Effects of organic and inorganic polyamine cations on the structure of human serum albumin

The presence of several high affinity binding sites on human serum albumin (HSA) makes it a possible target for many organic and inorganic molecules. Organic polyamines are widely distributed in living cells and their biological roles have been associated with their physical and chemical interactions with proteins, nucleic acids, and lipids. This study is designed to examine the effects of spermine, spermidine, putrescine, and cobalt [Co(III)]-hexamine cations on the solution structure of HSA using Fourier transform IR, UV-visible, and circular dichroism (CD) spectroscopic methods. The spectroscopic results show that polyamine cations are located along the polypeptide chains with no specific interaction. The order of perturbations is associated with the number of positive charges of the polyamine cation: spermine > Co(III)-hexamine > spermidine > putrescine. The overall binding constants are 1.7 × 10^4, 1.1 × 10^4, 5.4 × 10^3, and 3.9 × 10^3M^-1, respectively. The protein conformation is altered (IR and CD data) with reductions of (alpha) helices from 60 to 55% for free HSA to 50-40% and with increases of (beta) structures from 22 to 15% for free HSA to 33-23% in the presence of polyamine cations.