Dairy Chemistry And Biochemistry

MILK PROTEINS 195

Figure 4.26 Calcium-binding loop in bovine a-lactalbumin (modified from Berliner et a/., 1991).

4.9 Blood serum albumin

Normal bovine milk contains a low level of blood serum albumin (BSA)
(0.1-0.4gl-'; 0.3-1.0% of total N), presumably as a result of leakage from
blood. BSA is quite a large molecule (molecular mass c. 66 kDa; 582 amino
acids); its amino acid sequence is known. The molecules contain 17
disulphides and one sulphydryl. All the disulphides involve cysteines that are
relatively close together in the polypeptide chain, which is therefore organ-
ized in a series of relatively short loops, some of which are shorter than
others (Figure 4.27). The molecule is elliptical in shape and is divided into
three domains.
In blood, BSA serves various functions but it is probably of little
significance in bovine milk, although it does bind metals and fatty acids; the
latter characteristic may enable it to stimulate lipase activity.

4.10 Immunoglobulins (Ig)

Mature milk contains 0.6-lg Igl-' (c. 3% of total N) but colostrum
contains up to 1OOgl-', the level of which decreases rapidly postpartum
(Figure 4.2).