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

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 (Tm), at pH 2.0–8.0. The bovine lactoferrin
(bLF) showed highest thermal stability with a Tm of
90 ± 1�C at pH 7.0 whereas caprine lactoferrin (cLF)
showed a lower Tm 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 Tm values of both holo bLF and holo cLF were
reduced showing Tm 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 pHwas 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 a-helix structure
together with an increase in the content of unordered
(aperiodic) structure, while b 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.