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Milk Composition & Synthesis
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Milk Synthesis

Protein Profiles in Milks of Various Species

The image below is of a polyacrylamide elecrophoretic gel used to separate proteins by molecular mass. This particular type of gel involves reducing and denaturing the proteins. This results in the disassociation of multi-peptide proteins such as the immunoglobulins. In addition,the process essentially causes each protein to have the same mass-to-charge ratio. That means each protein will migrate through the gel in response to an electrical potential accross the gel. Rate of migration through the gel is dependent on the molecular mass of the protein. After the gel is run, the proteins are visualized by staining with a dye. Larger mass proteins will migrate through the gel slower than smaller proteins and will be nearer the top of the gel. Mammary secretion samples shown here are collected from (by lane) :

  • 1 - a dry cow
  • 2 - lactating cow
  • 3 - lactating goat
  • 4 - lactating rabbit
  • 5 - lactating mouse
  • 6 - Tammar Wallaby phase 2 of lactation
  • 7 - Tammar Wallaby phase 3 of lactation
  • 8 - mammary tissue from a bat.

The protein bands (in kilodaltons or kDa) in lane 1 (from the top) are lactoferrin (~80 kDa), serum albumin (~66 kDa), heavy chain of IgG (the faint broad band at ~55 kDa), -casein and a-casein (between 32 and 25 kDa), -lactoglobulin (~18 kDa), and a-lactalbumin (~14 kDa).

The bands in the lactating cow sample (lane 2) correspond to -casein and a-casein (between 32 and 25 kDa), -lactoglobulin (~18 kDa), and a-lactalbumin (~14 kDa), from the top respectively.

Note that the goat has a similar band profile (lane 3), although the caseins migrate a little differently compared with the cow caseins (in lane 2).

Rabbit milk (lane 4) proportionallyhas a lot of serum albumin (~66 kDa) and caseins (around 29 kDa), but less a-lactalbumin that shows up with this stain.

Mouse milk (lane 5) has an extra casein band at about 48 kDa in addition to the less dense casein bands around 29 kDa, while the mouse a-lactalbumin is actually closer to about 16-18 kDa and does not stain very dark here.

Tammar Wallaby phase 2 milk sample represents the period shortly after birth of the joey, while the phase 3 milk sample represents the phase when the Joey has left the pouch but is still nursing; note the switch to a somewhat different profile of proteins between phase 2 and 3.

The bat milk appears to have a lot of casein and probably the -lactoglobulin (~18 kDa) and a-lactalbumin (~14 kDa) bands, plus an extra band at about 50 kDa. I am unsure of the exact identity of the bat "milk" bands.

Milk Composition & Synthesis