Effect of acyl chain unsaturation on the conformation of model diacylglycerols: a computer modeling study.

In a previous modeling study we identified an angle iron-shaped conformation of docosahexaenoic acid and showed that an sn-1-stearoyl diacylglycerol (DG) that contained an sn-2-docosahexaenoyl group in this conformation could adopt a highly regular shape. In the present study we compared the properties of this DG with those of sn-1-stearoyl DGs that contained other unsaturated fatty acyl groups in the sn-2 position. The major findings were that: 1) sn-1-stearoyl DGs that contain polyenoic fatty acids in the sn-2 position can assume regular shapes, and 2) these shapes differ depending on the location of the double bonds. sn-2-Polyenoic fatty acyl groups with a double bond sequence that begins close to the carboxyl ester bond are associated with one type of regular shape, while sn-2-polyenoic fatty acyl groups with a double bond sequence that begins toward the middle of the chain are associated with another. Such shapes would not have been predicted by current ideas relating membrane fluidity to unsaturation. In contrast, another finding of the present study, that sn-1-stearoyl-2-oleoyl DG can adopt, at best, only a highly irregular shape is in good agreement with the results of previous investigators.

Effect of acyl chain unsaturation on the packing of model diacylglycerols in simulated monolayers.

In a companion study of the effects of acyl chain unsaturation on a series of model sn-1,2-diacylglycerols (DGs) we showed that individual DGs could adopt one of three energy-minimized conformations depending on the number and location of cis double bonds in the sn-2 chain. Here we show that each of these conformations promotes a distinct type of packing arrangement in a simulated DG monolayer. One conformation, shown by sn-1-18:0 DGs containing an sn-2 22:6(n-3)-, 20:4(n-6)-, or 20:3(n-9)- group, determines a regular packing that resembles a known hybrid subcell, HS2, of crystalline hydrocarbon chains. The second conformation, shown by DGs containing an sn-2 18:0-, 18:2(n-6)-, or 18:3(n-3)- group, determines a regular packing that resembles a second known, distinct hydrocarbon subcell, HS1. The third conformation, that of 18:0/18:1(n-9) DG, determines a much looser, less energetically favorable packing. Stable heterogeneous packings are possible for DGs that have similar conformations, but mixed packings of DGs that have dissimilar conformations are less stable. These results raise the possibility that differences in sn-2 acyl chain unsaturation among membrane sn-1,2-diacylglycerophospholipids may promote the formation of different domains.

Computer-based modeling of the conformation and packing properties of docosahexaenoic acid.

We used a molecular modeling approach to search for a conformation of docosahexaenoic acid (DHA) that might uniquely influence acyl chain packing in cell membranes. Studies of DHA models containing six cis double bonds and five intervening methylene groups identified two conformations of special interest. Both had nearly straight chain axes formed by methylene carbon alignment. In one, the carbons of the six double bonds projected outward from the methylene axis in two nearly perpendicular planes to form an angle iron-shaped molecule. In the other, the double-bond carbons projected outward from the axis at nearly 90 degree-intervals to form a helix. Studies of packed arrays of these hexaenes with or without saturated hydrocarbons showed that tight intermolecular packing arrangements were possible, particularly in the case of the angle iron-shaped molecules. The planar surfaces of two or more such molecules could be brought into contact "back to back," while the interplanar "V groove" of each molecule could come into close apposition with a saturated chain. Because a similar mixed chain packing arrangement was found also for 1,2 diacylglycerols, these results raise the possibility that DHA may, in certain circumstances, promote tight, regular acyl chain packing arrays in DHA-rich membranes.

Characterization of apolipoprotein E-rich high density lipoproteins in familial lecithin:cholesterol acyltransferase deficiency.

We have isolated and charachterized a subfraction of high density lipoproteins, rich in apolipoprotein E, from the plasma of patients afflicted with familial lecithin:cholesterol acyltransferase deficiency. Prepared by successive ultracentrifugal flotation, affinity chromatography on heparin-agarose, and affinity chromatography on conconavalin A-agarose, the subfraction contained disc-shaped lipoproteins that measured 14--40 nm in diameter and 4.4--4.5 nm in thickness. The major components were apolipoprotein E, phosphatidylcholine, and unesterified cholesterol, though other apolipoproteins and lipids were present in small amounts. A second subfraction of high density lipoproteins, isolated during the chromatography, contained apolipoproteins A-I and A-II, but no apolipoprotein E. This subfraction included disc-shaped lipoproteins, 13--24 nm in diameter, as well as small round particles, 5.7 nm in diameter. Both subfractions contained similar proportions of total protein relative to lipid, similar amounts of unesterified cholesterol relative to phosphatidylcholine, and a similar distribution of phosphatidylcholine fatty acid.

Plasma lipoproteins in familial lecithin: cholesterol acyltransferase deficiency: effects of dietary manipulation.

To study the metabolism of the abnormal plasma lipoproteins in familial lecithin:cholesterol acyltransferase deficiency we performed five dietary experiments designed to perturb their distribution and composition. Four patients with the disease were given successive diets that differed in triglyceride, carbohydrate, or cholestrol content, and after each dietary period the lipoproteins were analyzed by combinations of preparative and analytical ultracentrifugation, gel filtration, chromatography, and disc gel electrophorsis. Lowering the intake of long chain, dietary triglyceride descreased the concentrations of the large very low density lipoproteins, the large and intermediate low density lipoproteins, and the small high density lipoproteins by as much ad 79 %, but either increased or did not change the concentrations of the small very low and low density lipoproteins. Re-adding long chain triglycerdine to the diet generally reversed these effects, but increasing the dietary cholesterol without lowering the dietary triglyceride only decreased the concentration of plasma cholesteryl ester. We conclude that the concentrations of the large very low and low sensity lipoproteins, the intermediate-sized low density lipoproteins, and the small high density lipoproteins are related to the absorption and subsequent transport of long chain dietary fatty acids. Since these lipoproteins are rich in unesterified cholesterol and lecithin, two polar lipids that form a substantial part of the surfaces of chylomicrons, components of chylomicron surfaces may accumulate in the patient's plasma following enzymic removal of chylomicron triglyceride and contribute to several of the abnormal lipoproteins.

Plasma lipoproteins in familial lecithin: cholesterol acyltransferase deficiency: effects of incubation with lecithin: cholesterol acyltransferase in vitro.

To study the effect of lecithin: cholesterol acyltransferase (LCAT) on the plasma lipoproteins of patients with familial LCAT deficiency, whole plasma or the lipoprotein fraction of d smaller than 1.006 g/ml (VLDL) was incubated in the presence of LCAT and subsequently examined by chemical, physical, and immunological techniques. The following occured upon incubating either hyperlipemic or nonlipemic plasma: The concentrations of polar lipids decreased, particulary in the large molecular weight lipoprotein subfraction of d 1.019-1.063 g/ml (LDL2) and in the lipoprotein fraction of 1.06301.25 g/ml (HDL). The concentration of cholesteryl ester (CE) increased, particularly in the VLDL and in the lipoprotein fractions of d 1.006-1.019 g/ml (LDL1) and LDL2. The concentration of arginine-rich apolipoprotein decreased in the HDL and increased in the VLDL and LDL1. The concentrations of the C-apoliproteins appeared to change in the opposite direction. The concentration of apolipoprotein B in the LDL increased concomitantly with an increase in the concentration and flotation rsate of the small LDL2. The concentration apolipoprotein A-I in the HDL increased; and a major component in the HDL fraction became identical in apperance to normal HDL. Upon incubating a patient's isolated VLDL in the presence of LCAT, lipoproteins with properties similar to normal LDL2 were formed. These experiments show that the LCAT reaction can alter the apolipoprotein content and physical properties as well as the lipid content of the patient's lipoproteins.