MFGM proteins

Attempts to Isolate and characterize MFGM proteins have not been completely successful due to their strong tendency to associate in most protein dissociating buffers. Thus, their role in stabilizing milk emulsions is not entirely understood (6,14), even though it is assumed that they are an integral part of the MFGM system. 

Xanthine oxidase, which requires Fe, Mo and flavin adenine dinucleotide (FAD) as co-factors, is capable of oxidizing lipids via the production of superoxide radicals. It represents about 20% of the MFGM protein and part is readily lost from the membrane, e.g. on cooling isoelectric focusing... 

Several of the minor proteins of the MFGM have been isolated and partially characterized (Keenan and Dylewski, 1995). A systematic nomenclature has not been developed for the MFGM proteins and most are referred to by their relative electrophoretic mobility on SDS-PAGE and whether or not they are glycoproteins. The proteins of the MFGM represent approximately 1 % of the total proteins in milk. 

The ADSA Committee on Milk Protein Nomenclature (Eigel et al. 1984) presented a tentative nomenclature for the new enzyme membrane proteins. While the primary structures of these proteins have not been established, sufficient information exists to obtain an operational definition. The total protein complement of the membrane as observed is dependent upon the past history of the membrane from its formation to its analysis. Both the temperature and the time of storage before analysis can alter the membrane composition and physical state (Wooding 1971). In addition, plasmin has been shown to be associated with preparations of the membrane, and proteolytic products of the membrane protein have been observed in milk (Hoffman et al. 1979 Kanno and Yamauchi 1979). Therefore, one should use fresh warm raw milk for the study of the native MFGM protein. 

Figure 3.9. SDS gel electrophoresis of MFGM proteins prepared by different washing procedures. S reference proteins phosphorylase B, bovine serum albumin, ovalbumin, carbonic anhydrase, trypsin inhibitor, a-lactal-bumin. (Courtesy of J. Basch and H. M. Farrell, Jr.)...

ADPH was not identified as a major MFGM protein until recently because it co-migrates with the glycoprotein known as PAS 6/7. ADPH was known previously as adipocyte differentiation-related protein (ADRP) because it is expressed early during adipocyte differentiation and was believed to be expressed only in adipocytes (Jiang and Serrero, 1992). Since then, ADPH has been detected in a large number of tissues and cell types, where it invariably is associated with intracellular lipid droplets (Heid et al., 1996 1998 Brasaemle et al., 1997). 

About 40 proteins with Mr 15-240 kdal (milk fat globule membrane proteins, MFGM proteins) are involved in the make-up of the membrane. Although their nutritional value with regard to the caseins and whey proteins is low, they receive... 

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