What are the structure and the biological function of lactoferrin in human breast milk?

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Is There An Answer? is intended to serve as a forum inwhich readers to IUBMB Life may pose questions of thetype that intrigue biochemists but for which there may beno obvious answer or one may be available but not widelyknown or easily accessible.Readers are invited to e-mail f.vella@sasktel.net if theyhave questions to contribute or if they can provide answersto questions that are provided here from time to time. Inthe latter case, instructions will be sent to interestedreaders. Answers should be, whenever possible, evidence-based and provide relevant references.–Frank VellaLactoferrin (Lf) is an iron-binding glycoprotein of thetransferrin family that was first discovered in bovine milk andisolated later from human milk (1). In humans it is found inmost exocrine fluids, such as milk, saliva, bile, tears, andpancreatic fluid (2). The level in milk depends on the lactationperiod and may vary from 7 mg/ml in colostrum to 1 mg/ml inmature milk.Human breast milk Lf (hmLf) is a single polypeptide chainof 692 amino acids with a molecular mass of *80 kDa. Thechain is folded into two globular lobes (N-lobe, residues 1-333;C-lobe, residues 345-692) with very similar folds and linked bya three-turn a helix (Fig. 1) (3). Each lobe is organized intotwo a/b domains N1, N2 and C1, C2, and contains one iron-binding site and one glycosylated site to which N-linkedglycan residues are attached. Lf displays stronger affinity forand greater stability of iron binding than transferrin, due inpart to cooperative interactions between the two lobes. Thebinding of iron is accompanied by major conformationalchanges of the lobes. Thus, in its iron-free form, Lf becomes amore ‘open’ molecule which is more sensitive to proteolysisthan the iron-saturated ‘closed’ form. Unlike transferrin, Lf ishighly basic (pI *9) due in part to a unique basic region (fourarginines) in the N-terminal region of the molecule. Thesefeatures explain its ability to bind to a large variety of eithersoluble or membrane bound molecules.In the mammary gland Lf is synthesized by epithelial cellsand its expression is under the control of prolactin. Theprotein passes through the gastrointestinal tract and isremarkably resistant to proteolytic degradation especially inits iron-saturated form. The amount of Lf surviving digestionin the gastrointestinal tract as intact protein decreases with theage of the neonate (4). Intact protein and fragments thereofhave been found in the faeces of newborn infants (5). Apepsin-derived peptide called lactoferricin (Lfcin, residues 1–47) with antimicrobial activity can be generated from Lfin vitro (6) and the presence of peptides containing this domainin the human stomach has been reported (7). However,because of incomplete development of the digestive system ofnewborns it is not known to what extent this peptide isgenerated in vivo.It is generally accepted that human breast milk providesgood nutrition and protection against infection and isimportant for the normal development of the newborn (8).Does Lf contribute to any or all of these beneficial effects? Lfwas at first thought to play a role in intestinal absorption ofiron, due to its strong iron binding capacity, and the greaterbioavailability of iron and abundant concentrations of Lf inbreast milk. In addition, a specific receptor for Lf on the smallintestinal surface has been identified and cloned (9). It is aglycoprotein with a molecular mass of 114 kDa. However, theinvolvement of Lf in intestinal iron absorption is controver-sial. Thus, in vitro experiments on CaCo-2 intestinal cellstransfected with Lf receptor revealed a higher iron uptakecompared to mock-transfected cells (10). In contrast, studies inhuman infants showed that removal of Lf from breast milkenhanced the bioavailability of breast milk iron (11). Since thedivalent metal transporter 1 (DMT-1) molecule is essential for