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.