Sphingomyelinase activity associated with human plasma low density lipoprotein - Possible functional implications

Isolated human plasma low density lipoprotein (LDL) was observed to possess sphingomyelinase activity. Accordingly, the formation of ceramide was cata lyzed by LDL at 37 degrees C using tertiary Liposomes composed of sphingomy elin (mole fraction (x) = 0.2), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoch oline (x = 0.7), 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (x = 0.1 ), and either the fluorescent sphingomyelin analog Bodipy-sphingomyelin or [C-14]sphingomyelin as substrates. However, this activity was not present i n either very low density lipoprotein or the high density lipoprotein subfr actions HDL2 and HDL3. Oxidation of LDL abrogated its sphingomyelinase acti vity. Aggregation of the liposomes upon incubation with LDL was evident fro m the light scattering measurements. Microinjection of LDL to the surface o f giant liposomes composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholi ne (SOPC), N-palmitoyl-D-sphingomyelin (C16:0-sphingomyelin), and Bodipy-sp hingomyelin as a fluorescent tracer (0.75:-0.20:0.05, respectively) reveale d the induction of vectorial budding of vesicles, resembling endocytosis.