Structural characteristic, pH and thermal stabilities of apo and holo forms of caprine and bovine lactoferrins

Abstract

Apo and holo forms of lactoferrin (LF) from caprine and bovine species have been characterized and compared with regard to the structural stability determined by thermal denaturation temperature values (T_m), at pH 2.0-8.0. The bovine lactoferrin (bLF) showed highest thermal stability with a T_m of 90±1°C at pH 7.0 whereas caprine lactoferrin (cLF) showed a lower T_m value 68±1°C. The holo form was much more stable than the apo form for the bLF as compared to cLF. When pH was gradually reduced to 3.0, the T_m values of both holo bLF and holo cLF were reduced showing T_m values of 49±1 and 40±1°C, respectively. Both apo and holo forms of cLF and bLF were found to be most stable at pH 7.0. A significant loss in the iron content of both holo and apo forms of the cLF and bLF was observed when pH was decreased from 7.0 to 2.0. At the same time a gradual unfolding of the apo and holo forms of both cLF and bLF was shown by maximum exposure of hydrophobic regions at pH 3.0. This was supported with a loss in α-helix structure together with an increase in the content of unordered (aperiodic) structure, while β structure seemed unchanged at all pH values. Since LF is used today as fortifier in many products, like infant formulas and exerts many biological functions in human, the structural changes, iron binding and release affected by pH and thermal denaturation temperature are important factors to be clarified for more than the bovine species.