Antiarteriosclerosis activity due to disposal of oxidized cholesterol by plasma lecithin ; cholesterol acyltransferase reaction.

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Oxidative modification of low density lipoprotein (LDL) appears to play an important role in atherogenesis. Our previous studies demonstrated that Lecithin : cholesterol acyltransferase (LCAT ; EC 2.3.1.41) an important enzyme in the reverse cholesterol transport system was inhibited by oxidized LDL (ox-LDL). However the precise mechanism of inhibition of LCAT activity by ox-LDL is not clear still. In this study, we investigated the effect of ox-LDL on LCAT molecule and it's activator lipoprotein in HDL, apolipoprotein A-I (apo A-I). Purified LCAT and d>1.063 g/ml fraction of humanplasma as source of LCAT were incubated with LDL and ox-LDL for 1h at 4℃ and 37℃. LCAT was separated by gel permeation chromatography using Superose-12 gel filtration column through FPLC system. LCAT activity was found inhibited significantly only when it was separated from ox-LDL after incubation at 37℃ (inhibition was 20% and 85% of control in purified and plasma LCAT respectively). HDL was isolated form recombined plasma (d>1.063 g/ml fraction of plasma ＆ LDL/ox-LDL) by sequential ultrcentrifugation using NaBr to adjust density. This isolated HDL was then labeled by [^<14>C]-Cholesterol-BSA emulsion and used as substrate for LCAT.Activity was found 21.18% lower in substrate group, which was previously exposed to ox-LDL, compared to control. LCAT was more susceptible to adverse effect of ox-LDL compared to apo A-I in HDL.LCAT activity was found protected by DTNB but GSH was found not afforded any protection to LCAT activity from ox-LDL.We therefore concluded that oxidation product (s) of LDL was transferred to LCAT and apo A-I in HDL at 37℃ and tightly bound to it, which caused irreversible modification to these molecules. More than one type of oxidation product was involved in the inhibition of LCAT activity, which may be follow different mechanism.