Direct measurement of nitric oxide and oxygen partitioning into liposomes and low density lipoprotein.

Nitric oxide (*NO) has been proposed to play a relevant role in modulating oxidative reactions in lipophilic media like biomembranes and lipoproteins. Two factors that will regulate *NO reactivity in the lipid milieu are its diffusion and solubility, but there is no data concerning the actual diffusion (D) and partition coefficients (KP) of *NO in biologically relevant hydrophobic phases. Herein, a "equilibrium-shift" method was designed to directly determine the *NO and O2 partition coefficients in liposomes and low density lipoprotein (LDL) relative to water. It was found that *NO partitions 4.4- and 3.4-fold in liposomes and LDL, respectively, whereas O2 behaves similarly with values of 3.9 and 2.9, respectively. In addition, actual diffusion coefficients in these hydrophobic phases were determined using fluorescence quenching and found that *NO diffuses approximately 2 times slower than O2 in the core of LDL and 12 times slower than in buffer (DNOLDL=3.9 x 10(-6) cm2 s(-1),DO2LDL=7.0 x 10(-6) cm2 s(-1),DNObuffer=DO2buffer=4.5 x 10(-5) cm2 s(-1)). The influence of *NO and O2 partitioning and diffusion in membranes and lipoproteins on *NO reaction with lipid radicals and auto-oxidation is discussed. Particularly, the 3-4-fold increase in O2 and *NO concentration within biological hydrophobic phases provides quantitative support for the idea of an accelerated auto-oxidation of *NO in lipid-containing structures, turning them into sites of enhanced local production of oxidant and nitrosating species.