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

Nitric oxide ((NO)-N-.) 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)-N-. reactivity in the lipid milieu are its diffusion and solubility, but there is no data concerning the actual diffusion (D) and partition coefficients (K-P) of (NO)-N-. in biologically relevant hydrophobic phases. Herein, a equilibrium-shift method was designed to directly determine the (NO)-N-. and O-2 partition coefficients in liposomes and low density lipoprotein (LDL) relative to water. It was found that (NO)-N-. partitions 4.4- and 3.4-fold in liposomes and LDL, respectively, whereas O-2 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)-N-. diffuses similar to2 times slower than O-2 in the core of LDL and 12 times slower than in buffer (D-NO(LDL) = 3.9 x 10(-6) cm(2) s(-1), D-O2(LDL) = 7.0 x 10(-6) cm(2) s(-1), D-NO(buffer) = D-O2(buffer) = 4.5 x 10(-5) cm(2) s(-1)). The influence of (NO)-N-. and O-2 partitioning and diffusion in membranes and lipoproteins on (NO)-N-. reaction with lipid radicals and auto-oxidation is discussed. Particularly, the 3-4- fold increase in O-2 and (NO)-N-. concentration within biological hydrophobic phases provides quantitative support for the idea of an accelerated auto-oxidation of (NO)-N-. in lipid-containing structures, turning them into sites of enhanced local production of oxidant and nitrosating species.