Thermal stability of the human blood serum acid alpha(1)-glycoprotein in acidic media

Thermal stability of human alpha(1)-acid glycoprotein and its desialyzed form were studied in the pH range of 1.5-5.2, i.e. about its pI. Circular dichroism, fluorescence and UV-absorption were used to determine the conformational changes and their reversibility in the temperature range 25-80 degrees C. These changes were tested in a three step process-heating, cooling and a second heating. Principal component analysis was applied for analyzing the spectral sets obtained in these experiments. Fully reversible behavior of Trp residues, as characterized by fluorescence spectroscopy, was observed during the heating process at all pH values. Nevertheless, three different types of the protein motion (reversible, irreversible and rearrangement of the protein core) were determined by UV-absorption spectroscopy. Thus, an environment of Tyr and Phe is modified or reversibly rearranged during the heating process in acid media. These types of alpha(1)-acid glycoprotein behavior were not significantly affected by desialyzation.