Sphingomyelinase-to-LDL molar ratio determines low density lipoprotein aggregation size: biological significance

The subendothelial retention of low density lipoproteins (LDL) is believed to be the central pathogenic event in atherosclerosis, as stated by the response-to-retention hypothesis. Sphingomyelinase, an enzyme present in the arteries, has been proven to promote LDL aggregation. This study investigates the hypothesis that the extent of LDL aggregation is determined by the molar ratio of sphingomyelinase (SMase)-to-LDL, rather than the absolute concentrations. A mass action model is used to describe the aggregation process, and binding and dissociation rate constants are determined by fitting of dynamic light scattering data. The model predicts aggregate sizes that agree well with experimental observations. This study also tests the hypothesis that monocyte uptake of LDL correlates with aggregate size. LDL aggregates of three specific sizes (75, 100, and 150 nm) were incubated with J774A.1 cells and the net accumulation of LDL was monitored by measuring changes in the cellular cholesterol and protein content. Relative to a control sample, cholesterol accumulation was enhanced for aggregate sizes of 75 and 150 nm. The intermediate size aggregates, 100 nm, led to a very striking result demonstrating that cholesterol accumulation was markedly greater than the other samples, and was sufficient to cause cell death. These results underscore an important role of colloidal aggregation, and the influence of LDL aggregate size, in atherosclerosis.