β-Lactoglobulin and α-lactalbumin as potential modulators of mammary cellular activity

Summary

It has been observed that β-lactoglobulin of cow's milk inhibits the hydrolysis of p-nitrophenyl phosphate by phosphoprotein phosphatases from bovine spleen and lactating mammary gland. Kinetic studies indicate that the inhibition of p-nitrophenyl phosphate hydrolysis involves binding to the enzyme by β-lactoglobulin. Alkaline denaturation of the β-lactoglobulin molecule decreases the inhibition. This inhibition is reversed by Ca²⁺-binding to sites on β-lactoglobulin with K_D equal to 3 mM, or by salt concentration >100 mM.

α-Lactalbumin functions in a similar fashion with both the mammary and spleen enzymes. Since both these whey proteins and the milk phosphatase are secreted by Golgi, this in vitro reaction may serve as a model system for enzyme regulation in mammary secretory vesicles where the [Ca²⁺]_free concentration approaches 5 to 8 mM. Formation of enzyme-protein complexes in Golgi could in turn be effected by the binding of a ligand such as Ca²⁺ to “weak” sites. Thus protein—protein interactions which limit the activity of an enzyme may in turn be modulated by Ca²⁺ binding to the protein.