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Abstract
The influence of various lipoproteins, apolipoproteins, and lipoprotein lipids on the initial rate of cholesterol esterification (IRE) by the enzyme lecithin:cholesterol acyltransferase (LCAT) has been studied in vitro in sera from normolipemic and hyperlipemic human subjects. This was achieved by adding isolated lipoprotein density fractions and families to fresh sera, on the one hand, and by specific adsorption of distinct lipoprotein classes with antibodies and polyanions on the other. The results were as follows: 1) Addition of all known lipoprotein density classes to normal serum, except for HDL2, had no influence on the IRE but reflected to some degree the mode of assay system. Addition of HDL2 inhibited the LC AT and the inhibitor was chloroform methanol extractable. The addition of LpC, a minor fraction from HDL of all normal sera, markedly enhanced the IRE. 2) Removal of VLDL plus LDL by antibodies or dextran sulfate had little if any effect. After precipitation of all LpA with anti-AI, approximately 50% of the LCAT activity remained in the supernatant and was active even in the assays where only endogenous substrate was provided. In the precipitated LpA the rest of the activity was found, despite the fact that antigenic determinants of apoAI were blocked by antibodies. The removal of LpC from the HDL-class was followed by a complete abolishment of LCAT activity. This activity, however, could partly be restored by adding exogenous substrate. 3) The IRE was found to be increased in persons with primary and post-prandial hypertriglyceridemia. In these cases the concentration of the LpC in HDL was also increased. These findings suggest that LpC of HDL plays a major role in the esterification of cholesterol in human serum under physiological conditions.