Dairy Chemistry And Biochemistry

MILK PROTEINS 191

Octamer

(pH3.5-5.5)

0

Monomer

(pH < 3.5)

Dimer

(PH 5.5-7.5)

I

0

Monomer

(pH > 7.5)

Figure 4.24 Effect of pH on the quaternary structure of 8-lactoglobulin.

Above pH 7.5, bovine 13-lg undergoes a conformational change (referred
to as the N PR transition), dissociates to monomers and the thiol group
becomes exposed and active and capable of sulphydryl-disulphide inter-
change. The association of p-lg is summarized in Figure 4.24.

4.7.7 Physiological function

Since the other principal whey proteins have a biological function, it has
long been felt that p-lg might have a biological role; it appears that this role
may be to act as a carrier for retinol (vitamin A). p-Lg can bind retinol in
a hydrophobic pocket (see Figure 4.23), protect it from oxidation and
transport it through the stomach to the small intestine where the retinol is
transferred to a retinol-binding protein, which has a similar structure to p-lg.
p-Lg is capable of binding many hydrophobic molecules and hence its
ability to bind retinol may be incidental. Unanswered questions are how
retinol is transferred from the core of the fat globules, where it occurs in
milk, to p-lg and how humans and rodents have evolved without p-lg.