[Advances in the knowledge about human milk proteins]

The mammary gland and maternal milk are the product of millions of years of evolution that resul ted in an optimal composition that sustains the growth and development of newborns and infants. Maternal milk supports the growth, adaptation and survival of this immature organism. Recent studies have detected 1606 different proteins in human milk, most of them synthesized in the acini of the glandular tissue while others originate from distant organs such as the lymphoid tissue and the digestive tract. Maternal milk enzymes modify its proteins and liberate peptides with antimicrobial, antihypertensive or stimulatory activities. This proteolytic activity occurs at specific sites in peptide chains. To prevent the extemporaneous activation of these proteolytic enzymes, that would result in inflammatory processes, maternal milk also contains inhibitory peptides that together with the stimulatory peptides conform a complex regulatory system. Some enzymes in maternal milk main tain their activity in the gastrointestinal tract of infants and compensate for the decreased activity of digestive tract enzymes in newborns. Thus, the milk enterokynase stimulates the release of pancreatic proteases as it induces the liberation of cholecystokynin/pancreozymin. The bile salt-activated lipase of human milk is activated in the duodenum by the infants' bile salts and partially compensates for the low levels of pancreatic lipase in newborns. These milk enzymes probably contribute to the nutrition of premature infants as they increase the availability of amino acids and peptides in their upper gastrointestinal tract; furthermore, as their intestinal epithelium is more permeable to peptides and partially digested protein this may help induce immune tolerance. The most relevant issues in the physiology and composition of the maternal milk are presented in this review.