Glycoproteins amphibian

Because L-Xaa2-containing peptides have never been found in amphibian skin extracts, the epimerization mechanism probably involves a quantitative inversion of the chirality of the a-carbon of the amino acid residue, rather than a racemization, which would yield an equimolar mixture of L and D isomers [16,17]. Enzymes catalyzing the formation of D amino acids are so far known only in yeast [18]. From Bombina skin secretions Kreil et al. [19] recently purified a 52-kDa glycoprotein which catalyzes the reaction Ile-Ile-Gly to Ile-D-allo-Ile-Gly. The partial conversion of He to D-allo-Ile in peptide linkage proceeds without the addition of cofactors. 

Several new constituents of glycoproteins such as poly-Sia and Kdn have been discovered in fish egg glycoproteins. The occurrence of Kdn, a new member of the sialic acid family, has been reported in one bacterial strain [68], two amphibian speeies [69,70] and various mammalian tissues [71,84—86]. Use of monoclonal antibodies [71,72] and specific enzymes [73-75] that eleave Kdn linkages may facilitate new discoveries not only in the area of fundamental biology but also in oneology and pathology. 

Avidin, a glycoprotein in the egg white of many birds and amphibians. Chicken avidin (Mr 66 kDa) consists of four identical subunits (without carbohydrate Mr 14 kDa 128 aa). Avidin binds four molecules of the vitamin biotin with high affinity (dis-... 

AvkNn. Tetrameric glycoprotein (Mr 66000), occurring in the albumen of bird and amphibian eggs and in the genital tracts of all animals. Denaturization occurs on boiling and irradiation. 

Avidin a basic glycoprotein in the egg whites of many birds and amphibians. The primary structure of chicken A. is known M, 66,000, pi 10, 10.5 % threonine. There are 4 identical subunits of M, 14332 (without the carbohydrate) (128 amino acids). A. forms a stoichiometric, noncovalent complex with 4 molecules of the vitamin, biotin the complex is not attacked by proteolytic enzymes and therefore not resorbed. Feeding of A. or raw egg white can therefore result in experimental biotin deficiency (see Vitamins, biotin). A. is denatured and thus inactivated by heating. Like the unrelated lysozyme and conalbumin (see Siderophilins), A. protects the egg white against bacterial invasion. 

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