Avidin

Avidin is a glycoprotein found in raw egg white. It combines stoichiometrically with biotin. The toxic effect of uncooked egg white which causes a syndrome similar to that of vitamin B deficiency (Boaz 1924) led to the discovery of the vitamin biotin (Gyorgy 1931, 1939). The toxic factor, first isolated by Eakin et al. (1940, 1941) who named it avidin, combines with the essential growth factor resulting in a "non-digestible" avidin-biotin complex which is not absorbed from the intestine or from the surrounding medium by microorganisms. Avidin plays an important role in biotin function studies and in the study of several enzymes in which biotin is a coenzyme. Avidin or avidin subunits bound to a matrix have been utilized for affinity purification (Berger and Wood, 1975; Green and Toms, 1973).

Characteristics of Avidin from Egg White:

Molecular weight and subunits: 68,000 (Green 1964) composed of four essentially identical polypeptide chains (Green 1964; DeLange and Huang 1971).

Composition: Avidin is a basic glycoprotein of known carbohydrate and amino acid content (Melamed and Green 1963; DeLange 1970; Huang and DeLange 1971). The sequence of amino acids in the 128 residue subunit has been reported (DeLange and Huang 1971). The carbohydrate moiety, 4 glucosamines and 5 mannoses per subunit are attached at asparaginyl residue 17 (DeLange and Huang 1971).

Extinction coefficient: =15.5 (Melamed and Green 1963).

Binding properties: The best preparations bind 15.1 ug of d-biotin per milligram of protein corresponding to 4 moles of biotin bound per mole of protein. The dissociation constant for biotin is approximately 10^-15 M (Green 1963a). The biotin-avidin complex is dependent upon media ionic strength, but relatively stable over a wide pH range and to heat. It is necessary to effect a reversible or irreversible denaturation to free biotin. 6-8 M guanidinium chloride at pH 1.5 and autoclaving at 120°C (15 minutes) respectively are examples of dissociation conditions (Green 1970). The binding of biotin analogs by avidin has been summarized (Green 1970).