Copper and gas-p smoke inhibit plasma lecithin :ch by different mechanisms rol acyltransferase activity

Cigarette smokers have reduced levels of plasma high density lipoprotein (HDL) compared to nonsmokers and are at risk of premature cardiovascular disease. Previous work from this laboratory has shown that exposure of human plasma to gas- phase cigarette smoke (CS) inhibited the activity of 1ecithin:cho- lesterol acyltransferase (LCAT), the enzyme that catalyzes the formation of cholesteryl ester in HDL and thereby promotes HDL maturation. As CS contains free radicals that could poten- tially oxidize plasma lipoproteins, we examined the involvement of lipid peroxidation in LCAT inhibition. Results obtained with CS were compared with those obtained by initiating lipid per- oxidation with copper ions. Exposure of dialyzed human plasma to an equivalent of one-eighth of a cigarette at 15-min intervals resulted in a progressive loss of LCAT activity (50 and 90% reductions by 1 and 6 h, respectively). A similar pattern of LCAT inhibition was produced with copper (0.5 mM) where 50 and 97% reductions were observed at 1 and 6 h, respectively. To determine whether LCAT inhibition was related to lipid peroxi- dation, lipoprotein fractions corresponding to VLDL-IDL, LDL, and HDL were isolated from plasma exposed to CS or copper and analyzed for changes in TBARS, the polyunsatu- rated fatty acid arachidonate relative to palmitate (20:4/16:0 ratio), and vitamin E concentrations. Exposure of plasma for 6 h to CS had no effect on the levels of TBARS and 20:4/16:0 ratio; however, 6 h copper treatment (0.5 mM) caused a 3.0-, 4.0-, and 1.4-fold increase in TBARS and a 17, 25, and 13% reduction in the 20:4/16:0 ratio in VLDL-IDL, LDL, and HDL fractions, respectively. In addition, a complete depletion of lipoprotein vitamin E was observed with CS, whereas copper decreased vitamin E levels by approximately 50% in each frac- tion. Supplementation of plasma with either vitamin C (85 pM) or butylated hydroxytoluene (BHT, 0.45 mM) was unable to protect LCAT from CS. In contrast, BHT completely protected LCAT activity from inhibition by copper. We conclude that unlike copper, CS-induced inhibition of plasma LCAT activity was unrelated to free radical-induced lipid peroxidation. The in- hibition of LCAT activity by cigarette smoke may contribute to the development of atherosclerosis by impairing HDL metabo- lism and the reverse cholesterol transport process. - Bielicki, J. K., M. R. McCall, J. J. M. van den Berg, F. A. Kuypers, and T. M. Forte. Copper and gas-phase cigarette smoke inhibit plasma 1ecithin:cholesterol acyltransferase activity by different mechanisms. J. Lipid Res. 1995. 36: 322-331. Elevated levels of plasma high density lipoprotein (HDL) are associated with reduced risk of atherosclerosis (1-3). The beneficial effects attributed to this lipoprotein are probably linked to the ability of HDL to remove ex- cess cholesterol from peripheral cells (4, 5) and to facili- tate transport of cholesterol to the liver: a process termed reverse cholesterol transport (6-8). A central component of the reverse cholesterol transport process is 1ecithin:cho- lesterol acyltransferase (LCAT) which catalyzes the for- mation of cholesteryl esters in HDL (8, 9). This esterifica- tion reaction may contribute to the efflux of cholesterol from cells by maintaining a concentration gradient for the diffusion of excess cellular cholesterol to HDL (8, 10). In the circulation, LCAT converts smaller HDL particles into larger, cholesteryl ester-rich HDL. The delivery of HDL cholesteryl esters to the liver for catabolism and subsequent excretion from the body can occur by several mechanisms including the selective uptake of cholesteryl ester independent of HDL apolipoproteins, uptake of en- tire HDL particles, and transfer of cholesteryl ester via cholesteryl ester transfer protein to VLDLiLDL which are cleared from the circulation via the apoB receptor pathway. Cigarette smoking has been identified as an indepen- dent risk factor in the development of atherosclerosis (11-14), yet the underlying mechanism(s) responsible for this association is incompletely understood. Cigarette smoke contains a number of free radicals including nitric oxide, superoxide anion, and organic peroxyl radicals