MISCELLANEOUS NITROGEN AND SULFUR COMPOUNDS 293 



SOME VITAMINS 



Several important vitamins are nitrogen and/ or sulfur compounds which do not fit 

 conveniently under any larger structural category. Some of these will be mentioned briefly 

 here; but since each one is a special case and we are concerned with discussing general 

 categories of compounds, the general references should be consulted for methods of iso- 

 lation and characterization. Also, for paper chromatography of the vitamins see (58), for 

 biosynthesis (59). 



Thiamine or vitamin Bj probably occurs to some extent in all plants, if not in all 

 cells. Thiamine pyrophosphate (cocarboxylase) is the form in which this vitamin serves 

 as a coenzyme in the decarboxylation of pyruvic acid to acetate, acetyl-coenzyme-A, or 

 other products and in the transketolase reaction. The exact mechanisms of these reac- 

 tions are still under active investigation. Pathways of thiamine biosynthesis are likewise 

 obscure although it is probable that the pyrimidine and thiazole portions are synthesized 

 separately and then combined. Many aspects of thiamine chemistry and physiology are 

 presented in a symposium publication (60). 



NH 



^ /= 





CH3 



^J 



¥ ' V 



-CHjCHgOPOgH 



OPO3H2 



thiamine pyrophosphate 



Lipoic acid or 6, 8-thioctic acid has a rather simple structure. It exists in a re- 

 duced dithiol form which is readily oxidized to a disulfide. 



CH2 CHfCHjP^COOH ^ CHg CH(CH2)4C00H + 2[h] 

 SH SH S S 



This vitamin functions along with thiamine pyrophosphate as a coenzyme in decarboxylation 

 and acyl group transfer. It may also play a role in photosynthesis where it has been sug- 

 gested that light energy is used to reduce the disulfide form to the dithiol which can then 

 transfer hydrogen to pyridine nucleotides. 



Biotin serves as a coenzyme in several carbon dioxide fixation reactions such as the 

 formation of oxalacetate from phosphoenolpyruvate and the formation of /3-methylglutaconyl- 

 CoAfrom senecioyl-CoA (61). The actual coenzyme form a biotin may be biocytin, e-N 

 biotinyl-L-lysine. In many cases biotin appears to be bound so tightly to its enzyme pro- 

 tein that it might more aptly be described as a prosthetic group rather than a diffusible co- 

 enzyme. 



