in so far as cystine was needed 

 by the animal.^'' 



The demonstration of gro^vth-stimulating activity of homocystine in rats 

 on a methionine-poor diet was difficult to understand, inasmuch as cystine 

 was ineffective.^^ The explanation of the methionine-like action of homo- 

 cystine was soon found to depend upon the presence of choline in the diet. 

 Homocystine replaced methionine as a growth factor in young rats if the 

 water-soluble vitamins were pro\dded in the form of concentrates of milk 

 and rice bran but not if purified vitamins were used.^- In the latter instance 

 markedly fatty livers were observed and the addition of choline to the 

 mixture of purified vitamins permitted homocystine to function as a source 

 of methionine.^* Choline was isolated from the concentrates of milk and 

 rice bran vitamins. 



The concept of transmethylation or transfer of intact methyl groups was 

 established by du Vigneaud and his collaborators in a series of experiments 

 in which isotopically labeled compounds were employed. Choline containing 

 deuterium-labeled methyl was isolated from the carcasses of rats fed a 

 choline-deficient diet supplemented with methionine containing deuterium 

 in the sulfur methyl. ^^ By the same procedure the methyl of creatine was 

 shown to come from methionine."^ In addition, the transfer of methyl from 

 choline to creatine"^ and to methionine"" occurred if labeled choline and 

 homocystine replaced methionine in the diet. The transfer of methyl to 

 guanidoacetic acid to form creatine, however, was irreversible."^"^'' This im- 

 pressive evidence of transfer of intact methyls was given additional support 

 by the experiment in which choline was isolated after feeding rats doubly 



58 T. B. Osborne and L. B. Mendel, J. Biol. Cheni. 20, 351 (1915).- 



59 W. C. Rose and T. R. Wood, ./. Biol. Chem. 141, 381 (1941). 



60 M. Womack and W. C. Rose, J. Biol. Chem. 141, 375 (1941). 

 «i A. White and E. F. Beach, J. Biol. Chem. 122, 219 (1937). 



62 V. du Vigneaud, H. M. Dyer, and M. W. Kies, /. Biol. Chem. 130, 325 (1939). 



63 V. du Vigneaud, J. P. Chandler, A. W. Moyer, and D. M. Keppel, J. Biol. Chem. 

 131, 57 (1939). 



64 V. du Vigneaud, M. Cohn, J. P. Chandler, J. R. Schenck, and S. Simmonds, /. 

 Biol. Chem. 140, 625 (1941). 



65 V. du Vigneaud, Biol. Symposia 5, 234 (1941). 



66 S. Simmonds, M. Cohn, J. P. Chandler, and V. du Vigneaud, J. Biol. Chem. 149, 

 519 (1943). 



6' V. du Vigneaud, J. P. Chandler, and A. W. Moyer, J. Biol. Chem. 139, 917 (1941). 

 67a S. Simmonds and V. du Vigneaud, Proc. Soc. Exptl. Biol. Med. 59, 293 (1945). 



