Identification of lactoferrin complexes in bovine mammary secretions during mammary gland involution

Part of the antimicrobial activity of lactoferrin resides in its ability to bind to bacteria. The complexing of lactoferrin with other proteins could alter its activity. This study identified the presence of lactoferrin complexes in mammary secretions during mammary gland involution and determined the proportion of free and complexed lactoferrin in mammary secretions. Mammary secretions were collected from Holstein cows on d 7, 14, and 21 of involution, Proteins were fractionated from defatted, filtered mammary secretions by sucrose density gradient ultracentrifugation and by gel filtration chromatography. Proteins contained in separated fractions were identified by SDS-PAGE. The presence of lactoferrin was confirmed by immunoblot analysis. Lactoferrin was present as complexed forms of high molecular mass in mammary secretions at each day of involution. The majority of lactoferrin was present in complexes of higher molecular mass rather than as monomers. A majority of lactoferrin existed in fractions of approximately 250 kDa, although peaks of lactoferrin at 150, 300, and 800 kDa were also found. The presence of lactoferrin complexes may result from interactions with casein or immunoglobulins or from the formation of lactoferrin multimers in the secretions. The interaction of lactoferrin with other proteins in mammary secretions during involution may affect the antimicrobial properties of lactoferrin.