Thermal unfolding of albumin in aqueous solutions proceeding under different
physico-chemical conditions has been investigated using differential scanning calorimetry
(DSC). The dependence of the observed conformational restructuring on the kind of
albumin (human, bovine), its form (fatty acid free and nondefatted), the properties of
solvent (water, ethanol solutions), ionic strength, pH, protein concentration and experimental
conditions has been discussed in this work. The endothermic unfolding transition
has been shown to be modified by time changes and changes induced by such environmental
factors as radio frequency radiation or UV radiation.
Considering the process of albumin thermal unfolding within the equilibrium thermodynamics,
a deconvolution of DSC traces have been performed using the appropriate
mathematical models. Structurally independent subunits revealed during thermal denaturation
of albumin has been found pH dependent. At pH range corresponding to the N
form of albumin these subunits could be correlated with three albumin domains for human
albumins and nondefatted bovine albumin. Under the same conditions two subunits
have been revealed for fatty acid free bovine albumin: C-terminal fragment containing
domain III and the greater part of domain II and the N-terminal fragment containing
domain I and the smaller part of domain II.
DSC study of albumin in ethanol solutions has revealed stronger binding of ethanol
to defatted than to nondefatted albumin. The interaction of ethanol with fatty acid binding
sites located in subdomain IIA has been confirmed. Ethanol has been observed to
be a stabilizer of the folded state of albumin at a lower concentration contrary to the
high denaturant concentration where its binding to the unfolded protein predominates.
The obtained results indicate that the influence of radiofrequency radiation (from
several to tens MHz) on albumin unfolding events could be detected using ultrasensitive
microcalorimeter. That influence is not observed immediately, however, the differences
between DSC profiles for irradiated and nonirradiated albumin solutions have appeared
during their storage. The changes in irradiated samples outpace nonirradiated ones.
Calorimetric and spectroscopic results have shown the conformational restructuring
of albumin under UV irradiation. The differences in response to UV radiation between
nondefatted and fatty acid free albumins have been found. Albumin devoid of endoge-nous fatty acids has been suggested to be more susceptible to aggregation caused by
UV A—C as well as 254 nm UVC radiation. DSC curve deconvolution results allow to
conclude that the C-terminal fragment of albumin macromolecule, containing domain
III, is the most liable part to UV radiation.
The studies presented by the author have revealed the fundamental role of the presence
of fatty acids for the thermal stability, conformational rearrangement and binding
properties of albumin macromolecule.