Dairy Chemistry And Biochemistry

MILK PROTEINS 209

CYIOPLASM

lntrlcate network ol channels

ANS SLAT ION SEGREGATION MODIFICATION CONCENTRATION PACKAGING STORAGE EXOCUOSIS]

Figure 4.35 Schematic representation of the intracellular transport of proteins in mammary

cells (from Mepham, Gaye and Mercier, 1982).

Alternatively, protein granules are transported through the lumina of a

contiguous sequence of vesicles, so that only the most apical vesicle fuses

with the apical membrane (Figure 4.34). The process has been called

compound exocytosis.

Thus, the synthesis and secretion of milk proteins involves eight

steps: transcription, translation, segregation, modification, concentration,

packaging, storage and exocytosis, as summarized schematically in Figure

4.35.

4.14.7 Secretion of immunoglobulins

Interspecies differences in the relative importance of colostral Igs are
discussed in section 4.10. The IgG of bovine colostrum is derived exclusively
from blood plasma. It is presumed that cellular uptake involves binding of
IgG molecules, via the Fc fragment (Figure 4.28), to receptors situated in
the basal membranes; just prior to parturition, there is a sharp increase in
the number of such receptors showing a high affinity for IgG,, which is
selectively transported into bovine colostrum. The intracellular transport
route has not been described with any degree of certainty, but the most