Variability of Serum Proteins in Chinese and Dutch Human Milk during Lactation

Abstract

To better understand the variability of the type and level of serum proteins in human milk, the milk serum proteome of Chinese mothers during lactation was investigated using proteomic techniques and compared to the milk serum proteome of Dutch mothers. This showed that total milk serum protein concentrations in Chinese human milk decreased over a 20-week lactation period, although with variation between mothers in the rate of decrease. Variation was also found in the composition of serum proteins in both colostrum and mature milk, although immune-active proteins, enzymes, and transport proteins were the most abundant for all mothers. These three protein groups account for many of the 15 most abundant proteins, with these 15 proteins covering more than 95% of the total protein concentrations, in both the Chinese and Dutch milk serum proteome. The Dutch and Chinese milk serum proteome were also compared based on 166 common milk serum proteins, which showed that 22% of the 166 serum proteins differed in level. These differences were observed mainly in colostrum and concern several highly abundant proteins. This study also showed that protease inhibitors, which are highly correlated to immune-active proteins, are present in variable amounts in human milk and could be relevant during digestion.

Keywords: digestive tract; immune-active proteins; mammary gland; protease inhibitors; proteases.

Figure 1

Total bicinchoninic acid (BCA) serum protein concentrations (g/L) in Chinese human milk per mother over a 20-week lactation period.

Figure 2

Serum protein composition in human milk of seven Chinese mothers over a 20-week lactation period, based on BCA equivalent values (g/L). The number after the M indicates the mother, and the numbers after the W (1 to 20) indicates the number of weeks postpartum.

Figure 3

Averaged BCA equivalent values (g/L) of serum proteins for human milk from seven Chinese mothers categorized per biological function over a 20-week lactation period.

Figure 4

Calculated Pearson correlation coefficients between the different functional groups of serum proteins in Chinese and Dutch human milk, using the summed BCA equivalent values (g/L) over lactation. (A) Chinese human milk and (B) Dutch human milk.

Figure 5

Comparison of the common serum proteins in Chinese and Dutch human milk during lactation. Green squares indicate the proteins displayed in Table 1. For each serum protein in Chinese and Dutch human milk over the course of lactation, a regression line was fitted, summarizing the profile for each protein into an intercept (representing week 1) and slope (representing rate of change over lactation). These profiles were used for comparison between Chinese and Dutch human milk, and the p-values for differences between them were plotted. (A) Significantly different proteins in Chinese and Dutch human milk over the course of lactation, based on difference in slope; (B) significantly different proteins in Chinese and Dutch human milk at week 1, based on intercept; and (C) no significant difference.