Methionine Content of the Animal and Plant Organism 573 



labelled with the sulphur isotope ^^S, also contained the carbon isotope ^^C in 

 the a and ß positions in the carbon chain of the methionine, showed that in the 

 cystine isolated from the hair of the rats there was absolutely no labelled carbon, 

 while 80% of the sulphur of the cystine consisted of the isotopic sulphur of the 

 methionine. 



Du Vigneaud et al. [7] and Dyer & du Vigneaud [5] showed that the organism 

 can synthesize the L-methionine required for growth from l- and DL-homocystine 

 and homocysteine when there are adequate donors of methyl groups, choline 

 and betaine in the diet. 



Wood & Gutman [22] resolved »S-benzyl-DL-homocysteine by isotopic label- 

 ling, it being an intermediate product in the synthesis of methionine. This 

 confirms the mechanism of transformation of homocysteine into methionine 

 in vitro. 



Rose [14], White & Beach [20], Womack et al. [21] and Close [10] consider 

 that the transformation of methionine into cysteine in the living organism is 

 irreversible and that the requirements of the organism for methionine carmot be 

 covered by cysteine. 



Great interest attaches to the work of Horowitz [9] who discovered the ability 

 of the mould Neurospora crassa to transform cysteine into methionine. Radio- 

 active sulphur when administered to Neurospora crassa is incorporated into 

 methionine. 



Table i 



The average content of methionine, cysteine and cystine in the serum of old rats, 



guinea pigs, rabbits and dogs treated with methionine and cysteine (50 mgjkg of body 



weight daily for 16 days; rabbits received 25 mgjkg body weight). 



I 



