VITAMIN AND GROWTH FACTOR RESEARCH 



named the crystalline substance lipoic acid, and shortly there- 

 after by Patterson et al. (46), who later renamed the product 

 thioctic acid. Chemical characterization and synthesis (9,25) 

 showed the substance to be 6,8-dithiooctanoic acid (formula XV) ; 

 it can exist in both the cyclic structure and as an open-chain 

 dithiol, the interconversion of the two being explicitly involved in 

 its enzymatic role as a cofactor in the oxidative decarboxylation 

 of pyruvate to acetate and other a-keto acids to the corresponding 

 carboxylic acids containing one less carbon atom (18,51). 

 Such oxidative decarboxylation also requires thiamin pyro- 

 phosphate, and it appears an unsettled point whether the two 

 are combined into a single coenzyme (51). 



Most nutritional investigations with higher animals have 

 failed to show a growth response to added lipoic acid. However, 

 DeBusk and Williams (14) have recently observed both increased 

 growth and increased efficiency of food utilization in young rats 

 and chicks fed trace amounts of the substance. The reasons for 

 the differing results are not yet known. If these results are 

 confirmed, this will provide another instance of a vitamin dis- 

 covered, isolated, and characterized as a microbial growth factor 

 before its role in the nutrition of higher animals was suspected. 

 However, in a sense it is academic whether the substance is or 

 is not a vitamin, i.e., required in the diet of animals. There is no 

 doubt that it is a biocatalyst of great importance in animal as in 

 microbial and plant life, and the genetic accident that determines 

 whether the substance can or cannot be synthesized in animals, 

 though it has important implications for nutrition, hardly changes 

 the intrinsic biochemical importance of the substance. 



OTHER MICROBIAL GROWTH FACTORS 



A variety of additional substances found essential for growth 

 of one or another microorganism during the past few years is 

 listed in Table I. Although they are obviously important for the 

 organisms that require them, little is known of their metabolic 

 significance. D-Alanine is required for growth of many lactic 

 acid bacteria, but only in the absence of vitamin Bg. It is now 



105 



