Studies on the degradation of lipoprotein‐X

Abstract

Abstract. 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.

Formation of high density lipoprotein2-like particles during lipolysis of very low density lipoproteins in vitro.

“…In control experiments in which lipoprotein lipase was omitted from the incubation mixture, the VLDL contained the full range of subfractions as observed with normal plasma (13). The median Sf value for our substrate VLDL was about 100, and more than 95% of the VLDL exhibited So values between 60 and 400 (Fig.…”

“…For all iodination experiments, the molar ratio of iodine to apoprotein was less than 0. (13). Ultracentrifugation was performed with a Beckman L2-65B ultracentrifuge and a Beckman Ti-14 rotor at 42,000 rpm for 45 min at 14'C.…”

Plasma triglycerides determine low density lipoprotein composition, physical properties, and cell-specific binding in cultured cells.

“…1 A and B should approximate that of VLDL, the major triglyceride pool in fasting plasma. These values are slightly less than that of VLDL, which is typically between 0.6 and 0.9, with an average value being 0.8 (38,39). The slight difference between the measured ratio for LDL and the observed value for VLDL may be due to intrinsic differences in the affinities of LDL and VLDL for triglyceride and cholesterol ester (40).…”

Lipoproteins in familial hyperalphalipoproteinemia.

“…We demonstrated that HDL 2 -like particles could be produced from HDL 3 by incubation of normal plasma in the absence of lipoprotein lipase activity. 31 This interconversion was increased in the presence of LP-X, 32 a source of free cholesterol and phosphatidyl choline, and could be inhibited by p-OH mercuribenzoate, suggesting that HDL 3 -HDL 2 conversion was mediated by LCAT. 31 However, LCAT activity was found to be normal in hyperalphalipoproteinemia, 78 implying that LCAT-mediated conversion of HDL 3 to HDL 2 may be quantitatively of minor importance in hyperalphalipoproteinemia.…”