102 DAIRY CHEMISTRY AND BIOCHEMISTRYmembrane directly but rather the electron-dense cytoplasmic face material;
which constituents of the latter recognize and interact with constituents on
the droplet surface are not known. Immunological and biochemical studies
have shown that butyrophilin and xanthine oxidase, two of the principal
proteins in the MFGM, are major constituents of the electron-dense
material on the cytoplasmic face of apical plasma membrane. Butyrophilin,
a hydrophobic, transmembrane glycoprotein that is characteristic of milk-
secreting cells, is concentrated highly at the apical surface of these cells; it
binds phospholipids tightly, and is believed to be involved in mediating
interaction between lipid droplets and apical plasma membrane. Xanthine
oxidase is distributed throughout the cytoplasm, but appears to be enriched
at the apical cell surface.
In the secretion process, milk fat globules usually are enveloped compact-
ly by apical plasma membrane, but closure of the membrane behind the
projecting fat droplet occasionally entrains some cytoplasm as a so-called
crescent or signet between the membrane and the droplet surface. These
crescents can vary from thin slivers of cellular material to situations in which
the crescent represents a greater volume than does the globule core lipid.
Except for nuclei, cytoplasmic crescents contain nearly all membranes and
organelles of the milk-secreting cell. Globule populations with a high
proportion of crescents exhibit a more complex pattern of proteins by
SDS-PAGE than low-crescent populations. Presumably, the many addi-
tional minor bands arise from cytoplasmic components in crescents. Cres-
cents have been identified in association with the milk fat globules of all
species examined to date, but the proportion of globules with crescents
varies between and within species; about 1% of globules in bovine milk
contain crescents.
Thus, the fat globules are surrounded, at least initially, by a membrane
typical of eukaryotic cells. Membranes are a conspicuous feature of all cells
and may represent 80% of the dry weight of some cells. They serve as
barriers separating aqueous compartments with different solute composition
and as the structural base on which many enzymes and transport systems
are located. Although there is considerable variation, the typical composi-
tion of membranes is about 40% lipid and 60% protein. The lipids are
mostly polar (nearly all the polar lipids in cells are located in the mem-
branes), principally phospholipids and cholesterol in varying proportions.
Membranes contain several proteins, perhaps up to 100 in complex mem-
branes. Some of the proteins, referred to as extrinsic or peripheral, are
loosely attached to the membrane surface and are easily removed by mild
extraction procedures. The intrinsic or integral proteins, about 70% of the
total protein, are tightly bound to the lipid portion and are removed only
by severe treatment, e.g. by SDS or urea.
Electron microscopy shows that membranes are 79 nm thick, with a
trilaminar structure (a light, electron-sparse layer, sandwiched between two