Archives ofDisease in Childhood 1992; 67: 657-661 657 REGULAR REVIEW Arch Dis Child: first published as 10.1136/adc.67.5.657 on 1 May 1992. Downloaded from Biological role of lactoferrin Lourdes Sanchez, Miguel Calvo, Jeremy H Brock Lactoferrin is an iron-binding protein closely bearing in mind when considering its possible related in structure to the serum iron transport biological function. protein transferrin. Unlike transferrin, only Human milk contains 3-6-12-5 ,umol/l of traces are normally present in serum, and it is iron, and of this only 60-70% is in the whey instead found mainly in milk and other external fraction, the remainder being in the lipid secretions, and in the secondary granules of fraction (11-20%) or bound to casein (2-14%). " neutrophils. Although lactoferrin was first iso- As a consequence, milk lactoferrin is only 6-8% lated 30 years ago, its biological role remains saturated with iron, presumably because of the unclear. Some aspects of its function were difficulty in gaining access to iron in the lipid discussed about 12 years ago in this journal,' fraction or casein micelles. and this review will attempt to reassess the function of lactoferrin in the light of the large amount of new information that has accrued Association of lactoferrin with other since then. molecules Knowledge of the structure of lactoferrin has Lactoferrin is very prone to binding to other been advanced by recent x ray crystallographic macromolecules. These include other milk studies, and the structure and iron binding proteins such as IgA, casein, secretory compo- properties of lactoferrin are reviewed in detail nent, albumin, lysozyme, and f1-lactoglobulin.'2 elsewhere.2 Briefly lactoferrin, like transferrin, Lactoferrin also binds to DNA.'3 A report of reversibly binds two ferric ions, for which multiple forms of lactoferrin in human milk, synergistic binding of an anion, usually bicar- some of which exhibit ribonuclease activity,'4 bonate or carbonate, is necessary. However, its may arise from an interaction between lactoferrin affinity constant for iron is 300 times greater and milk ribonuclease. Bovine milk ribonuclease http://adc.bmj.com/ than that of transferrin, and even in the is, like lysozyme, a small basic protein (mole- presence of a competing iron chelator such as cular weight 13 600),'5 and some ribonuclease citrate it can retain iron down to pH 3 or less activity in human milk was also associated with while transferrin loses it at pH 5. Unlike a molecule of this size. 14 So far no clear transferrin, lactoferrin is strongly basic. Human biological role has been established for this lactoferrin has been cloned and sequenced4 and propensity of lactoferrin to interact with other the recombinant protein expressed in baby proteins. hamster kidney cells.5 on September 28, 2021 by guest. Protected copyright. Biological functions of lactoferrin Lactoferrin in milk BACTERIOSTATIC ACTIVITY Human milk is particularly rich in lactoferrin, Almost all bacteria require iron for growth, and the concentration ranging from about 7 g/l in because of their iron sequestering properties, colostrum to about 1 g/l in mature milk, though the iron free (apo) forms of lactoferrin and it may rise again towards the end of lactation.6 7 transferrin are able to impede iron utilisation by A similar pattern is seen in the cow, but bacteria. A large number of studies, reviewed importantly the concentration in mid-lactation elsewhere,'6 have demonstrated a bacteriostatic Department of Food milk is very much lower, only about 0-1 g/l.8 effect and in some cases a bactericidal effect of Technology and Lactoferrin concentrations were normal in iron lactoferrin in vitro on a wide range of micro- Biochemistry, Veterinary Faculty, deficient mothers,9 but lower in mothers who organisms, including Gram positive and Gram University of Zaragoza, were generally malnourished,'0 which suggests negative bacteria, aerobes, anaerobes, and Spain that protein energy malnutrition rather than yeasts. However, mechanisms other than simple Lourdes Sanchez Miguel Calvo iron influences lactoferrin synthesis in the iron withholding may be involved in the anti- University Department mammary gland. Although lactoferrin is found bacterial action of lactoferrin, such as blockade of Immunology, in the milk of some other species, it is completely of microbial carbohydrate metabolism'" or Western Infirmary, absent in others, such as the rat, rabbit, and destabilisation of the bacterial cell wall, perhaps Glasgow dog.6 The milk of the rat and rabbit do however through binding of calcium and magnesium.'8 Jeremy H Brock contain significant amounts of transferrin, but Lactoferrin may synergise with other anti- Correspondence and reprint requests to: the milk of the dog contains neither protein, bacterial proteins such as lysozyme, which is Dr Lourdes Sanchez, even though the iron content is exceptionally also present in milk. In this case even the iron Department of Immunology, Western Infirmary, high.6 The fact that lactoferrin is absent from saturated form is active, and inhibition results Glasgow Gll 6NT. the milk of some species is a point worth from agglutination by lactoferrin of bacteria 658 Sanchez, Calvo, Brock whose cell wall has been modified by lysozyme. 19 unproved until the wealth of in vitro data Antibodies can also enhance the bacteriostatic showing antimicrobial activity is supported by action of lactoferrin,20 probably by blocking convincing in vivo data, particularly from production or uptake of microbial siderophores. clinical trials. The possibility that lactoferrin Arch Dis Child: first published as 10.1136/adc.67.5.657 on 1 May 1992. Downloaded from Lactoferrin may also possess antiviral activity, might exert a systemic protective effect also as it could protect mice against polycythaemia deserves further investigation, as studies in due to Friend virus.2' This appears to be an mice have shown that lactoferrin has a protective indirect effect, perhaps via a reduction in target effect in experimental E coli septicaemia that is cell proliferation, and is probably related to the independent of its iron content.27 proposed inhibitory role of lactoferrin in myelopoiesis (see below). In vivo, lactoferrin in milk might exercise its ROLE OF LACTOFERRIN IN INFLAMMATION AND inhibitory effect on microbial growth in the THE IMMUNE SYSTEM mammary gland, in the intestine ofthe newborn, A number of studies suggest that lactoferrin or both. Its role as a defence against infection in may mediate some of the effects of inflammation the human mammary gland appears not to have and have a role in regulating various components been investigated, but in cattle concentrations of the immune system. It was proposed some of lactoferrin increase during intramammary years ago that lactoferrin released by degranu- infection,22 suggesting a possible role as a lating neutrophils mediated the hyposideraemia mammary non-specific defence mechanism. of inflammation by removing iron from plasma Conditions in the bovine mammary gland im- transferrin and short circuiting it to macrophages mediately before parturition and during involu- of the reticuloendothelial system, where it was tion favour antimicrobial activity of lactoferrin incorporated into ferritin.28 However, iron as concentrations of bicarbonate are higher and uptake by macrophages from lactoferrin is at those of citrate lower than in milk.22 23 Thus best extremely slow,29 and there is no recycling lactoferrin might perform a role in preventing of the protein as occurs with transferrin, as infection of the mammary gland, particularly at lactoferrin that has bound to monocytes cannot parturition and involution. subsequently rebind to these cells.30 In addition, It has frequently been suggested that the the rate of exchange of iron between transferrin antimicrobial activity of lactoferrin plays a part and lactoferrin at physiological pH is likely to in the selection of the intestinal flora of the beextremely slow, and finallyneutrophil-derived newborn and preventing colonisation by entero- lactoferrin differs from milk lactoferrin, which pathogenic organisms. The conditions in the was used in most studies, in lacking the intestine ofthe newborn may be more favourable terminal fucose residues in its glycan chains that for lactoferrin than those in the lactating mam- are required for binding to macrophages.3' It mary gland, as citrate is rapidly absorbed and has also been found in mice that interleukin-1 intestinal fluid has a high concentration of induces hypoferraemia even in the presence of bicarbonate, although this function might be neutropenia,32 suggesting that lactoferrin is adversely affected by the proteolytic enzymes unimportant. Thus it now seems unlikely that http://adc.bmj.com/ present in the intestine, as discussed below. lactoferrin plays a significant part in the hypo- Nevertheless, despite the wealth of in vitro ferraemia of inflammation. data, attempts to establish an in vivo anti- Lactoferrin might also contribute to the microbial role for lactoferrin have generally bactericidal activity of neutrophils by two yielded disappointing results. The much quoted opposing mechanisms. In the apo form it may experimental study of Bullen et al,20 which perform an iron withholding function and suggested a protective effect of lactoferrin in prevent growth of phagocytosed bacteria.33 On on September 28, 2021 by guest. Protected copyright. newborn guinea pigs infected with Escherichia the other hand iron-lactoferrin may provide