Human serum albumin (HSA) is a major transporter for delivering several endogenous compounds including fatty acids in vivo. Even though HSA is the primary target of fatty acid binding, the effects of cationic lipid on protein stability and conformation have not been investigated. The aim of this study was to examine the interaction of human serum albumin (HSA) with helper lipids-cholesterol (Chol) and dioleoylphosphati-dylethanolamine (DOPE)-and with cationic lipids-dioctadecyldimethylammonium bromide (DDAB) and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), at physiological conditions, using constant protein concentration and various lipid contents. Fourier transform infrared (FTIR), circular dichroism (CD), and fluorescence spectroscopic methods were used to analyze the lipid binding mode, the binding constant, and the effects of lipid interaction on HSA stability and conformation. Structural analysis showed that cholesterol and DOPE (helper lipids) interact mainly with HSA polypeptide polar groups and via hydrophobic moieties. Hydrophobic interactions dominate the binding of cationic lipids to HSA. The number of bound lipids (n) calculated was 1.22 (cholesterol), 1.82 (DDAB), 1.76 (DOPE), and 1.56 (DOTAP). The overall binding constants estimated were K-Chol = 2.3 (+/- 0.50) x 10(3) M-1, K-DDAB = 8.9 (+/- 0.95) x 10(3) M-1, K-DOTAP = 9.1 (+/- 0.90) x 10(3) M-1, and K-DOPE = 4.7 (+/-0.70) x 10(3) M-1. HSA conformation was stabilized by cholesterol and DOPE with a slight increase of protein alpha-helical structures, while DOTAP and DDAB induced an important (alpha --> beta) transition, suggesting a partial protein unfolding.