Enzymes in Human Milk

Abstract

Milk proteins are a complex and diverse source of biological activities. Beyond their function, intact milk proteins also act as carriers of encrypted functional sequences that, when released as peptides, exert biological functions, including antimicrobial and immunomodulatory activity, which could contribute to the infant's competitive success. Research has now revealed that the release of these functional peptides begins within the mammary gland itself. A complex array of proteases produced in mother's milk has been shown to be active in the milk, releasing these peptides. Moreover, our recent research demonstrates that these milk proteases continue to digest milk proteins within the infant's stomach, possibly even to a larger extent than the infant's own proteases. As the neonate has relatively low digestive capacity, the activity of milk proteases in the infant may provide important assistance to digesting milk proteins. The coordinated release of these encrypted sequences is accomplished by selective proteolytic action provided by an array of native milk proteases and infant-produced enzymes. The task for scientists is now to discover the selective advantages of this protein-protease-based peptide release system.

Figure 1

Ion abundances in a) intact human milk and b) gastric aspirates from human infants one hour after consuming the same human milk. Samples were run on an Agilent (Santa Clara, CA) nano-LC-chip-Q-TOF MS/MS (Chip-Q-TOF) with an Agilent chip C18 column. Total ion abundance extracted chromatograms are shown for representative samples.

Figure 2

Annotated Ion chromatogram of samples from human stomach aspirates. Peptides were identified by tandem mass spectrometry and mapped to specific proteins according to libraries established through stepwise construction (). Samples were run on an Agilent (Santa Clara, CA) nano-LC-chip-Q-TOF MS/MS (Chip-Q-TOF) with an Agilent chip C18 column.

Figure 3

Schematic of production and transfer of enzymes into milk within the lactating mammary gland. Enzymes can reach milk by direct protein synthesis by the epithelial cell, by translocation from blood and from secretion from immune cells within the mammary gland and milk itself.