On the binding of dolichol by human serum

Serum dolichols in chronic cholestatic liver diseases

BACKGROUND/AIMS

Dolichols are long-chain polyisoprenoid alcohols. It has been suggested that they modify membrane fluidity, stability and permeability. Some lysosomal diseases are associated with elevated serum dolichol levels. Liver has been suggested to play an important role in the regulation of serum dolichol levels and biliary excretion of dolichols has been proposed to be the main elimination route for dolichols from the body. The possible effect of liver diseases on serum dolichol, however, is not known.

METHODS

We therefore studied the effect of early or intermediate primary biliary cirrhosis, primary sclerosing cholangitis and alcoholic liver cirrhosis on serum dolichol concentration. Furthermore, serum dolichol content was measured in patients with end-stage primary biliary cirrhosis, primary sclerosing cholangitis and chronic active hepatitis, waiting to be transplanted.

RESULTS

As compared to age-adjusted controls, serum dolichol was significantly increased in early and intermediate primary biliary cirrhosis (451+/-56 ng/ml vs. 225+/-13 ng/ml, p<0.0001) and primary sclerosing cholangitis (315+/-16 ng/ml vs. 224+/-7 ng/ml, p<0.0001). However, in alcoholic liver cirrhosis serum dolichol was unaffected. Serum dolichol content was also significantly elevated in patients with end-stage primary biliary cirrhosis (844+/-210 ng/ml vs. 225+/-13, p<0.001) and chronic active hepatitis (594+/-198 vs. 224+/-7 ng/ml, p<0.02). Furthermore, in patients with liver diseases serum dolichol concentration correlated positively with serum high density lipoprotein (HDL)-cholesterol (r = +0.50, p<0.0001).

CONCLUSIONS

Serum dolichol levels are elevated in all stages of chronic cholestatic liver diseases but not in alcoholic liver cirrhosis. Impaired biliary excretion of dolichols appears to be the primary explanation for this finding.

Blood dolichols in a patient with abetalipoproteinaemia

Dolichol and dolichyl derivatives have an important function as glycosyl carriers in the assembly of the N-asparaginyl-linked oligosaccharide core region of glycoproteins. Dolichols are synthesized through the cholesterol biosynthesis pathways in all mammalian organs and are present in all tissues, and are also associated with lipoproteins in the blood circulation. However, the origin and metabolic pathway of blood dolichols remain unknown. Abetalipoproteinaemia is a disorder of the secretion of very low-density lipoprotein (VLDL) from the liver and of chylomicrons from the intestine into the blood circulation. Therefore, examination of blood dolichols in this disorder may provide valuable information on their origin and metabolic pathway. Dolichols were exclusively associated with the high-density lipoprotein (HDL) fraction (80.7 +/- 6.3% of total dolichols) in control human blood. Serum from a patient also contained dolichols in the HDL fraction (82.8% of total dolichols). The total amount of dolichols was higher in the patient (207.0 ng/mL) than in the controls (106.2 +/- 22.7 ng/mL, n = 14). The compositions of dolichols were very similar to each other. These results indicated that, at least in the patient with abetalipoproteinaemia, the HDL-associated dolichols were possibly derived from the liver not through other lipoproteins but through dolichol transfer protein, or were possibly taken up and carried by HDL from peripheral tissues.

Changes in plasma dolichol levels, transport, and hepatic delivery during rat liver regeneration

During the proliferative process that follows partial hepatectomy in the rat, the dolichol content increases in both plasma and liver. Its transport in the blood by lipoproteins also changes. The difference in the distribution of dolichols of various chain lengths in plasma and in the liver is further enhanced during liver regeneration. The dolichol released by perfused liver shows a homologue distribution more similar to that observable in blood than in the liver, thus confirming the importance of the liver as a regulatory site for the blood dolichol supply.

The shuttling of dolichol between VLDL and HDL: involvement of a protein factor from lipoprotein-deficient human serum

The occurrence of a dolichol transfer factor in LPDS has been demonstrated using three different transfer assays. Applying a three step purification procedure, the transfer factor could be enriched 4000-5000-fold with a recovery of 1-2%. SDS-gel electrophoresis revealed a molecular weight of 64 kDa. Kinetics as well as the influence of a series of effectors were studied. Transfer was not accompanied by a concurrent esterification and the HDL3 subpopulation showed the highest acceptor capacity. The transfer factor also affected liposomal stability based on calcein fluorescence dequenching upon release. The characteristics of this dol-TP are discussed in view of these other plasma LTPs. Dol-TP might play a role in dolichol transfer from VLDL to HDL, observed in vivo.

Discharge of newly-synthesized dolichol and ubiquinone with lipoproteins to rat liver perfusate and to the bile

An effective system for perfusing rat liver using complete tissue culture medium and washed calf erythrocytes as oxygen carriers was devised. Infusion of taurocholate and glucose proved necessary to maintain stable metabolic activity and bile secretion during a 6-hr period. Perfusate pO2, pCO2 and pH values were monitored continuously and found to be stable. Electron microscopic examination revealed the maintenance of normal hepatic structure, even after 6 hr. Normal rates of protein and urea synthesis, no leakage of cytoplasmic enzymes, and continuous bile acid production demonstrated the functional integrity of this system. Using [3H]mevalonic acid as precursor, dolichol, dolichyl phosphate, ubiquinone and cholesterol were found to be continuously synthesized in this perfused liver system. All these lipids appeared in the perfusate, indicating discharge through the ER-Golgi system. The lipoproteins of the perfusate were isolated by ultracentrifugation and characterized with respect to size distribution and lipid composition. Dolichol was found in VLDL, LDL and HDL fractions, with the highest concentration present in the latter. In rat and human blood plasma this lipid was mainly associated with HDL. The ubiquinone in the perfusate was primarily associated with the VLDL fraction, while in rat plasma it was found more evenly distributed among all the three lipoprotein fractions studied. Dolichol, ubiquinone and cholesterol were also discharged to the bile, whereas dolichyl phosphate was not. Thus, newly-synthesized dolichol and ubiquinone are transported out of the hepatocyte to the blood and to the bile.

On the binding of dolichol by bovine liver supernatant

Experimental evidence is presented that a bovine liver pH 5.1 supernatant possesses binding capacity towards dolichol. Optimal binding is found at physiological pH and at 5 degrees C. At higher temperature the binding is drastically reduced. After binding, the labelled ligand cannot be chased by unlabelled dolichol. Scatchard analysis indicates a single class of binding sites (Bmax = 3.6 pmol/mg protein) with an apparent Kd of 1.8 X 10(-11) M. Only dolichol and dolichyl derivatives drastically reduce the binding phenomenon. The involvement of a protein-like structure is inferred from ammonium sulphate precipitation and proteolysis experiments. Exclusion chromatography and gel electrophoresis under nondenaturating conditions indicate a high molecular weight of the binding complex. Upon SDS electrophoresis, bound [3H]dolichol comigrates with a single protein band (Mr approximately equal to 25,000).