[Studies on the structure and metabolism of lipoprotein-X (LP-X), the abnormal plasmalipoprotein in cholestasis (author's transl)]

Abstract

Recent results regarding the pathophysiology of hyperlipoproteinemia in cholestasis are reported. The isolation of an abnormal lipoprotein (Lipoprotein-X; LP-X) from the plasma of cholestatic patients was achieved by a combination of various physico-chemical techniques. Most of the plasmacholesterol in these patients is transported in form of this abnormal lipoprotein which is very rich in phospholipids and unesterfied cholesterol. LP-X represents a vesicle with a mean diameter of 700 A. Albumin takes part as a structural protein of the particle. Besides albumin, which seems to be located in an internal water compartment or to be covered with lipids. Apo-C and Apo-D are present as surface proteins. The lack of Apo-B in LP-X, the major apoprotein of normal low density lipoproteins, seems to be the reason for a disturbed endogenous feedback mechanism of hepatic cholesterol synthesis, which is strongly increased in cholestasis. The high specificity and power of the LP-X test as clinical-chemical parameter to demonstrate or exclude cholestasis finds its explanation in our knowledge about the origin of this abnormal lipoprotein in cholestasis. LP-X is formed when a lipoprotein normally excreted with the bile refluxes into the plasma stream to convert into LP-X. This formation depends only on certain physico-chemical requirements and is independent of an energy-providing or enzymatically regulated process. The biological halflife of LP-X is similar to that of normal plasmalipoproteins. However, enzymes of postheparin plasma as well as the lecithin: cholesterol acyltransferase do not seem to play a major role in the catabolism of lipoprotein-X, but only change some of the physicochemical characteristics of this vesicle.