Original Article

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

Correspondence to: Jeffrey S. Cohn.

Gamma-LpE (-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-ß₁-LpA-I, as an initial acceptor of cell-derived cholesterol. In order to further characterize the presence of -LpE in human plasma, we have separated -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 -LpE were routinely detected in human plasma, ranging in mean particle diameter from 9.5 to 16.5 nm. The largest proportion of -migrating apoE was associated with ₂-LpE having a diameter of 13.0 nm. Neither the amount of -LpE apoE (representing less than 1–2% of total plasma apoE) nor the number of -LpE subfractions was different in serum vs. plasma, or was affected by the presence of agents able to inhibit protein dimerization. -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°C (90 min) caused a significant decrease in plasma -LpE (>80%) that was not dependent on LCAT or CETP activity. Storage (at -70°C) of hypertriglyceridemic but not normolipidemic plasma resulted in an increase in -LpE. Freezing of postprandial plasma samples, containing increased amounts of triglyceride-rich lipoproteins (TRL) enriched in apoE, also caused an increase in -LpE. Incubation of VLDL (d < 1.006 g/ml) with lipase resulted in the production of -migrating apoE. These results demonstrate that: 1) different -LpE subfractions exist in human plasma; 2) the amount of apoE associated with -LpE subfractions is dependent on in vitro conditions of plasma storage; and 3) TRL can act as a source of -LpE apoE in vitro.—Krimbou, L., M. Tremblay, H. Jacques, J. Davignon, and J. S. Cohn. In vitro factors affecting the concentration of gamma-LpE (-LpE) in human plasma. J. Lipid Res. 1998. 39: 861–872.

Supplementary key words: HDL, cholesterol efflux, apoE

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