Type of Document Master's Thesis Author Deeney, Jude T. URN etd-11142012-040236 Title Micro lipid droplet precursors of milk lipid globules Degree Master of Science Department Biochemistry and Nutrition Advisory Committee
Advisor Name Title Keenan, Thomas W. Committee Chair Bevan, David R. Committee Member Gregory, Eugene M. Committee Member Smith, David F. Committee Member Keywords
Date of Defense 1985-01-05 Availability restricted AbstractThe lipid in milk (milk fat) is found in the form of droplets
known as milk lipid globules (MLG). These milk lipid globules are
encompassed by a unit membrane known as the milk lipid globule
membrane (MLGM) which is derived from the apical plasma membrane of
the mammary epithelial cell during secretion. In lactating mammary
epithelial cells, immediate precursors of milk lipid globules appear
to be cytoplasmic lipid droplets (CLD). These cytoplasmic lipid
droplets have diameters >l μm and are characterized by an electron
dense, granular surface coat. A previously unrecognized group of
structures with diameters <.5 μm, which resemble cytoplasmic lipid
droplets in matrix and surface coat appearance, has been observed.
The surface coat of these triacylglycerol containing structures,
termed micro lipid droplets (μLD), was similar to that of cytoplasmic
lipid droplets in enzyme and polypeptide composition. Morphological
evidence suggested that these small structures may originate from
rough endoplasmic reticulum (RER) and fuse with cytoplasmic lipid
droplets. Immunochemical studies showed homology of certain proteins
among the rough endoplasmic reticulum, micro lipid droplets and
cytoplasmic lipid droplets, which supported the possibility of an endoplasmic reticulum origin of these droplets. The rate of
incorporation of [l—
l4C]—palmitate and [1,2,3-3H]—glycerol into
lipid of RER, μLD, CLD and MIG fractions suggested a possible
translocation pathway of triacylglycerols from the rough endoplasmic
reticulum to cytoplasmic lipid droplets. The micro lipid droplets
seem to provide triacylglycerols to support growth of cytoplasmic
lipid droplets. In addition, morphological evidence suggested that
these micro lipid droplets can be secreted directly in a manner
similar to cytoplasmic lipid droplets, providing for the small lipid
globules in milk. Little is known concerning the biochemical processes
of milk lipid secretion but it is thought that butyrophilin, a
glycoprotein found in milk lipid globule membrane, may play a role.
After treatment of mammary epithelial cells with tunicamycin,
butyrophilin content of this membrane is reduced. Thus a method for
the study of the physiological role of this glycoprotein is proposed.
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