II. CHEMISTRY 555 



tioiis, behavior on paper i'lirt)matograms, rates of hydrolysis resistance 

 toward commercial enzymes and inactivation by egg white. 



2. Other Forms of Bound Biotin 



Free biotin as well as biocytin are water-soluble dialyzable compounds. 

 In contrast, biotin as it occurs in tissues and as it participates in various 

 specific metabolic reactions appears to be bound to protein or to peptides 

 or similar higher molecular compounds, perhaps by peptide linkages.**^ 

 Through enzj^matic hj'drolysis of liver protein with crystalline pepsin, 

 soluble bound protein compounds w'ere obtained, the chemical nature of 

 which has not yet been elucidated. The purest of these components con- 

 tained 50 atoms of nitrogen per mole of biotin.*^ 



The biotin activity present in acetone-dried beef liver may be almost 

 quantitatively extracted with a buffer solution of pH 3 and dilute XaOH. 

 Two distinct fractions containing biotin were thus obtained. The substance 

 soluble at pH 3 possessed biotin activity for yeast and showed no further 

 increase of activity by acid hydrolysis. This acid-soluble active compound 

 is dialyzable. On the other hand the material extracted with dilute alkali is 

 nondialyzable and exhibits biotin activity only after treatment with acid. 

 It may represent a soluble biotin-protein complex, named biotoprotein.** 



A "saline-soluble," high-molecular, undialyzable, microbiologically ac- 

 tive (for yeast) biotin complex was found in egg yolk.^^ 



3. AVIDIN 



The most interesting bound form of biotin is its combination with a 

 special protein-like constituent of egg w'hite named avidin.^"- ^^ Avidin 

 represents the "toxic" component of raw egg white responsible for the 

 production of egg white injury (p. 527). 



The induced nature of egg w'hite injury was first suggested by experi- 

 ments in which feces obtained from rats suffering from egg white injury 

 were fed to like animals. The feces were found to be non-potent as source of 

 biotin but wei'e highly active after steaming. It was assumed that some 

 constituent of egg white combines with and holds in non-absorbal)le form 

 the curative factor which originated either from the diet alone or was 

 excreted into the digestive tract. The non-absorption of the curative factor 

 would then be responsible for the egg white injury.^^ 



«6 J. P. Bowden and W. H. Peterson, J. Biol. Chem. 178, 533 (1949). 

 " Chang Wei-Shen and W. H. Peterson, J. Biol. Chem. 193, 5<S7 (1951). 



88 K. Hofmann, D. F. Dickel, and A. E. Axelrod, J. Biol. Chem. 183, 481 (1950). 



89 P. Gyorgy and C. S. Rose, Proc. Sac. Expll. Biol. Med. 49, 294 (1942). 



90 R. E. Eakin, E. E. Snell, and R. J. Williams, J. Biol. Chem. 136, 801 (1940). 

 "' R. E. Eakin, E. E. Snell ,and R. J. Williams, /. Biol. Chem. 140, 535 (1941). 



92 II. T. Parsons, J. Gardner, and C. T. Walliker, J. Nutrition 19, Suppl. 19 (1940). 



