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Milk Proteins Are Predigested Within the Human Mammary Gland

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Abstract

Previous work demonstrates that proteases present in human milk release hundreds of peptides derived from milk proteins. However, the question of whether human milk protein digestion begins within the mammary gland remains incompletely answered. The primary objective of this study was to determine whether proteolytic degradation of human milk proteins into peptides begins within the mammary gland. The secondary objectives were to determine which milk proteases participate in the proteolysis and to predict which released peptides have bioactivity. Lactating mothers (n = 4) expressed their milk directly into a mixture of antiproteases on ice followed by immediate freezing of the milk to limit post-expression protease activity. Samples were analyzed for their peptide profiles via mass spectrometry and database searching. Peptidomics-based protease prediction and bioactivity prediction were each performed with several different approaches. The findings demonstrate that human milk contains more than 1,100 unique peptides derived from milk protein hydrolysis within the mammary gland. These peptides derived from 42 milk proteins and included 306 potential bioactive peptides. Based on the peptidomics data, plasmin was predicted to be the milk protease most active in the hydrolysis of human milk proteins within the mammary gland. Milk proteases actively cleave milk proteins within the mammary gland, initiating the release of functional peptides. Thus, the directly breastfed infant receives partially pre-digested proteins and numerous bioactive peptides.

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Acknowledgements

The authors thank Melinda Spooner for assisting in collecting the human milk samples and Cora J. Dillard for editing the manuscript. SDN and DCD planned the study. SDN collected the samples. SDN and RB conducted the experiments and data analysis. SDN, RB and DCD prepared the manuscript. All authors read and approved the final manuscript. The authors acknowledge the Mass Spectrometry Center at Oregon State University, which is supported in part by the National Institute of Health grant S10OD020111 (CSM).

Funding

Supported by the K99/R00 Pathway to Independence Career Award, Eunice Kennedy Shriver Institute of Child Health & Development of the National Institutes of Health (R00HD079561) (DCD).

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Authors and Affiliations

  1. Nutrition Program, School of Biological and Population Health Sciences, College of Public Health and Human Sciences, Oregon State University, 118G Milam Hall, Corvallis, OR, 97331, USA

    Søren D. Nielsen, Robert L. Beverly & David C. Dallas

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  1. Søren D. Nielsen
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  2. Robert L. Beverly
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  3. David C. Dallas
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Correspondence to David C. Dallas.

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Ethical Approval

This present study was approved and carried out in accordance with the guidelines from the Institutional Review Board at Oregon State University. All mothers gave their informed consent.

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The authors declare that they have no conflict of interest.

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Nielsen, S.D., Beverly, R.L. & Dallas, D.C. Milk Proteins Are Predigested Within the Human Mammary Gland. J Mammary Gland Biol Neoplasia 22, 251–261 (2017). https://doi.org/10.1007/s10911-018-9388-0

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  • DOI: https://doi.org/10.1007/s10911-018-9388-0

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