II. CHEMISTRY 557 



Irradiation with visible light inactivates avidin in solutions of egg white 

 or of purified concentrates, the latter more easily. Biotin bound to the 

 a\'idin may be released. The presence of riboflavin accelerates the action 

 of visible light. ^'^ Biotin could also be liberated from the avidin-biotin com- 

 plex by oxidation with 0.45% H2O2. 



By means of radioactive biotin the reaction between avidin and biotin 



was found to be practically complete but to some extent reversible with a 



measurable dissociation of the complex. ^"^ The radiobiotin was prepared 



with radioactive C14 in the ureido position. ^"^ On the basis of equilibrium 



f4VB')- 

 measurements, it was found that an expression of the type K = \ 



with a modification for the incomplete dissociation of the intermediate AB 

 describes the system over a considerable range of concentrations. An 

 average value was found for K = 0.0 X 10~-\ at 25° in 0.2 M ammonium 

 carbonate and other media, corresponding to a degree of dissociation of 

 0.08% at 10~^ M complex. The dissociation was found to increase with 

 temperature and seems to increase in certain specific media. 



The technique of Sanger'*'- • '"•■' has been applied to avidin for the charac- 

 terization and determination of amino groups. It has been shown that in 

 a\ddin preparations, free from nucleoprotein and nucleic acid, all the lysine 

 e-amino groups'^ react and that there are three N-terminal alanine residues 

 per mole.'""* 



Avidin which is toxic when given enterally is of high therapeutic value 

 in egg white injury when administered parenterally.^^ This is explained by 

 liberation of the biotin consistently present in avidin concentrates. The 

 mechanism of the splitting of the avidin-biotin complex in a parenteral 

 medium has not yet been explained. It is certainlj' at variance with its 

 stability in the intestinal tract, and it may be related to oxidative, perhaps 

 fermentative, reactions in the parenteral medium. ^^ 



The onlj^ source of avidin so far known is egg white (hen, turkey, duck, 

 and goose) and also the egg jelly of frogs. It is from the oviduct tissue, prob- 

 ably from its mucosal lining, that biotin-binding avidin is secreted in hen 

 and frog."'^ Xo explanation has been found for the presence of this anti- 

 vitamin in the oviduct and in the egg. 



The a\ddin combinability of biotin, its analogs, and related compounds 

 has been used for their chemical characterization. The "relative affin- 



><"> H. F. Launer ami H. Fraenkel-Conrat, ./. Biol. Chevi. 193, 125 (1951). 



1" D. B. Melville, J. G. Pierce, and C. W. H. Partridge, J. Biol. Chem.1%0, 299 (1949). 



">2 F. Sanger, Biochem. J. 39, 507 (1945). 



'" R. R. Porter and F. Sanger, Biochem. J. 42, 287 (194S). 



'"^ H. Fraenkel-Conrat and R. R. Porter, Biochem. J. 49, Ixxviii (1951). 



"»s R. Hertz and W. H. Sebrell, Science 96, 257 (1942j. 



