Anion-induced refolding of human serum albumin under low pH conditions

We studied the effect of various anions (of acids and salts) on the acid de natured state of HSA by near-UV circular dichroism (CD), far-UV CD, 1-anili nonaphthalene-8-sulfonate (ANS) binding, tryptophan fluorescence and therma l transition. Addition of different acids and salts caused an induction of alpha-helical structure as evident from the increase in the mean residue el lipticity (MRE) value at 222 nm and loss of ANS binding sites exhibited by the decrease in the ANS fluorescence intensity at 480 nm. However, the conc entration range of acids/salts required to bring about the transition varie d greatly among different acids and salts. Among various acids/salts tested , K3Fe(CN)(6) was found to be most effective whereas HCl and KCI were least effective in inducing the properties close to native structure. Further, t hey followed the electroselectivity series. The near-UV CD spectra showed a n increase in MRE towards the native state, whereas the tryptophan fluoresc ence emission spectra produced a red shift of about 6 nm on addition of KCl O4. The temperature-induced transition in the presence of 40 mM KClO4 monit ored by ellipticity measurements at 222 nm was characterized by the presenc e of an intermediate state in the temperature range 30-50 degrees C having abundant secondary structure. These results suggest that human serum albumi n at low pH and in the presence of acids or salts exists in a partially fol ded state characterized by native-like secondary structure and tertiary fol ds. (C) 2000 Elsevier Science B.V. All rights reserved.