Predicting the important enzymes in human breast milk digestion

Abstract

Human milk is known to contain several proteases, but little is known about whether these enzymes are active, which proteins they cleave, and their relative contribution to milk protein digestion in vivo. This study analyzed the mass spectrometry-identified protein fragments found in pooled human milk by comparing their cleavage sites with the enzyme specificity patterns of an array of enzymes. The results indicate that several enzymes are actively taking part in the digestion of human milk proteins within the mammary gland, including plasmin and/or trypsin, elastase, cathepsin D, pepsin, chymotrypsin, a glutamyl endopeptidase-like enzyme, and proline endopeptidase. Two proteins were most affected by enzyme hydrolysis: β-casein and polymeric immunoglobulin receptor. In contrast, other highly abundant milk proteins such as α-lactalbumin and lactoferrin appear to have undergone no proteolytic cleavage. A peptide sequence containing a known antimicrobial peptide is released in breast milk by elastase and cathepsin D.

Figure 1

Representation of the total number of cleaved milk proteins per predicted enzyme. Each enzyme name on the X-axis is plotted against the total number of proteins it cleaves on the Y-axis.

Figure 2

Illustration of antimicrobial-like peptides released in vivo from β-casein in human milk. The peptide QELLLNPTHQIYPVTQPLAPVHNPISV shown at the top of the figure in gray has been discovered to be an antimicrobial peptide. This peptide is found at the C-terminus of β-casein (positions 199–225). Four overlapping peptides found to be released in vivo in human milk are represented under the β-casein sequence. The in vivo cleavage positions are indicated with gray lines. The enzymes predicted to be responsible for these cleavages are presented beside the gray lines.