Effects of Cortisol and Tetrahydrocortisol on the Secondary Structure of Apolipoprotein A-I as Measured by Fourier Transform Infrared Spectroscopy
The interaction of the steroid hormone cortisol and its metabolite tetrahydrocortisol (THC) with two serum proteins – apolipoprotein A-I (apoA-I) and human serum albumin (HSA) was studied in aqueous solutions at physiological pH. Using the tryptophan fluorescence quenching method, it was found that the binding constant of hormones to proteins increases in the order: apoA-I−THC < HSA−THC < apoA-I−cortisol < < HSA−cortisol. The results confirm the existing point of view that albumin and apoA-I play the important role in the transport of active forms of steroid hormones. However, the use of Fourier transform infrared spectroscopy revealed the most pronounced and multidirectional changes in the secondary structure of proteins under the influence of an inactive form of the hormone − THC. Upon incubation of apoA-I in the presence of THC, a dose-dependent decrease in the content of α-helices and an increase in β-turns were observed. On the contrary, in HSA incubated with THC, the number of α-helices increased, and β-turns decreased. Similar changes were found in the structure of apoA-I incubated with cortisol. The secondary structure of HSA in the presence of cortisol did not change. It is assumed that conformational changes in apoA-I under the influence of steroid hormones can lead to a significant change in the acceptor and regulatory properties of the protein.