Quantitative studies of lipoprotein-X in familial lecithin: cholesterol acyltransferase deficiency and during...

In eight patients with familial lecithin: cholesterol acyltransferase (LCAT) deficiency the plasma concentration of the abnormal lipoprotein LP-X ranged from 43 mg/100 ml to 251 mg/100 ml with a mean of 127 mg/100 ml. This is above the mean level of LP-X found in a group of patients with intrahepatic cholestasis (49 mg/100 ml) and below the mean level found in patients with extrahepatic cholestasis (341 mg/100 ml). Following blood transfusions, an increase in LCAT activity and a decrease in the plasma concentration of LP-X were observed. This decrease was not a simple dilution phenomenon. Quantitation of LP-X during long-term incubation of a whole plasma system demonstrated that LP-X was a source of free cholesterol (FC) in the LCAT reaction. More free cholesterol was derived from LP-X during cholesterol esterification than corresponding to the percentage of the plasma free cholesterol found in the LP-X fraction. The present investigation did not answer the question whether LP-X only was an easily accessible source of free cholesterol for cholesterol esterification or whether LP-X acted directly as a substrate for the LCAT reaction.