Diffusion of nitric oxide into low density lipoprotein.

A key early event in the development of atherosclerosis is the oxidation of low density lipoprotein (LDL) via different mechanisms including free radical reactions with both protein and lipid components. Nitric oxide ((NO)-N-.) is capable of inhibiting LDL oxidation by scavenging radical species involved in oxidative chain propagation reactions. Herein, the diffusion of (NO)-N-. into LDL is studied by fluorescence quenching of pyrene derivatives. Selected probes 1-(pyrenyl)methyltrimethylammonium (PMTMA) and 1-(pyrenyl)-methyl-3-(9-octadecenoyloxy)-22,23-bisnor-5-cholenate (PMChO) were chosen so that they could be incorporated at different depths of the LDL particle. Indeed, PMTMA and PMChO were located in the surface and core of LDL, respectively, as indicated by changes in fluorescence spectra, fluorescence quenching studies with water-soluble quenchers and the lifetime values (To) of the excited probes. The apparent second order rate quenching constants of (NO)-N-. (k(NO)) for both probes were 2.6-3.8 x 10(10) M-1 s(-1) and 1.2 x 10(10) M-1 s(-1) in solution and native LDL, respectively, indicating that there is no significant barrier to the diffusion of (NO)-N-. to the surface and core of LDL. Nitric oxide was also capable of diffusing through oxidized LDL. Considering the preferential partitioning of (NO)-N-. in apolar milieu (6-8 for n-octanol:water) and therefore a larger (NO)-N-. concentration in LDL with respect to the aqueous phase, a corrected kNO value of similar to0.2 x 1010 M-1 s(-1) can be determined, which still is sufficiently large and consistent with a facile diffusion of .NO through LDL. Applying the Einstein-Smoluchowsky treatment, the apparent diffusion coefficient ONO) of (NO)-N-. in native LDL is on average 2 x 10(-5) cm(2) s(-1), six times larger than that previously reported for erythrocyte plasma membrane. Thus, our observations support that (NO)-N-. readily traverses the LDL surface accessing the hydrophobic lipid core of the particle and affirm a role for (NO)-N-. as a major lipophilic antioxidant in LDL.