710 THE BIOCHEMISTRY OF B VITAMINS 



a methyl donor and not as a lipotropic agent, since fatty livers do not 

 develop on a 1 per cent coramine diet. 



Ethionine (S-Ethylhomocysteine). It was early shown 53 that ethionine 

 is toxic to rats on a low methionine diet and that the toxicity is prevented 

 by the simultaneous administration of methionine. Methionine also re- 



NH 2 



C 2 H 6 — S— CH 2 — CH 2 — CH— COOH 

 ethionine 



verses the growth inhibition produced in Escherichia coli by ethionine. 54 

 The latter compound was used by Stekol and Weiss 55 in an effort to 

 determine whether the inhibition of growth in rats is the result of a block 

 in the utilization of methionine per se, or whether there is interference 

 with the utilization of some of the metabolites which normally originate 

 from methionine. They found that choline alone can alleviate the growth 

 inhibition caused by ethionine just as methionine can. The other sub- 

 stances tested, cystine, homocystine and cystathionine, were without 

 effect on the inhibition. It is suggested that the increased need for choline 

 which is created by the administration of ethionine may result in a 

 diversion of the labile methyl group for greater synthesis of choline, thus 

 decreasing the amount of methionine available for growth. The possibility 

 of the incorporation of ethionine into the tissue protein of the rat was 

 also considered, but it was stressed that further experimentation must 

 be done before a rigorous interpretation of the data will be possible. 



In this discussion no attempt has been made to treat the many in- 

 hibitors of enzyme systems in which choline is involved, but only those 

 which may have a bearing on the status of choline as a doubtful member 

 of the B vitamin complex. 



Inositol 



The role of inositol as a growth factor for yeast was discovered in 

 1928 by Eastcott, 56 who isolated it from tea and recognized it as the 

 active constituent of Lukas' "Bios I." Subsequent work has shown that 

 it is a complementary growth factor for some strains of yeast; i.e., it is 

 relatively ineffective alone, but in combination with other B vitamins, it 

 often causes a striking increase in growth. 57, 5S The amount of inositol 

 necessary to produce these growth effects is from 100 to 1000 times larger 

 than the effective concentrations of the other B vitamins, and the effects 

 obtained are largely dependent on the particular strain of yeast under 

 investigation. Inositol is also a complementary growth factor for various 



