Activation of lecithin-cholesterol acyltransferase by apolipoprotein D: comparison of proteoliposomes containing apolipoprotein D, A-I or C-I.
To study the activation of lecithin-cholesterol acyl transferase (LCAT) (phosphatidylcholine:sterol O-acyltransferase, EC 2.3.1.43) by apolipoprotein D in comparison to apolipoproteins A-I and C-I, proteoliposomes with a phosphatidylcholine/free cholesterol molar ratio of 24:1, containing 10-300 micrograms/ml of apolipoproteins were used. The proteoliposomes were prepared by the cholate dialysis technique. In all proteoliposome preparations we found rouleaux structures and stacked discs. The particles formed with apolipoprotein A-I were the most homogeneous, followed by apolipoprotein D- and apolipoprotein C-I-containing particles. Apolipoprotein A-I was the most potent LCAT activator in our system followed by apolipoproteins C-I and D. The fractional esterification rate observed with apolipoprotein D-containing substrates amounted to 15-48% that of apolipoprotein A-I-containing ones. Neither apolipoprotein A-I- nor C-I-containing proteoliposomes gave linear reaction kinetics with LCAT. Even during the first 15-30 min of incubation, the kinetics deviated strikingly from linearity at all apolipoprotein concentrations. In contrast, proteoliposomes containing apolipoprotein D exhibited linear reaction kinetics up to 60-90 min. At low apolipoprotein A-I concentrations (5 micrograms/ml), the addition of apolipoprotein D to the incubates resulted in significantly higher esterification rates as compared to substrates containing apolipoprotein A-I only. This was not the case using substrates with high apolipoprotein A-I concentrations (50 micrograms/ml). From our results we speculate that apolipoprotein D may have some stabilizing effect on the enzyme LCAT.
D000596:Amino Acids / Q000032:analysis; D016632:Apolipoprotein A-I; D053302:Apolipoprotein C-I; D001053:Apolipoproteins / Q000494:pharmacology*; D001054:Apolipoproteins A / Q000494:pharmacology*; D001056:Apolipoproteins C / Q000494:pharmacology*; D053399:Apolipoproteins D; D002788:Cholesterol Esters / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation / Q000187:drug effects; D004951:Esterification; D006801:Humans; D007122:Immunoelectrophoresis; D007700:Kinetics; D008081:Liposomes; D008854:Microscopy, Electron; D007862:Phosphatidylcholine-Sterol O-Acyltransferase / Q000378:metabolism*; D010713:Phosphatidylcholines
D000596:Amino Acids; D016632:Apolipoprotein A-I; D053302:Apolipoprotein C-I; D001053:Apolipoproteins; D001054:Apolipoproteins A; D001056:Apolipoproteins C; D053399:Apolipoproteins D; D002788:Cholesterol Esters; D008081:Liposomes; D010713:Phosphatidylcholines; D007862:Phosphatidylcholine-Sterol O-Acyltransferase