In vitro factors affecting the concentration of gamma-LpE (γ-LpE) in human plasma

Gamma-LpE (gamma-LpE), a sphingomyelin-rich lipoprotein that contains apolipoprotein (apo) E as its only protein component, has been proposed to play a role in cellular cholesterol efflux by acting, like pre-beta1-LpA-I, as an initial acceptor of cell-derived cholesterol. In order to further characterize the presence of gamma-LpE in human plasma, we have separated gamma-LpE by two-dimensional non-denaturing polyacrylamide-gradient gel electrophoresis and detected its presence by immunoblotting with 125I-labeled polyclonal anti-apoE antibody. Five species of gamma-LpE were routinely detected in human plasma, ranging in mean particle diameter from 9.5 to 16.5 nm. The largest proportion of gamma-migrating apoE was associated with gamma-LpE having a diameter of 13.0 nm. Neither the amount of gamma-LpE apoE (representing less than 1-2% of total plasma apoE) nor the number of gamma-LpE subfractions was different in serum vs. plasma, or was affected by the presence of agents able to inhibit protein dimerization. Gamma-LpE subfractions were present in the plasma of patients having different apoE phenotypes (i.e., apoE 2/2, 3/3, or 4/4). Incubation of plasma at 37 degrees C (90 min) caused a significant decrease in plasma gamma-LpE (>80%) that was not dependent on LCAT or CETP activity. Storage (at -70 degrees C) of hypertriglyceridemic but not normolipidemic plasma resulted in an increase in gamma-LpE. Freezing of postprandial plasma samples, containing increased amounts of triglyceride-rich lipoproteins (TRL) enriched in apoE, also caused an increase in gamma-LpE. Incubation of VLDL (d < 1.006 g/ml) with lipase resulted in the production of gamma-migrating apoE. These results demonstrate that: 1) different gamma-LpE subfractions exist in human plasma; 2) the amount of apoE associated with gamma-LpE subfractions is dependent on in vitro conditions of plasma storage; and 3) TRL can act as a source of gamma-LpE apoE in vitro.