Sphingomyelinase activity associated with human plasma low density lipoprotein.

Area of Science:

• Biochemistry
• Lipid Metabolism
• Cell Biology

Background:

• Low density lipoprotein (LDL) is a key carrier of cholesterol in plasma.
• Sphingomyelinase activity has been implicated in cellular lipid processing.
• The enzymatic properties of native LDL are not fully elucidated.

Purpose of the Study:

• To investigate the presence and characteristics of sphingomyelinase activity in isolated human plasma LDL.
• To determine if other major lipoprotein classes share this activity.
• To explore the impact of LDL oxidation and liposome interactions on this enzymatic function.

Main Methods:

• Enzymatic assays using fluorescent or radiolabeled sphingomyelin substrates in liposomes.
• Incubation of LDL with defined liposome compositions at 37°C.
• Assessment of LDL sphingomyelinase activity in very low density lipoprotein (VLDL) and high density lipoprotein (HDL) subfractions.
• Evaluation of LDL activity following oxidation.
• Liposome aggregation measurements using light scattering.
• Microinjection studies with giant unilamellar vesicles (GUVs) to observe cellular interactions.

Main Results:

• Isolated human LDL demonstrated significant sphingomyelinase activity, catalyzing ceramide formation from sphingomyelin-containing liposomes.
• This activity was absent in VLDL, HDL(2), and HDL(3) subfractions.
• Oxidation of LDL completely abolished its sphingomyelinase activity.
• LDL incubation induced liposome aggregation.
• Microinjection of LDL into GUVs triggered vesicle budding, mimicking endocytosis.

Conclusions:

• Human plasma LDL possesses intrinsic sphingomyelinase activity, contributing to ceramide production.
• This activity is specific to LDL and sensitive to oxidative modification.
• LDL-induced liposome aggregation and vesicle budding suggest a role in lipoprotein-cell interactions and potential endocytic pathways.