Degradation of High-Density Lipoproteins (HDL) In Vitro

Immunoperoxidase localization of apolipoprotein D in human enterocytes and hepatocytes

In this study we prepared a pure apolipoprotein D and obtained a specific antiserum to it. The purified apolipoprotein D migrated as a single band of Mr = 29,000 but appeared as five isoforms on isoelectrofocusing. The antiserum did not cross-react with other apolipoproteins. Immunoenzymatic staining revealed the presence of apolipoprotein D in the perinuclear area of the cytoplasm of isolated normal hepatocytes and HepG2 cells. Apolipoprotein D was also localized in intestinal epithelium and in liver cells. The intracellular distribution of apolipoprotein D was similar to that of apolipoprotein B. Our results indicated that apolipoprotein D, like many other circulating apolipoproteins, is synthesized in enterocytes and hepatocytes.

Apolipoproteins as the basis for heterogeneity in high-density lipoprotein2 and high-density lipoprotein3. Studies by isoelectric focusing on agarose films

A method is described for the isoelectric focusing (IEF) of lipoproteins on thin films of agarose. Within a pH gradient of 4.60-5.30 both high-density lipoproteins 2 and 3 (HDL2 and HDL3) are resolved into more than 10 fractions which could be stained either for protein or for lipids. The isoelectric focusing patterns for HDL2 and HDL3 are similar although HDL2 appears richer in the more alkaline bands. Narrow film strips from the IEF separation of HDL2 and HDL3 were interfaced with various agarose plates containing antisera against apolipoproteins apoAI, apoAII and apoCIII either alone or in combination, to provide two-dimensional IEF immunoelectrophoresis patterns. This technique demonstrated that apoAI and apoAII were present throughout the IEF gel for both subclasses of HDL. It also provided evidence for the existence of lipoproteins containing both apoAI and apoAII and other lipoproteins present in the alkaline region of the gel which contained apoAI but no apoAII. ApoCIII was found mostly in acidic lipoproteins and was not distributed identically in HDL2 and HDL3. The lipoproteins separated by IEF on agarose were also analysed by two-dimensional IEF-SDS electrophoresis and the individual apolipoproteins were identified by reaction with antibodies to apolipoproteins AI, AII, CI, CII, CIII, D, and E. This technique confirmed that in IEF of HDL, apoAI extended throughout the spectrum of lipoproteins whereas apoE was only present in alkaline lipoproteins and apoD was only present in acidic lipoproteins. IEF on agarose of either HDL2 or HDL3 allowed us to collect eight different fractions, which have the same pI in either lipoprotein class. The apolipoprotein composition of each isolated band was analysed by electroimmuno-assays for apolipoproteins AI, AII, CI, CII, CIII, D, and E and the results expressed as the ratio of the measured apolipoprotein to measured apoAI. In both HDL2 and HDL3, acidic lipoprotein fractions were enriched in apoAII, apoCIII and apoD. ApoCII and apoCII were not similarly distributed in HDL2 and HDL3 subfractions whereas the apoCI distribution was similar in both classes. Noteworthy in all experiments was the difference in the distributions of apoCI, apoCII, and apoCIII in HDL2 and HDL3, which indicated that the existence of a lipoprotein containing simultaneously CI, CII and CIII can only account for a small fraction of these apolipoproteins. Therefore these experiments substantiate the theory of the protein basis of HDL heterogeneity and suggest that the majority of apolipoproteins are present in complexes which upon IEF result in lipoprotein fractions of identical pI for both HDL2 or HDL3.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitation of human serum apolipoprotein A-I by electroimmunoassay. Studies on some techniques for standardization of the assay and determination of serum apolipoprotein A-I levels in a random population sample of middle-aged men

Serum apolipoprotein A-I levels have been determined in a population sample of seventy-six randomly selected men aged 41-48 years. Electroimmunoassay with a monospecific rabbit antiserum to human apoA-I was used. Different routes to standardization of the electroimmunoassay were exploited. Standardization was finally carried out with purified apoA-I and confirmed with LpA-I. The mean apoA-I level was 1.75 +/- 0.20 g/1 in the populaton sample, with a normal distribution of the values. A highly significant, positive correlation to alphalipoprotein cholesterol was observed. ApoA-I levels were negatively correlated to tobacco consumption. Significant correlations were also observed to the levels of serum cholesterol and physical activity during leisure time, but no significant correlations to serum triglycerides or body weight index were found.

Plasma cholesterol esterification and plasma lipoproteins in bile-duct-ligated dogs

To study the lipoprotein changes in cholestasis while the capacity for plasma cholesterol esterification was normal, the common bile duct was ligated in dogs and plasma investigated 8 h and 48 h later. The plasma concentration of cholesteryl esters was slightly increased, concomitant with a tendency toward an increase in the activity of lecithin:cholesterol acyltransferase (LCAT). The content of cholesteryl esters in the main lipoprotein classes was normal. Marked alteration in the low density lipoproteins (LDL) and the high density lipoproteins (HDL) took place and were essentially similar 8 h and 48 h after bile duct ligation. In LDL, (density 1.006--1.019 g/ml) and LDL2 (density 1.019--1.063 g/ml) an increase in the content of polar lipids was observed, and in LDL2 heterogeneity in particle size was demonstrated by gelfiltration on 2% agarose and by electron microsopcy. Large myelin structures, flattened disc-shaped particles, and particles with the appearance of normal LDL2 were present. HDL isolated after operation was characterized by a decreased protein/lipid ratio and an increased content of phospholipids. By gelfiltration on Sephadex G-200 and by electron microscopy changes in particle size were observed, with the presence of disc-shaped particles with a tendency in form rouleaux. These results demonstrate that marked lipoprotein changes occur as early as 8 h after bile duct-ligation in dogs and indicate that a deficient LCAT mechanism is present in cholestasis even with normal or high plasma LCAT activity.

Isolation and partial characterization of a glycine- and serine-rich polypeptide from human serum high-density lipoproteins (HDL)

Lately, several minor polypeptides characterized by a high content of glycine and serum have been described in human serum lipoproteins. By column chromatography we have isolated a glycine- and serine-rich polypeptide from totally delipidized high-density lipoproteins, apo-HDL. The partial characterization of this polypeptide by total amino acid composition and by immunology utilizing monospecific antisera to polypeptides of human serum lipoproteins, would indicate a unique polypeptide. This low molecular weight (4900 Daltons) glycine- and serine-rich polypeptide was characterized by a mobility on polyacrylamide gel electrophoresis, similar to that of polypeptide. A-I, a blocked NH2-terminal amino acid and a terminal COOH-fragment consisting of R-Ser-Ala-Gly-Gly. There are similarities between this polypeptide and the protein moiety of a cholesterol ester-rich lipoprotein fraction, HDLsup, obtained by in vitro incubation of HDL subfractions and described in earlier publications by our group. The identity between these two polypeptides, cannot be confirmed by the present study.

Studies on the pre-alpha-lipoprotein of human serum. I. characterization of an albumin-containing lipoprotein

In this study an albumin-containing lipoprotein is isolated by column affinity chromatography and electrophoresis. Its electrophoretic mobility is faster than that of albumin in agarose gel. Immunologically it reacts with anti-albumin and anti-apolipoprotein-A-I. No reaction is obtained against antibodies to any of the other previously known lipoportein families: Apo-AII, Apo-B, Lp(a), Apo-C, LpX, Apo-D or Apo-E ('the arginine rich peptide'). It differs from purified albumin in having higher amounts of glycine, serine and glutamic acid. Moreover, the molecular weight is estimated to 80,000 as compared to 67,000 for albumin.

Studies on human serum high-density lipoproteins. VI. Studies on a cholesterol ester-releasing reaction in vitro

High-density lipoproteins (HDL) turn turbid during in vitro incubation, concomitant with the formation of a cholesterol ester-rich lipoprotein, designated HDL-sup. The increase in turbidity (A) formed in relation to incubation time (t) is an asymptotic function: A=Uo(1 - e-k1t), where Uo is the amount of HDL with the property of releasing HDL-sup and k1 the velocity constant of the reaction. The increase in turbidity and formation of HDL-sup was not related to cholesterol ester content of the incubated fraction nor to exogenous factors like bacterial growth. The in vitro incubation was accompanied by a cholesterol esterification with a mean production of 8 nmol cholesterol ester/mg HDL protein, but also by a more pronounced degradation of phosphatidyl choline, 148 nmol/mg HDL protein. These data indicate that the lipid changes are induced by a two-step lecithin:cholesterol acyltransfer (LCAT) reaction. This reaction caused in HDL lipids a consumption of surface material and an increase in 'lipid core', presumably leading to a weakening and disruption of the lipoprotein surface with a recombination of 'lipid core' material in the form of HDL-sup.

Studies on human serum high-density lipoproteins. V. Isolation and characterization of a cholesterol ester-rich lipoprotein after in vitro incubation

Purified high-density lipoprotein (HDL), obtained by preparative ultracentrifugation at density 1.063-1.19 g/ml in the cold, were subfractionated by hydroxyl apatite column chromatography. Two of the obtained subfractions (subfractions II and III) turned turbid after incubation at 37 degrees C for 12 h. The turbid material was recovered in the supernatant of D 1.006 g/ml after centriguation at 30,000 g for 2 h. The lipoprotein fraction causing the turbidity was composed of 94% cholesterol ester and 2% apolipoprotein (by weight). On polyacrylamide gel electrophoresis the apolipoprotein moiety appeared as one polypeptide with the electrophoretic mobility of polypeptide A-I, revealed a blocked NH2-terminal amino acid, and had a total amino acid composition that differed from that of A-I and of the arginine-rich polypeptide.