Enzymatic and
lipid transfer reactions involved in reverse
cholesterol transport were studied in healthy and
lecithin:cholesterol acyltransferase (LCAT), deficient subjects. Fasting plasma samples obtained from each individual were labeled with [3H]
cholesterol and subsequently fractionated by gel chromatography. The radioactivity patterns obtained corresponded to the elution volumes of the three major ultracentrifugally isolated
lipoprotein classes (
very low density lipoproteins (VLDL),
low density lipoproteins (
LDL), and
high density lipoproteins (HDL)). In healthy subjects, the LCAT activity was consistently found in association with the higher molecular weight portion of HDL. Similar observations were made when exogenous purified LCAT was added to the LCAT-deficient plasma prior to chromatography. Incubation of the plasma samples at 37 degrees C resulted in significant reduction of unesterified
cholesterol (FC) and an increase in esterified
cholesterol (CE). Comparison of the data of FC and CE mass measurements of the
lipoprotein fractions from normal and LCAT-deficient plasma indicates that: (i) In normal plasma, most of the FC for the LCAT reaction originates from
LDL even when large amounts of FC are available from VLDL. (ii) The LCAT reaction takes place on the surface of HDL. (iii) The product of the LCAT reaction (CE) may be transferred to either VLDL or
LDL although VLDL appears to be the preferred acceptor when present in sufficient amounts. (iv) CE transfer from HDL to lower density
lipoproteins is at least partially impaired in LCAT-deficient patients. Additional studies using
triglyceride-rich
lipoproteins indicated that neither the capacity to accept CE from HDL nor the lower CE transfer activity were responsible for the decreased amount of CE transferred to VLDL and
chylomicrons in LCAT-deficient plasma.