Ovotransferrin; not only the egg’s gatekeeper of Iron

First identified in 1944 and named Conalbumin, this protein is now better known as Ovotransferrin. Accounting for 12-13% of the protein content of egg white, Ovotransferrin has key roles in protecting the developing chick embryo from micro-organismal growth. The first line of defence for the growing chick is, of course, the shell, a physical barrier supported by an inner mesh of glycoproteins which act as a filter. The egg-white is next in line, employing innate antimicrobials including LYSOZYME and iron-binding Ovotransferrin.

The control of iron, its location, and concentration is of utmost importance for the thriving of life on Earth; animals, plants, and micro-organisms. Iron (with the chemical symbol Fe) is the fourth most common element in the Earth’s crust and the most abundant transition metal in the human body. However, despite its abundance in nature, the concentration of iron is often a growth-limiting factor for living organisms. This seeming paradox arises as only the soluble forms, those that are not bound by oxygen in the air, are readily available to be taken up by cells. As a result, elegant evolutionary mechanisms have developed to capture, store, and scavenge iron.

Transition metals (including iron) have essential roles in biological systems, particularly at the active sites of enzymes due to their reactive properties. Eukaryotic organisms (us, chickens, and all with membrane-bound organelles) have used this to protect themselves from invading micro-organisms, in what has been termed nutritional immunity. This is the strategy of starving micro-organisms of iron, by the use of proteins that bind to it. For the developing chick in the egg, and indeed the fully grown chicken, this is Ovotransferrin’s role.

The role of Ovotransferrin and its derived peptides

For mammals, there are two soluble members of the iron-binding transferrin family, serum transferrin, and lactoferrin. Serum transferrin binds to all of the free iron in the blood, transporting it to the bone marrow for the production of haemoglobin. Lactoferrin is found in bodily secretions, where it is involved in nutritional immunity. For avian species, Ovotransferrin carries out both functions; regulating iron absorption, the immune response, and inflammation for the growing chick and grown chicken.

Ovotransferrin is a two-lobed protein composed of 686 amino acids, with each lobe capable of reversibly binding one Fe3+ ion. Its antibacterial function is two-fold, being both bacteriostatic and bactericidal. The iron sequestering property is bacteriostatic, rendering bacteria incapable of moving due to the requirement of iron for the generation of energy by mitochondria. Secondly, its cationic (positively charged) nature drives Ovotransferrin attachment to bacterial membranes; destabilising, permeabilising, and rupturing them. Beyond this antibacterial function, Ovotransferrin also possesses antifungal activity, particularly against the Candida genus. Of over 100 Candida strains tested, only C. Krusei showed resistance to Ovotransferrin.

It is not only the intact Ovotransferrin protein that possesses interesting properties for industrial applications. Peptides generated from the hydrolysis (digestion) of Ovotransferrin, either by the gastrointestinal tract, food processing, or within the egg as it ages, are capable of potent antimicrobial functions. A 92 amino acid peptide fragment (termed OTAP-92) which has lost its iron-binding capability, exhibits a wider antimicrobial spectrum than the native, intact form. OTAP-92 impairs bacterial function by disrupting the cytoplasmic membrane, with proven efficacy against E. Coli, and Staphylococcus Aureus. Other identified bioactive peptides show antihypertensive properties, toxicity to cancer cells, and antioxidant properties in addition. Ovotransferrin also has immunomodulatory properties, being capable of modifying the responses of macrophages and heterophils in chickens. Its levels are rapidly increased upon infection, which can drive phagocytosis and induce IL-6 release, which is both pro-and anti-inflammatory in its function.

Industrial Ovotransferrin Applications: underexploited yet highly capable

The industrial exploitation of Ovotransferrin remains in its infancy despite the powerful antibacterial, antifungal, and positive immune modulation functionalities being recognised since the 1970s.  Ovotansferrin shows much promise as a functional food as the core component of protective edible films. This extends the shelf life of foods, including noodles, fried bean curd, and fresh meats such as chicken breast. Aerosolised Ovotransferrin has also been shown to be an effective antibacterial and antiviral in large-scale turkey farming.

Recent work has examined Ovotransferrin’s potential in food-grade Pickering emulsions when complexed with gum arabic. Given the limited options for edible, surfactant-free emulsifiers, Ovotransferrin has significant potential for use in the production of foods including ice cream, mayonnaise, whipped cream, margarine, and batter. These properties also render Ovotransferrin of high value to the pharmaceutical and cosmetic industries, where again, limited biocompatible options are available. Evidence is emerging of Ovotransferrin-lysozyme complexes also being a potential emulsion-based delivery tool for hydrophobic nutraceuticals, which could improve the bioavailability of non-polar vitamins and nutrients, including vitamins A, D, E, carotenoids, flavonoids, and coenzyme Q10.

The potential medical and pharmaceutical applications of Ovotransferrin, and peptides derived from its hydrolysis are equally broad. Its antiviral, antibacterial, and antifungal properties, alongside evidence of antioxidant, anti-cancer and antihypertensive functionality, make it highly applicable. Evidence shows that Ovotransferrin may be used as a supplementing agent in humans (similarly to human lactoferrin), supporting iron-binding and transport capacity for those with iron deficiencies. Ovotransferrin also shows promise in bone repair and osteoporosis by reducing bone resorption whilst promoting bone formation.

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