556 BIOTIN 



By the microbiological method it has been demonstrated^^ that the tis- 

 sues of chicks on a diet causing egg white injury were deficient in biotin, 

 despite the abundance of this vitamin in the diet. Further it has been 

 shown^" that commercial or fresh egg white is capable of inactivating biotin 

 in vitro, oAving to the formation of a fairly stable compound of biotin vnth. 

 a special constituent of egg white. This compound, avidin, has been purified 

 and obtained in crystalline f orm^^ • ^^ By definition one unit of avidin com- 

 bines with 1 7 of biotin. With the assumption that one molecule of biotin 

 combines with one molecule of protein, the molecular weight of avidin has 

 been calculated to be 43,500. Crj^stalline avidin is slightly less active than 

 highly purified amorphous preparations: 4000 units per gram as compared 

 with 7000 units per gram. The isoelectric point of avidin is claimed to be 

 at pH 10.95 



Avidin, or egg white containing avidin, is denatured and inactivated by 

 heat. This explains why egg white injury may be produced only with raw 

 or slightly cooked but not with heat-denatured egg ^vhite. 



Purified avidin produced effects in rats similar to those caused by dried 

 egg white. ^^ The avidin-biotin complex (AB) is easil}'^ split by steaming 

 for a short time (30 to 60 minutes) at 100°. Rats fed a diet contained cooked 

 egg white plus avidin excreted only negligible amounts of free biotin, 

 whereas after the feces had been steamed large additional amounts of biotin 

 became free." 



Solutions of purified avidin lose activity on standing, more readily in 

 dilute than in concentrated solution. In any concentration the rate of 

 destruction is much greater at 38° than at ice-box temperature, and, except 

 in the most concentrated solutions, even room temperature will cause 

 considerable acceleration of the rate. Dilute solutions of egg white are much 

 less affected over this temperature range.''* When solutions of egg white" 

 or of avidin concentrates were brought to pH 1.8 with HCl, their avidin 

 activity was almost but never quite completely destroyed. The aAddin-biotin 

 complex is much more stable than avidin itself. Biotin already bound to 

 the avidin was not released by the treatment with acid^* or trypsin, pan- 

 creatin, and papain, or with liver, kidney, muscle, and blood. ^^ The high 

 resistance of the avidin-biotin complex to enzymes might contribute to the 

 fact that it passes through the alimentary tract practically unchanged. 



»3 R. E. Eakin, W. A. McKinley, and R. J. Williams, Science 92, 224 (1940). 



9^ D. Pennington, E. E. Snell, and R. E. Eakin, ./. Am. Chem. Soc. 64, 469 (1942). 



85 D. W. Woolley and L. G. Longsworth, ./. Biol. Chem. 142, 285 (1942). 



96 P. Gyorgy, C. S. Rose, R. E. Eakin, E. E. Snell, and R. J. Williams, Science, 93, 



477 (1941). 

 «' P. Gyorgy and C. S. Rose, Science 94, 261 (1941). 



98 P. Gyorgy, C. S. Rose, and R. M. Tomarelli, J. Biol. Chem. 144, 169 (1942). 



99 P. Gyorgy and C. S. Rose, Proc. Soc. Exptl. Biol. Med. 53, 55 (1943). 



