Copper and gas-phase cigarette smoke inhibit plasma lecithin:cholesterol acyltransferase activity by different mechanisms.

Abstract

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 lecithin:cholesterol 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 potentially 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 peroxidation 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 peroxidation, 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 polyunsaturated 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 fraction. Supplementation of plasma with either vitamin C (85 microM) 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 inhibition of LCAT activity by cigarette smoke may contribute to the development of atherosclerosis by impairing HDL metabolism and the reverse cholesterol transport process.