Discovery and Structure
Human LF is a cationic glycosylated protein consisting of 691 amino acids (9) folded into two globular lobes (80 kDa bi-lobal glycoprotein) (10), that are connected by an α-helix (11, 12). Bovine LF contains 689 amino acids (13). LF was first discovered and isolated from bovine milk in 1939 (14), and is a member of the transferrin family (60% amino acid sequence identity with serum transferrin) (11). LF and transferrin have similar amino acid compositions, secondary structures (including their disulphide linkages), and tertiary structures, although they differ in terms of biological functions (11, 15, 16) (see Figure 2). There are also three different isoforms: LF-α is the iron-binding isoform, while LF- β and LF-g both have ribonuclease activity but do not bind iron (11, 17). When it is iron-rich it is referred to hololactoferrin and when iron-free apolactoferrin (18). The tertiary structures of the two forms are significantly different: apolactoferrin is characterized by an open conformation of the N-lobe and a closed conformation of the C-lobe, while both lobes are closed in the hololactoferrin (18). Human LF and bovine LF possess high sequence homology and have very similar antibacterial, antifungal, antiviral, antiparasitic, anti-inflammatory, and immunomodulatory activities (19–21). Consequently, it is common to give the bovine form rather than say a recombinant human form as a supplement. Bovine LF is also deemed a “generally recognized as safe” substance by the Food and Drug Administration (FDA, USA), and is commercially available in large quantities (19).
Figure 2  Crystal structures of bovine lactoferrin (PDB code = 1BLF), human lactoferrin (1B0L), and rabbit serum transferrin (1JNF). Adapted from Vogel (10). Pink spheres represent ferric iron (Fe3+) binding sites. Due to its similarities to transferrin, which is the main iron transporting molecule in serum (22, 23), α-LF possesses iron binding capabilities (24, 25), and it can chelate two ferric irons (Fe3+) (26). LF binds one ferric iron atom in each of its two lobes; however, an important attribute is that it does not release its iron, even at pH 3.5. This is of importance as this property assures iron sequestration in infected tissues where the pH is commonly acidic (27). In the context of its iron-binding capabilities, it means that when it binds ferric and siderophore-bound iron, it limits the availability of essential iron to microbes (27).
In healthy individuals, iron is largely intracellular and sequestered within ferritin or as a co-factor of cytochromes and FeS proteins, and as haem complexed to hemoglobin within erythrocytes. Circulating iron is rapidly bound by transferrin (28, 29). When erythrocytes lyse and hemoglobin or haem is released into the circulation, their hemoglobin is captured by haptoglobin, and haem by hemopexin (30). Here, circulating serum ferroxidase ceruloplasmin is of importance, as LF can bind to ceruloplasmin, such that a direct transfer of ferric iron between the two proteins is possible (31). A direct transfer of ferric iron from ceruloplasmin to lactoferrin prevents both the formation of potentially toxic hydroxyl radicals (32) and the utilization of iron by pathogenic bacteria. LF is therefore an important player in preventing bacteria from acquiring and sequestering iron, which [with the possible exception of Borrelia burgdorferi (33)]; they require for growth and virulence. LF also acts as biomarker, as it is commonly upregulated when the host is suffering from various kinds of disease. See Table 1 for selected references.
Table 1  Lactoferrin as a major player in host defense and iron binding, and its use as biomarker for various diseases.
Area of action   References
Protecting neonates via breast milk  (34–41)
LF in cervicovaginal mucosa and female reproductive tract; antibacterial, antifungal antiparasitic, antiviral  (42–45)
LF in the airways  (46, 47)
Mucosal surfaces, allergen-induces skin infections  (48)
Neutrophil extracellular trap (NET) production  (49)
Saliva and its antimicrobial activities and iron binding  (50–52)
Saliva as biomarker for neurological diseases  (53–55)
Saliva as biomarker for periodontal disease and oral dryness  (56–59)