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Biological Chemistry

Mining The Milk Proteome

Functional Fluid: Comparison of whey from colostrum and mature milk highlights the complex role of whole proteins

by Sarah Webb
November 29, 2010

BOVINE BURDEN
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Eating less dairy foods could reduce excess nitrogen runoff that creates coastal dead zones
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A cow nurses her newborn calf.

A complex mixture of proteins, fat, and carbohydrates, milk is the perfect food for a mammalian mother to feed her baby. But this fluid provides more than nutrients: Milk confers a variety of health benefits, including immune protection to offspring. In a new study of colostrum—the milk produced for a newborn calf—and mature milk from cows, researchers have nearly doubled the number of known whey proteins, providing insights that could aid the development of new food products (J. Proteome Res., DOI: 10.1021/pr100884z).

Over the past 15 years, researchers have used proteomics to catalog the collection of low-abundance proteins in milk, looking for chemical clues to its biological effects. Qiang Zhang and his colleagues at Mead Johnson Nutrition, an infant formula manufacturer in Evansville, Indiana, wanted to know how the protein composition of milk changes from the colostrum to mature milk for a three-month-old calf. After separating the watery whey from fat and insoluble casein, the researchers isolated the proteins using ion exchange chromatography. They then digested the proteins with the protease trypsin into peptides and analyzed them using tandem mass spectrometry.

Of the nearly 300 whey proteins identified in this study, the researchers found more than two thirds of them in both colostrum and mature milk. Although other research had compared these two types of milk, this paper is the first to identify a large enough sample of proteins to definitively show that the two are largely similar, Zhang says. A number of these newly discovered, shared proteins are proteases or protease inhibitors. Zhang thinks that the proteases could activate latent milk proteins and the inhibitors could help those activated proteins survive the digestive enzymes of the stomach. He and his colleagues also examined 32 proteins that showed significantly different concentrations in colostral whey versus that of mature milk, many of which are antibodies or other immune proteins, Zhang adds.

The research is "another significant step in understanding the complexity of milk," says Tom Wheeler of AgResearch, a New Zealand government-owned research company. To determine how these newly identified proteins could boost baby immune systems, he says, researchers must study their biological functions and how they interact with other structures within milk, including whole cells, fat globules, and casein micelles.

Zhang and his colleagues are currently working on a similar study of human breast milk. He says that those data should help researchers develop new infant formula that more closely mimics mother's milk.

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