Impact of the degree of prematurity and time post-partum on proteases and antiproteases in human milk and the infant stomach

Veronique Demers-Mathieu, Oregon State University, Corvallis, USA

Veronique Demers-Mathieu1, Søren Drud Nielsen1, and David Dallas1
1. Nutrition Program, School of Biological and Population Health Sciences, College of Public Health and Human Sciences, Oregon State University, Corvallis

Human milk contains a complex system of proteases and protease inhibitors. This proteolytic system provides an apparently controlled and protein-selective proteolytic digestion in the mammary gland and the infant stomach. This balance of proteases and antiproteases may allow for assistance with protein digestion during the early stages of the infant’s digestive development while also allowing immune-related proteins like lactoferrin and the immunoglobulins to remain intact and functional. Protease activity is likely to differ across the degree of prematurity because of the physiological maturation differences; however, the degree to which specific proteases and inhibitors differ across prematurity remain unexplored.

Our research aim was to determine the degree to which the concentration and activity of milk’s proteases (plasmin, kallikrein, thrombin, elastase, cathepsin D, cytosol aminopeptidase and carboxypeptidase B2) and protease inhibitors (α1-antitrypsin, α1-antichymotrypsin, α2-antiplasmin, plasma serine protease inhibitor and antithrombin III) differ between human milk and the infant stomach, as well as across the spectrum of prematurity and across time post-partum. Human milk and infant gastric samples were collected from two groups of mother-infant pairs: 10 early preterm (24–26 wks of gestational age at birth (GA) and 620–770 g birth weight) and 10 late preterm (27–32 GA wks and 900–2,210 g birth weight). The activity level of each milk protease was determined by fluorometric assays, and the concentrations of these enzymes as well as gastric pepsin and the protease inhibitors were determined by ELISA assays. Total protease activity, pH and protein concentrations were also determined in these samples. Concentrations of immune-related proteins (IgA and lactoferrin) were determined by ELISA assays. Carboxypeptidase B2 was the most abundant and active protease in human milk, kallikrein was the second and plasmin the third, followed by cathepsin D, thrombin and elastase. These milk proteases were far more active and abundant than pepsin in the preterm infant stomach (milk enzymes accounted for 83-88% of total protease activity in the stomach). The early preterm infant stomach contained lower pepsin and cytosol aminopeptidase concentrations and lower plasmin, elastase and thrombin activities than the late preterm infant stomach for at least 3 weeks postpartum. However, the late preterm infant stomach had higher α1-antitrypsin concentration than in the early infant stomach, which may affect overall protein digestive capacity (all other protease inhibitor concentrations were similar between early and late preterm infants). Lactoferrin and IgA concentrations were higher in the early than late preterm milk. Across time post-partum (1 wk vs. 3 wk), in human milk, activities of plasmin, thrombin and cytosol aminopeptidase and concentrations of α1-antitrypsin, α1-antichymotrypsin, α2-antiplasmin decreased while activities of kallikrein and elastase increased. In addition, in the stomach, activity of kallikrein and elastase increased across time post-partum. Lactoferrin concentration decreased across time; however, IgA concentration remained stable, suggesting some protection from gastric digestion. Future work will examine further how protein digestion changes across gestational age and time post-partum.

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