32 THE BIOCHEMISTRY OF B VITAMINS 



activity was not cocarboxylase or its combined form. 25 Goodhart and 

 Sinclair 26 have presented evidence for the existence in serum of thiamine 

 not in the form of diphosphothiamine bound to protein. 



The presence of thiamine-destroying principles in the tissues of fishes, 

 clams and ferns 27 complicates the extraction of the vitamin from these 

 sources, as well as the more fundamental problem of whether thiamine 

 has diverse modes of linkage in the tissues of various organisms. The 

 inconclusive investigations of Myrback 28 and Shonberg 29 and their respec- 

 tive associates with respect to the predominant presence or absence of an 

 oxidized form of thiamine (presumably thiamine disulfide, in which the 

 thiazole ring is cleaved and the sulfur oxidized) in bakers' yeast, have a 

 bearing upon the problem of thiamine utilizability mentioned above. 

 According to Myrback and his associates, bakers' yeast, when highly 

 aerated, contains no substantial amount of cocarboxylase or thiamine as 

 such, and this explains the lack of fermentation under these conditions 

 (Pasteur reaction). Shonberg and his associates deny the existence of the 

 oxidized form in substantial amounts. 



It should be evident that complete information regarding the various 

 ways in which thiamine is bound in tissues would constitute an important 

 contribution to understanding the catalytic functions which thiamine 

 performs. 



Riboflavin 



The history of riboflavin is closely associated with the flavoproteins of 

 which it is a part, and from which it may be split by extraction with O.liV 

 HC1. Warburg and Christian's "old yellow enzyme," 30 which antedated 

 exact knowledge regarding riboflavin, was the original flavoprotein dis- 

 covered, and from it riboflavin was subsequently obtained. 



Since that time a dozen or more specific flavoproteins have been isolated 

 in pure form from natural sources, or at least have been concentrated or 

 identified enzymatically. 31-44 Many of these have been obtained from 

 yeast, others from various animal tissues and from milk, Neurospora and 

 other molds. In addition, there is the L-amino acid oxidase of snake 

 venoms on which information is not available as to whether or not it is 

 a flavoprotein. 45 In the great majority of these flavoproteins, the pros- 

 thetic group is flavin adenine dinucleotide, made up as follows: isoalloxa- 

 zine-D-ribose-phosphate-phosphate-D-ribose-adenine. However, in three 

 of the flavoproteins, the "old yellow enzyme," the L-amino acid oxidase 

 isolated from rat kidney, and cytochrome-c reductase, 40 the prosthetic 

 group is a mononucleotide: isoalloxazine-D-ribose-phosphate. The flavo- 

 proteins are dissociable into the protein and prosthetic parts which readily 

 recombine to form the flavoprotein with its original enzymatic activity. 



