Studies on the degradation of lipoprotein-X.

. Three patients suffering from prolonged obstructive jaundice were studied after surgical removal of biliary obstruction. Four days after surgery the initial cholesterol esterification rate in the plasma rose from very low activities to values three to four times higher than those found in normal subjects. During this time lipoprotein-X decreased in concentration. The initial cholesterol esterification rate returned to normal values when plasma lipids and lipoproteins reached normal levels. During incubation of normal plasma with lipoprotein-X, changes in the lipid composition of the incubation mixture were observed which appeared to be catalysed by lecithin:cholesterol acyltransferase. The formation of esterified cholesterol and lysophosphatidylcholine exceeded that of control experiments when lipoprotein-X was replaced by buffer solution in the incubation mixture. Lipoprotein-X disappeared as monitored by electrophoresis on agar and measured after zonal ultracentrifugation, and lipoproteins with flotation behaviour of high density lipoprotein2, enriched in polar lipids, appeared. When plasma was incubated separately with lipoprotein-X1 and lipoprotein-X2in vitro, the T1/2 of these lipoproteins were 24 h and 9 1/2 h, respectively. Addition of lipoprotein-X2 to normal plasma resulted in an increased initial cholesterol esterification rate with a sigmoidal response curve. Lipoprotein-X1 increased the initial cholesterol esterification rate also but to a lesser extent. Our findings indicate that incubation of normal plasma with lipoprotein-X results in the disappearance of this abnormal lipoprotein and the appearance of a high density lipoprotein2-like particle mediated by enzymic cholesterol acyltransfer.