Understanding human milk proteins is important for improving newborn infants’ health because human milk is their main food source. Alpha-lactalbumin, a dominant protein in human milk, can be enzymatically degraded into small peptides that are beneficial to humans. However, alpha-lactalbumin is thought to be resistant to digestion in a newborn infant’s stomach, due to the newborn infant’s limited gastric acid secretion and low activity of the only gastric enzyme pepsin. In contrast, our research group recently found peptides released from alpha-lactalbumin at pH 4 (typical infant stomach pH), which suggested an alternative mechanism. This study focuses on whether cathepsin D, a maternal enzyme found in human milk, is responsible for the hydrolysis of alpha-lactalbumin, and how pH affects their interaction. Human alpha-lactalbumin was incubated with cathepsin D in triplicates at physiologically relevant pH ranging from 2 to 7 for 2 hours. Peptides were extracted by C18 solid-phase extraction. The concentration of total peptides was quantified by fluorometric peptide essay, and sequences of peptides were identified using mass spectrometry. Most alpha-lactalbumin-derived peptides were released at pH 3 and 4 groups. The cleavage sites of these peptides correspond to the reported cleavage patterns of cathepsin D, which are between hydrophobic amino acids such as leucine and phenylalanine. Furthermore, several peptides derived from alpha-lactalbumin matched the <i>in vivo</i> data published previously analyzing digested human milk samples obtained from infant stomach. The findings of this study provide mechanistic understanding of how alpha-lactalbumin interacts with cathepsin D at weak acidic pH, and emphasize the important role of milk enzymes in maintaining infant health.

Interaction between alpha-lactalbumin and human milk enzyme cathepsin D reveals specific peptide release at infant stomach pH

2019 Spring National Meeting

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