176 Sialic acid species-variation

In general, the transferrins are all very similar with molecular weights reported in the range from approximately 70,000 to 90,000 g., and with nearly identical, if not identical, metal binding sites. In addition to variations in contents of amino acids, which normally occur in homologous proteins from different organs or species, the transferrins also vary in the amounts of carbohydrates and, of particular current interest, in the amounts of sialic acid. 

Birds. Early studies showed the presence of sialic acids in the serum of hen (Bohm and Baumeister, 1956) and mucous from pheasant and goose (Wessler and Werner, 1957). Feeney et al. (1960) studied the chemical and the physical characteristics of avian egg white proteins and found sialic acids in the proteins of 25 species belonging to six orders of Aves. There are marked variations in sialic acid content between bird species, but they all appear to contain the same sialic acid, V-acetylneu-raminic acid. Only a trace amount of free sialic acid was detected. 

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