II. CARNITINE 583 



('KM'ti'oiiic cxcital ion. I ii tlic ahsciicc of lifilit, rcoxidal ion ol the lice ladicals 

 would ocfur, tlu()ii,<i;li DPN, (laxoprotcins, and other oxidants, including 

 linally CO-j . 



Such a proposal imphos that (he "reduction" of hpoic acid (disulfide) by 

 electronic excitation proceeds more rapidly than its reduction by pyruvate. 

 Since the /s'l/ of the operating lipoic acid coenzyme has not yet been re- 

 ported, a comparison cannot be made; however spectrophotometric evi- 

 dence is presented*^ ■ "* to show that the energy needed to rupture the S — S 

 bond in (>,8-thioctic acid (80,000 to 40,000 cal. per mole) is approximately 

 equal to the energ}^ available for photosynthesis when one quantum of light 

 is absorbed. By contrast, 5,8-thioctic acid and other disulfides required 

 00,000 to 70,000 cal. per mole for similar cleavage. The peculiar instability 

 of G,8-thioctic acid was thought to be due to strain within the ring. This is 

 further suggested by the yellow color of the latter compound; the other 

 disulfides examined were colorless. Further indirect evidence supporting 

 this concept was provided by model systems, whereby zinc porphyrin or 

 tetralin could be used to reduce the disulfide photochemically. Finally, w hen 

 algae were treated with G,8-thioctic acid, oxygen production was acceler- 

 ated, using (luinone as the oxidizing agent. 5 ,8-Thioctic acid was ineffective. 

 These suggestions, if verifiable by direct experiments, will consititue a 

 major step toward a full understanding of the photosynthetic process. 



The interest that the discovery of lipoic acid has engendered will doubt- 

 less continue for some time; in addition to further studies on the chemistry 

 of the vitamin, the enzymic aspects of the problem deserve major atten- 

 tion. The finding in lipoic acid of a catalytically active thiol has emphasized 

 again the importance of this class of compounds to biochemical systems. 

 The reaUzation that CoA and other thiols possess acetylating power in 

 in vitro systems and that the thiol bond is probably of the "high-energy" 

 variety'" "^^ focuses interest upon the lipoic acid sj^stem as a possible parti- 

 cipant in energy conservation and transport from pyruvate to ATP and 

 other eventual acceptors. 



II. Carnitine (Vitamin Bt) 



In 1947 Fraenkel and Blewett'*' obser\ed that 3^east extracts contained 

 two apparently unfamiliar growth factors which were required by the 

 mealworm Tenebrio molilor. One of the factors, adsorl)able on Xorit, was 

 found to be folic acid; the other, a "filtrate factor," was regarded as a new 



60 F. Lynen and E. Reichert, Angew. Chem. 63, 47 (1951). 

 *' F. Lynen and E. Reichert, Ann. 574, 1 (1951). 

 "J. Baddiley and E. .M. Thain, ./. Chem. Soc. 3425 (1951). 

 " T. Wieland and E. Bakelmann, Angew. Chem. 64, 59 (1952). 

 "G. Fraenkel and M. Blewett, Biochem. J. 41, 469 (1947). 



