Most of the lipid present in lactating mammary gland cytosol was
associated with a high molecular weight aggregate isolated from cytosol by gel
exclusion chromatography or by density gradient centrifugation. The major
polypeptide constituent of this lipoprotein aggregate was the monomer of fatty
acid synthase (FAS). The major milk lipid globule proteins, butyrophilin (8u)
and xanthine oxidase (XO) , as well as the small GTP-binding protein ARF, also
were present. This lipoprotein complex was abundant in cytosol from lactating
but not from involuting mammary glands. HPTLC analysis of lipids extracted
from the low density FAS (LDFAS) complex demonstrated the presence of the
five major milk phospholipids as well as triacylglycerols, cholesterol,
unesterified fatty acids, and diacylglycerols. 32P-labeled phospholipids present
in cytosol could be transferred to microlipid droplets (MLD) and endoplasmic
reticulum (ER), in vitro, and could be precipitated along with FAS, and other
polypeptide constituents of the LDFAS complex. Complexed FAS could be
separated from noncorrlplexed FAS by density gradient centrifugation, native
PAGE, and gel exclusion chromatography. A large amount of phospholipid consistently was retained with the complexed form of FAS. These results
suggest that FAS migrates to a low density fraction by virtue of its association
to other proteins and lipids.
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