Lactotransferrin

The transferrins are a group of specific metal-binding proteins, the best characterized of which are serotransferrin (present in blood plasma, milk, spinal fluid and semen), ovotransferrin (conalbumin present in avian and reptile egg white) and lactotransferrin (present in milk, pancreatic juice, tears and leucocytes). 

Human colostrum and milk contain 6-8 mg ml and 2-4 mg ml lactotransferrin, respectively, representing about 20% of the total protein in the latter bovine colostrum and milk contain about 1 and 0.02-0.35 mg ml respectively. The concentration of lactotransferrin in human milk decreases slightly during lactation but appears to increase slightly in bovine milk and very markedly during the dry period. 

Lf have been reviewed by Lonnerdal and Iyer (1995) and Hutchens and Lonnerdal (1996). 

Lactotransferrins have been isolated from the milks of several species, including human and bovine, and some have been well characterized, including determination of their amino acid sequence. Some of the isolation procedures have industrial-scale potential the preparations obtained from such procedures usually contain both Lf and LPO. 

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Figure 5.4 (a) Stereo ribbon representation of hFBP with Fe3+ ligands shown. 6-sheets are gold, a-helices are cyan, and other structures are dark blue, (b) N-lobe of lactotransferrin. Secondary structure elements are coloured as in hFBP, except for grey regions, which are those most different from hFBP. From Bruns et al., 1997. Reproduced by permission of Nature Publishing Group. 

One of the important developments in dairy technology in recent years has been the fractionation of milk into its principal constituents, e.g. lactose, milk fat fractions and milk protein products (caseins, caseinates, whey protein concentrates, whey protein isolates, mainly for use as functional proteins but more recently as nutraceuticals , i.e. proteins for specific physiological and/or nutritional functions, e.g. lactotransferrin, immunoglobulins). 

Bayard investigated a number of glycoproteins (aracid glycoprotein, fetuin, lactotransferrin, transferrin, and ovomucoid) by N-deacetylation followed by deamination.176 They all gave the oligosaccharides 108 and 109, demonstrating the presence of common structural elements in these compounds. 

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). 

Fig. 9.—Structure of Glycans105 of Bovine-milk Lactotransferrin ( Red Protein ).

Spik, G., Coddeville, B., Mazurier, J., Bourne, Y., Cambillaut, C., and Montreuil, J. 1994a. Primary and three-dimensional structure of lactotransferrin (lactoferrin) glycans. Adv. Exp. Med. Biol. 357, 21-32. 

Stephens, S., Dolby, J.M., Montreuil, J., and Spik, G. 1980. Differences in inhibition of the growth of commensal and enteropathogenic strains of Escherichia coli by lactotransferrin and secretory immunoglobulin A isolated from human milk. Immunology 41, 597-603. 

The two lobes correspond to the N-terminal and C-terminal halves of the molecules and are tightly associated by non-covalent interactions. Also, both are joined by a connecting short peptide of 12 and 11 amino acids in sero- and lactotransferrin, respectively. The... 

As in the case of ovotransferrin, no function lias been ascribed to the lactotransferrin in milk. An attractive hypothesis is for a function in the nutritional utilization of iron, but no evidence favoring such a function has been found. As also with ovotransferrin, however, the lactotransferrin could have a function as an antimicrobial agent, because milk is low in iron and the transferrin is not saturated. 

The values for the N-terminal amino acids of five different transferrins are given in Table 6. Three out of the five have been reported to have alanine. The absence of an N-terminal amino acid in human lactotrans-ferrin (95) is an unexpected finding in view of the many physical and chemical properties human lactotransferrin and other transferrins have in common (22, 137). Confirmation of this absence is highly desirable. 

Table 3. Amino acid composition of human lactotransferrin...

As compared to the information on the carbohydrate contents of human serum transferrin, much less information is available on lacto-transferrin. In general, however, the available values for lactotransferrin are more similar to those for serum transferrin than to those for ovotrans-ferrin. 

The relationship of lactotransferrin to serum transferrins is more obscure. Derechin and Johnson (33) found that bovine lactotransferrin had two distinct immunologic components, one of which reacted similarly to blood serum transferrin and one which did not react with blood serum transferrin. Komatsu et al. (83) found immunologic cross reactivity between human serum transferrin and human lactotransferrin, but the cross reactivity was not strong. 

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