Serotransferrins

Conalbumin comprises 3-16% of the egg-white from various species of birds. It is structurally related to plasma serotransferrin, the polypeptide chain being coded by the same structural gene [27], but the carbohydrate moiety being different (see section 3.1.3). Because of the close relationship of these two proteins, the name ouotransferrin is now used in place of conalbumin. 

In the meantime, in 1947, Laurell and Ingelman[17] had independently purified the red protein from pig plasma and in the same year proposed the name transferrin which has since been adopted as the generic name of the proteins of this family serotransferrin (instead of siderophilin) present in blood and some external secretions, ovotransferrin (instead of conalbumin) in avian egg-white, lactotransferrin (also called lactoferrin) from milk, external secretions and leukocytes and melanotransferrin (instead of p97) in melanocyte and normal cell plasma membrane. A dozen mammalian and some frog, fish and insect serotransferrins were later isolated and characterized. 

In humans, the plasma concentration of serotransferrin, which is hiosynthesized by the liver, varies from 3 to 3.5 g per liter and it is worthwhile to mention that the protein is only 20-30% iron-saturated, an important fact considering the inhibitory power of bacterial growth provided hy aposerotransferrin acting via ferrideprivation (see above). 

Important variations of the plasma serotransferrin concentration are observed in physiological and pathological situations. They are often accompanied by dramatic modifications of glycan primary structure (see section 3.2). 

Since these first discoveries, lactotransferrin has been isolated from milk of numerous mammals goat [40], mare [41], monkey [42], mouse [43,44], rabbit [45] and sow [46,47]. 

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Figure 5.1 Schematic diagram of the lactoferrin molecule. The positions of carbohydrate attachment are marked with a star. O, ovotransferrin T, human serotransferrin L, human lactoferrin R, rabbit serotransferrin M, melanotransferrin A, the connecting helix B, the C-terminal helix. The disulfide bridges are indicated by heavy bars, and the iron and carbonate binding sites by filled or open circles, respectively. Reprinted from Baker et al., 1987. Copyright (1987), with permission from Elsevier Science.

Multisites in proteins are not uncommon. The removal of metal ions from such centers is likely to be involved. This is in fact illustrated by the iron removal from serotransferrin, see also Sec. 2.6. This protein is bilobal and each lobe contains an iron-binding site. These are 35 A apart and it is believed that direct interaction between the sites is absent. The two Fe s, designated a and b, are different and their removal is biphasic, although not markedly so. 

The lactoglobulin fraction consists mainly of immunoglobulins (Ig), especially IgGj, with lesser amounts of IgG2, IgA and IgM (section 4.10). The lactalbumin fraction of bovine milk contains three main proteins, / -lactoglobulin (j8-lg), a-lactalbumin (a-la) and blood serum albumin (BSA), which represent approximately 50, 20 and 10% of total whey protein, respectively, and trace amounts of several other proteins, notably lactotrans-ferrin, serotransferrin and several enzymes. The whey proteins of sheep, goat... 

Leger, D., Verbert, A., Loucheux, M.-H. and Spik, G. 1977. Study of the molecular weight of human lactotransferrin and serotransferrin. Ann. Biol. Anim. Biochim. Biophys. 17, 737-747 (French). 

H-N.m.r. Data far Anomeric Protons, Mannose H-2, and N-Acetyl Methyl Protons of Human Serotransferrin Asialoglycan (from Ref. 191)... 

Fig. 5. The glycoprotein analysis workflow proposed by Renkonen et al,43 As a proof of the principle, the authors showed an example of comprehensive approach for glycoproteomics of purified human plasma serotransferrin.

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