Lecithin: Cholesterol Acyltransferase Activity in Relation to Lipoprotein Concentration and Lipid Composition

Abstract

The known methods for measurement of the lecithin:cholesterol acyltransferase (LC AT; EC 2.3.1.43) activity in plasma do not distinguish well between influences of enzyme activity and lipoprotein substrate on esterification rate. An assay has therefore been developed consisting in quantitative determination of the main lipoprotein classes and their respective lipid pattern from 50 µl of plasma before and after incubation at 3 7 °C for 5 hours. With the aid of polyanions and divalent cations very low density (VLDL), low density (LDL), and high density lipoproteins (HDL) were stepwise precipitated. From the precipitated lipoprotein fractions and the final supernatant (VHDL) total lipids were extracted and quantificated gravimetrically. From each lipoprotein class lipids were separated into esterified cholesterol (CHE), triglycerides (TG), cholesterol (CH), free fatty acids (FFA), and phospholipids (PL) by thin-layer chromatography on microchro-matoplates and determined by densitometry after charring. After correction for recovery, the cholesterol esterification rate was determined from the difference of total free cholesterol before and after incubation. Plasma samples of 19 male healthy persons (age 15-61) were analyzed in duplicate. The mean esterification rate was 12.8 ± 2.7 mg CH/ 1/h (range 7.8-18.1). LCAT activity expressed as %CH esterified per hour was not correlated with mg HDL and negatively with mg VLDL r = 0.528 (p < 0.05)and with % of total lipid in VLDL r = -0.520 (p < 0.05). The influences of different lipoprotein concentrations on the esterification rate were evaluated by certain quotients. Thus, the quotient HDL/VLDL r = 0.566 (p < 0.05) and VHDL/VLDL r = 0.556 (p < 0.05) showed significant positive correlation. In addition, LC AT activity was highly dependent on lipid composition of lipoproteins. Thus, the relative %PL in HDL showed a weak positive correlation and the ratio CHE/CH in HDL a negative correlation with LCAT activity. The quotient TG + FFA/CH + CHE in VLDL r = 0.583 (p < 0.01) was significantly positively and the relative %CHE in VLDL r = -0.563 (p <0.05) significantly negatively correlated with fractional esterification rate. From these results the LCAT reaction is proposed to be a crossing of two pathways, the conversion of nascent HDL to normal HDL and the conversion of VLDL-remnants to IDL. Pathological and clinical disorders supporting this thesis are discussed.

Key Words:

• cholesterol metabolism
• high density lipoprotein
• lipoprotein metabolism
• very low density lipoprotein