CHEMISTRY OF MILK

Lesson 12. FAT GLOBULE MEMBRANE PROTEINS THEIR PROPERTIES AND ROLE

Module 3. Milk proteins

Lesson 12
FAT GLOBULE MEMBRANE PROTEINS THEIR PROPERTIES AND ROLE

12.1 Introduction

Proteomics which involves in a large scale study of proteins especially with reference to their structure and function it has become possible to know the presence of some minor milk protein which are having a significant role in the physiological and abnormal functioning of the human system. Milk-fat globules originate near the basal region of the secretory cells as small droplets of fat. They migrate through the cytoplasm, gradually increasing in size, as the synthesis of triacylglycerol proceeds. The milk fat globules are secreted from the apical surface of the cell, surrounded by a membrane thin bilayer, the Milk Fat Globule Membrane (MFGM). MFGM are formed by a unique and quantitatively small subcategory of milk proteins (approximately 2–4% of total protein in human milk), the content of which is still largely unknown.

12.2 Milk Fat Globule Membrane Proteins

The MFGM proteins are having the nutraceutical and biological importance. Realizing the importance of these proteins there is an increase in the studies on MFGM. The MFGM is a rich source of membrane proteins, and applied proteomic analysis of these membrane proteins, has highlighted some of the possible signaling and secretory pathways used by the mammary gland .MFGM glycoproteins seem to contribute to the prevention of pathogenic organisms infections, being able to act as specific bacterial and viral ligands in the stomach of newborns, to prevent the attack of the intestinal mucosa The diversity of the glycans found in MFGM is thought to enable the glycoproteins to perform this function in the acidic environment of the stomach. It has been noted that some forms of gastric diseases such as peptic ulcer, chronic type B gastritis and gastric cancer can be attributed to the colonization of gastric mucosa by Helicobacter pylori Nondefatted and defatted MFGM preparations, given orally, caused equal healing effect on H. pylori infection of gastric mucosa in BALB/CA mice, leading to the conclusion that the major role in inhibition of H. pylori infection is played by the protein

12.3 Composition of Fat Globule Membrane Proteins

The composition of the fat globule membrane is not consistent and varies considerably depending on different factors. The membrane is predominantly protein aciousin nature. The fat globule membrane contains approximately 50% protein and accounts for about 1% of the total protein of the milk. There is also wide variation in the nature of these proteins several enzymes have been reported to be associated with this membrane. AT least 25 different enzymes have been reported to be isolated from the fat globule membrane which includes the 5’nucleotidase, alkaline phosphatase,acid phosphatase , aldolase, xanthine oxidase. Although a major portion of the membrane material contains higher proportion of enzymes, presently it is not possible to estimate the ratio of enzymatic and non-enzymatic components. A part from the enzymes the proteins of the fat globule membrane protein exists as polypeptides of varying molecular weight. These micro lipid droplets are encircled by a special membrane composed of lipid bilayer and proteins. This membrane has been designated the milk fat/lipid globule membrane The MFGM originates from the plasma membrane of the mammary gland secretory cells by extrusion of lipid droplet from the cell. Because each lipid droplet in milk has its own membrane envelope,i.e., the MFGM, the lipid droplets are present in milk in a dispersed form and, therefore, do not aggregate with each other.Only after destruction of the structure of the MFGM through, for example, a mechanical force like churning, do the lipid droplets aggregate and subsequently form large fat clumps known as butter. Milk fat globule membrane is composed of proteins and lipids in a 1:1 weight ratio. Many of these proteins are present in the MFGM as glycoproteins.

The main protein of the MFGM is the glycoprotein butyrophilin (about 40% of the total proteins of the MFGM), and the second representative protein of the MFGM is xanthine oxidase, which comprises 12 to 13% of the total proteins. Other proteins are present in MFGM each at 5% or less. The physiological role of the MFGM proteins is not completely clear despite numerous research studies. However, there are some reasonable suggestions about the physiological function of these proteins in the cell because these proteins are an integral part of the plasma membrane of secretory mammary gland cell.

12.4 Role of Fat Globule Membrane Proteins

For the last 15 years, a great deal of knowledge hasbeen accumulated on health beneficial factors, protein andnon-protein, of bovine milk fat globule membrane (MFGM). Among thehealth beneficial components of the MFGM are cholesterolemia loweringfactor, inhibitors of cancer cell growth, vitamin binders, inhibitor of Helicobacter pylori, inhibitor of beta-glucuronidase of the intestinal Escherichia coli, xanthine oxidase as a bactericidal agent, butyrophilin as a possible suppressor of multiple sclerosis, andphospholipids as agents against colon cancer, gastrointestinal pathogens,Alzheimer’s disease, depression, and stress. All of the above compelus to consider bovine MFGM as a potential neutraceutical.