648 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



ethanolamine, HOCH 2 CH 2 NH 2 , may act as a structural unit in the synthe- 

 sis of choline, and hence be a limiting factor. This has later been found to 

 be the case. Moreover, Cantoni 637 obtained evidence which suggests that 

 methionine must be activated as a prerequisite for the transfer of its methyl 

 group. The rate at which the radioactive methyl group of dietary meth- 

 ionine is oxidized to C 14 2 has been shown by Mackenzie and du Vigneaud 638 

 to be increased by choline, and reduced to its original value by cystine. 



In the later work of Beveridge et al., 639M0 the differences between the lipo- 

 tropic effect of casein and of a corresponding amount of methionine were 

 related to the protein level of the diet. When casein made up less than 22% 

 of the diet, the activity of free methionine as a lipotropic agent exceeded 

 that of casein containing a similar amount of methionine. On the other 

 hand, conditions were reversed when the dietary casein exceeded 22%; 

 in this situation, casein exhibited a lipotropic effect superior to that of a 

 corresponding amount of free methionine. These investigators demon- 

 strated that the differences between the two rations were equalized when 

 the quantities of essential amino acids in the two diets were identical. The 

 lipotropic activity of a protein is determined not only by its methionine 

 and cystine contents but also by the nature and quantity of the sulfur-free 

 essential amino acids. The lipotropic effects of the essential amino acids 

 are not the result of a direct action, but rather of an effect on growth and 

 maintenance which influences the amount of new tissue, thus modifying 

 the quantity of methionine left available for lipotropic purposes. 



Treadwell et al. 6il have demonstrated that growth has first priority as 

 regards demand on the methionine available in the organism. When the 

 need for this function is satisfied, the remaining methionine can be used for 

 lipotropic purposes. Thus, it was found that, when rats were raised on a 

 diet containing 20% of arachin (which is deficient in methionine) and 0.5% 

 methionine, the rats grew normally but developed fatty livers. On the 

 other hand, when the proportion of methionine was increased to 1%, the 

 rats grew normally and the liver lipids were only slightly above normal. 

 Since less methionine is required for growth in older rats than in rapidly 

 growing animals, more of the ingested methionine remains, in the former 

 case, to act lipotropically. 642 



637 G. L. Cantoni, J. Biol. Chem., 189, 745-754 (1951). 



638 C. G. Mackenzie and V. du Vigneaud, /. Biol. Chem., 195, 489-491 (1952). 



639 J. M. R. Beveridge, C. C. Lucas, and M. K. O'Grady, /. Biol. Chem., 160, 505-518 

 (1945). 



640 J. M. R. Beveridge, C. C. Lucas, and M. K. O'Grady, J. Biol. Chem., 154, 9-19 

 (1944). 



"» C. R. Treadwell, H. C. Tidwell, and J. H. Gast, J. Biol. Chem., 156, 237-246 (1944). 

 642 M. G. Horning and H. C. Eckstein, J. Biol. Chem., 155, 49-53 (1944). 



