ClIAI'TKU 18 



NEW AND UNIDENTIFIED GROWTH FACTORS 



VERNON n. chi:li)i:lin 



I'age 

 1. Lipoic Acid (Tliioctic Acid, Pyruvate Oxidatioi) Factor, Protogen, Ace- 

 tate Replacement Factor) 575 



II. Carnitine (\'itaniin B-p) 58:i 



III. Peptides 585 



IV. Ly.xoflavin 588 



V. CoenzNme III 589 



VI. Factors Required in Unheated Growth Media 589 



VII. Guinea Pig Antistiffness Factor 591 



VIII. Miscellaneous Factors 593 



The body of information that has formed the basis for the preceding- 

 chapters has enabled us to obtain a clear outhne of the chemistry and func- 

 tion of se\'eral of the vitamins that have been discussed. Although the 

 pioneer work on vitamins was done only half a century ago, the intensity 

 of research, particularly during the past two decades, has so overcome the 

 time disadvantage that se\'eral vitamin systems are now as well or better 

 understood than manj^ of the more classical compounds of biochemical im- 

 portance. 



The purpose of the present discussion is mainly to provide the reader with 

 an up-to-date account of the newly suspected and recognized members of 

 the biocatalyst family — those which have been reported but have not yet 

 been sufficienth' developed to merit separate consideration. However, in 

 \-iew of the speed with which problems in this field are being carried forward, 

 the members of the "new factors" group must obviously be transient ones. 

 Compounds listed in the present article may within a few years be expected 

 either to emerge as fully characterized ^•itamins or to be dropped altogether 

 as duplications are discovered and the occasional artifact or combined effect 

 of known factors is recognized and eliminated. Xew members are mean- 

 while being added at a rapid rate. 



I. Lipoic Acid (Thioctic Acid, Pyruvate Oxidation Factor, 

 Protogen, Acetate Replacement Factor) 



This compound is selected for initial consideration because its chemistry 

 and some of its biological finictions have become well outlined and because 

 its development typifies both the intensity of study aiid the diversity of 

 systems that are being employed in the search for new growth factors. Two 



575 



