In a continued investigation of lecithin cholesterol acyltransferase reaction with micellar discoidal complexes of phosphatidylcholine, cholesterol, and various water soluble apolipoproteins, we prepared complexes containing human apo-E by the cholate dialysis method. These complexes were systematically compared to apo-A-I complexes synthesized under the same reaction conditions. Apo-E complexes (134 Ȧ in diameter) were slightly larger than apo-A-I complexes (110 Ȧ) but were very similar in terms of their protein and lipid content (2.4:0.10:1.0, egg phosphatidylcholine/cholesterol/apolipoprotein, w/w) and in the percentage of apolipoprotein in α-helical structure (72-74%). Concentration and temperature-dependence experiments on the velocity of the lecithin cholesterol acyltransferase reaction revealed differences in apparent K(m) values and small differences in apparent V(max) but very similar activation energies (18-20 kcal/mol). These observations suggest that differences in lecithin cholesterol acyltransferase activation by apo-A-I and apo-E are primarily a result of different affinities of the enzyme for the particles but that the rate-limiting step of the reaction is comparable for both complexes. Apo-E was found to be 18% as effective as apo-A-I in activating purified human lecithin cholesterol acyltransferase. Addition of free apo-A-I to apo-E complexes resulted in the exchange of bound for free apolipoprotein causing a slight increase in the reactivity with the enzyme when the incubation mixture was assayed. When the unbound apolipoproteins were removed by ultracentrifugation, reisolated complexes containing both apo-E and apo-A-I demonstrated an even greater increase in reactivity with the enzyme.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of Biological Chemistry|
|State||Published - 1985|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology